How much insulation.

[Jeremy Harris]

1 minute ago, Sensus said:

 

I'm basing my comments on the construction method that the OP has specified.

 

And even if you keep the cold bridging within acceptable limits, the fact that (roughly) 15% of the construction is timber stud, which as you acknowledge is a relatively poor insulator, any calculation of the U-value that fails to take into account the proportion of timber in the build-up is self-delusional.

 

Don't confuse the issue of cold bridging vs. basic heat loss performance (ie. the average heat loss per unit area of the thermal element).

 

Who's confusing cold bridging vs basic heat loss?  Not me, that's for sure!

 

As a retired senior principal scientist I'm reasonable aware of the basic physics of heat transmission.............................

[ProDave]

Play nicely gentlemen please.
 

[Jeremy Harris]

2 hours ago, Sensus said:

 

 

Your OP failed to even mention it, and your response to a post that said you need to take studs into account when calculating U-values responded by saying that it's irrelevant because there is no cold bridging going on.in your (quite different) frame construction.

 

You might be ex-Grand High Mufti of the BRE for all we know (or care)... I can only draw conclusions from what you have actually written.

 

As a retired senior principal scientist, I'm sure you're aware that there is no form of timber frame construction where no thermal bridging (ie. variation in psi-value across the area of the panel) occurs. What you mean is that the cold bridging is not of a significant degree, but that remains irrelevant when calculating the average U-value of the panel.

 

 

 

Pity you didn't actually read what I wrote, it sort of spoils the effect you were aiming for.

 

I didn't say there was NO cold bridging with Dave's build, what I wrote was this:

 

Quote

FWIW, I'm certain Dave has largely mitigated the stud cold bridging with the thick layer of wood fibre outside the studs

 

Note that the phrase I used was "largely mitigated".  There's an important and significant difference between what you allege I wrote and what I actually wrote.

 

Accuracy seems something that some here have a problem with, and I think it's important that if being critical one should at least make the effort to be accurate in that criticism.

 

As you're well aware, you can reduce cold-bridging to such a degree with a twin-stud wall that, for all practical purposes, it's not even worth the effort of calculating.

Edited February 4, 2017 by JSHarris

[TerryE]

@Sensus, you are correct in that wood has a relatively high R-value compared to most insulation materials so for a single-wall timber construction using the BRE standard method of computing the U-value, the bridging through a 38mm wide CLS member on 600mm centres will drop the composite U-value to 12½ - 15%, however this isn't the case for Larsen strut twin-wall designs where the twinwall construction largely mitigates these losses and the overall effective loss is about 4% of the overall figure for a notional wall made wholly out of insulation.

 

However, in common usage bridging refers not so much to a general lowering of efficiency, but to the creation of heatflow paths which result in cold spots on the surface and in particular those which reduce the surface temperature to the point where there is increased risk it being below the dew point for the room at its normal humidity, thus leading to condensation.  The Larsen strut design almost removes these entirely and you'd be hard pressed to find a temperature variation of more than 1°C over the internal surface of an MBC exterior wall.  Jeremy knows this well because he has a decent thermal camera and can see for himself.  So in this case Jeremy is correct in what he says.   

 

Perhaps it is a little dangerous to be rudely assertive in areas outside your expertise, especially as in this case when you are simply wrong, and the only person that such a tone of comment reflects badly on is yourself.   So please try to refrain from doing so.

 

PS. after Ian's comments:  I've amended the 7% figure to the figures given in BRE 443 section 4.5.1 and rounded up the Larsen strut figure rather than rounding to the nearest whole %.   As a general comment the 15% is guidance base on typical construction standards and includes some allowances for 2-D (commonly referred to as Psi) effects around the through members and other edge effects as a result of (poor) insulation packing.  The real figure can be better and often a lot worse.

[ADLIan]

I've been out for a couple of beers and come back to a bit of spat!!!

 

Back to the top of the page. BS EN (DIN) 6946 gives method for calculating U-values for walls, roofs etc and thermal resistance of air spaces/cavities. For U-values it includes the effect of repeating thermal bridges (timber studs in TF construction, mortar beds in aircrete blocks etc), mech fixings penetrating the insulation layer (wall ties in masonry, fixings through insulation in rendered insulated systems, flat roofs etc) and air gaps in the insulation layer.

 

BR 443 from the BRE gives guidance and how the Standard applies to our Building Regs/Standards. The thermal (not cold) bridge is accounted for but can be mitigated if there is an insulation layer internal or external to the bridged layer.

 

As for ProDave's calculation linked to above - it's wrong. Mainstream UK insulation manufacturers would give accurate numbers (multi foils excepted!)

 

Ian

 

 

[TerryE]

@ADLIan Ian,  I accept what you say for the guidance on conventional timber frames as per PRE 443 sect 4.5.1, but JSH's statements related to a Larsen Design which is even more insulating than the I-beam design discussed in 4.5.2 and out performs it by some margin which is why I said that the 15% (or the 12½%) figure given in 4.5.1 doesn't apply and the technique in section 4.5.2 is more appropriate.  Crank the numbers for a Larsen strut cassette.    

[Jeremy Harris]

31 minutes ago, ADLIan said:

I've been out for a couple of beers and come back to a bit of spat!!!

 

Back to the top of the page. BS EN (DIN) 6946 gives method for calculating U-values for walls, roofs etc and thermal resistance of air spaces/cavities. For U-values it includes the effect of repeating thermal bridges (timber studs in TF construction, mortar beds in aircrete blocks etc), mech fixings penetrating the insulation layer (wall ties in masonry, fixings through insulation in rendered insulated systems, flat roofs etc) and air gaps in the insulation layer.

 

BR 443 from the BRE gives guidance and how the Standard applies to our Building Regs/Standards. The thermal (not cold) bridge is accounted for but can be mitigated if there is an insulation layer internal or external to the bridged layer.

 

As for ProDave's calculation linked to above - it's wrong. Mainstream UK insulation manufacturers would give accurate numbers (multi foils excepted!)

 

Ian

 

 

 

I agree, it's what I dug through from the BRE around 4 years or so ago when doing the calcs (and looking for the "proper" way the building industry looks at things!).

It falls over a bit with Larsen truss type walls and roof construction, because the inner and outer timber frames are only connected with small section timber noggins (38mm x 89mm).  In our case there are only three of these noggins per wall truss and the trusses are on 400mm centres.  Additionally, in common with all Larsen truss type construction, the thermal path through the noggins is long, 300mm, and so the thermal bridging is actually extremely small, so small as to not be worth including in the calcs, because the other assumptions, like those on the surface emissivity and other surface effect losses, massively outweigh the tiny effect of these long and relatively small section noggins.

 

Terry's given the numbers for his house, which uses the same construction method as ours, at 3%.  For our house it was under half this, just because our design is different and our walls are a fair bit lower.

 

Ian, thanks for using the same terminology I used when describing Dave's construction!  Mitigation is indeed the approach his design takes.  Our systems a bit different, in that the idea was to effectively remove the thermal bridges, by making the heat path through them so long, and their cross section so small, that other assumptions in the standard methods were in error to a greater extent than the thermal bridging calcs.  A good example is the method in BS ISO 6946 for calculation of the thermal conductivity of sealed air gaps.  It's an approximation, that's good enough for building regs purposes, but has significant errors as the width of the void increases, as it doesn't take into account internal convection.  This matters not one jot in practice, for a normal form of construction, but it does introduce errors when dealing with a low U value construction method, especially when that construction method has negligible thermal bridging.

 

All these calculations are flawed, and at best approximations, but OK as long as common sense is applied and they are not assumed to be absolutes.

 

Edited to add: Sorry Terry, I was just hitting post on the above when the notification of your post saying much the same about Larsen truss type construction was posted.

 

Edited February 4, 2017 by JSHarris
Terry and I are saying basically the same thing at the same time!

[TerryE]

Picking up another point that Jeremy made in passing is the issue of decrement delay factors.  When your total wall losses are <400W during the peak winter months, another 13W or so are neither here nor there.  When your house is operating in this sort of domain, mitigating the effects of diurnal temperature variation are more significant in that the temperature difference across the walls can vary by 10°C over the day.   The BRE paper doesn't even raise this subject or provide a simple man's method of calculating this. 

 

A typical single-wall with PIR has DDF of the order of a few hours, so the house heating system has to track diurnal variations in order to maintain a consistent comfort level.  The MBC technique of using blown cellulosic filler not only gives complete and consistent packing of insulation, it also has a high c_p yelding a high DDF.  In our case this is enhanced by the thermal inertia of the external stone skin resulting in an overall DDF measured in days.  So we can pretty much use daily averages in our overall heat calcs, and from a design PoV for our temperature control system.

[NSS]

With so much hot air in circulation I wonder that any insulation is needed 

[Jeremy Harris]

Just now, Sensus said:

 

But  I thought you said that you weren't confusing cold bridging and basic heat loss?

 

It doesn't matter whether the cold bridging is 'largely mitigated'. The OP was asking about the calculation of U-values, not the calculation of Psi values.

 

If you're calculating U-values in thermal element with a layer that is composed of a significant proportion of timber, you need to take that timber into account. It really is that simple. It's a point you completely failed to mention in your original contribution to this thread, and  ADRIan was quite right to pick you up on it.

 

 

 

I really wish you'd not come across as so intent on creating an argument, rather than a more reasonable debate.

 

To summarise, the OP asked how to calculate the U value of a structure, I gave the simplistic way of doing it JUST FOR THE INSULATION AND SKIN MATERIALS.  The conversation continued and I offered to upload a simple U value calculator to my website and give a link, which I did, and with that link I wrote this text disclaimer, for the OP in this thread and anyone else reading it:

 

Quote

The U Value Calculator is again a bit “quick and dirty”, as it takes no account of thermal bridging within a structure and neither does it adjust for varying surface heat transmission.  The outer face of a wall or roof will, for example, have quite a wide variation in surface heat loss rate, and I’ve made no effort to put the surface thermal transmission corrections in to this simple calculator.  It assumes still air both sides and makes no corrections for varying surface emissivity.  This means it will, generally, give a lower (better) U value than the centre of a section with thermal bridging members, but also means that you could have a U value that’s higher (poorer). by perhaps as much as 15%. for a timber frame that has no mitigation against thermal bridging.

Our house is virtually thermal-bridge free, as it uses some clever design features to virtually eliminate all the normal cold bridges you get with a timber framed house, and the passive slab foundation removes the wall/foundation junction heat loss path that a standard form of construction will probably have to some extent.  It’s mainly for this reason that I didn’t take the time to model thermal bridging losses, as they didn’t apply to our particular house build method.

 

From my point of view, early in this thread (some four hours before your first contribution) I had explained the limitations and why I'd not bothered to include thermal bridging in the "quick and dirty" spreadsheet.  Many of the online U value calculators from the insulation suppliers take the same approach, but for different reasons, they almost certainly want to show the best U value their products can achieve and they also have no way of knowing how the customer will use the product.

 

The thread then deviated, as threads do, to Daves build and his thermal bridge mitigation strategy, which is, I believe,. a separate issue.

 

I don't disagree with what you've written above at all, BUT, you decided to take a rather argumentative approach and I don't think you actually followed what had happened some hours before you posted here.  Did you read the disclaimer and warning of the limitations I wrote on the link I gave yesterday morning?

 

Finally, our Larsen truss type timber frame has thermal bridges that are so tiny as to be down in the noise when it comes to all the other approximations that are made in BS ISO 6946.  And, FWIW, one of the timber frame houses we looked at, built by the same frame company we used, had zero thermal bridging, and I really mean zero.  It's not a system they use now, because it took longer to build and so was not as cost effective as using the modified Larsen truss method, but others have used the same technique of EWI on a structural frame for the same reason.  Personally I accepted the minuscule additional loss through the noggins in return for a lower price and quicker frame erection time.

 

 

 

 

[ProDave]

15% timber stud does not mean 15% of the frame has NO insulation. If you really wanted to be pedantic, you would look up the  λ value for timber and insert that instead for that 15% of the wall. 
 

[Jeremy Harris]

47 minutes ago, Sensus said:

 

I'm not intent on creating an argument, JS, really I'm not.,

 

My original contribution to this thread was merely to agree with a very valid and entirely correct point made by ADRIan that you need to take the studs into account when calculating the U-value of a timber frame.

 

It was you who then tried to obfuscate your way out of admitting an omission by introducing a lot of irrelevancies about thermal bridging. I know that it is typical of forums that there are egos involved who will never admit that they are anything less than totally perfect and omniscient, but the easy response would have been simply to acknowledge 'yes, you need to take the studs into account in the thermal layer in which they occur'.

 

If you're anally retentive and have a lot of time on your hands, you can work it out precisely, but as ADRIan pointed out, the standard methodology is to assume a rule-of-thumb of 15% timber stud for a conventional panelised timber frame construction.

 

U-value calculation is not complicated unless you choose to make it so.

 

If you read the disclaimer I wrote yesterday morning, before you posted in this thread, you will see that I clearly said that thermal bridging needed to be accounted for in a conventional frame, long before your first post here!  I actually wrote that it could increase the U value by around 15%.

 

I think the issue here is that I knew, some hours before you first posted, that I'd made thinks transparently clear in the link I gave for the OP.   Every other post by me after that was assuming that everyone had read that, and frankly I don't think some did, as if they had then they would see there was no obfuscation going on by me at all.

 

EDITED TO ADD:

 

As it seems that the disclaimer and statement of limitations that I posted in good faith yesterday morning has been overlooked by some, I've just taken the spreadsheet down, added a similar disclaimer highlighting the limitations in a row at the top of the spreadsheet and re-posted it in the same place.  Can I suggest that anyone who has already downloaded it please delete their copy and download the version with the disclaimer in, as I really don't want this topic to pop up again because someone has the older copy without the limitations explained.  I should add that I wrote the thing for my own benefit a few years ago, and never for one moment thought that it'd be argued over like this.

 

Perhaps if I'd thought to take the time to do this yesterday morning some of the unpleasantness here could have been avoided - I apologise for that like of foresight on my part, I really didn't envision this thread going the way it did, nor do I understand at all why it has.

Edited February 5, 2017 by JSHarris
Edited at around 10:28, 5/2/2017 to add the lower comment

[Jeremy Harris]

13 minutes ago, ProDave said:

15% timber stud does not mean 15% of the frame has NO insulation. If you really wanted to be pedantic, you would look up the  λ value for timber and insert that instead for that 15% of the wall. 
 

 

You're right, Dave, but I'm pretty sure the 15% here is the assumption made in the BS method if psi isn't calculated.  It's rather like the ventilation rate assumption if an air test isn't performed in SAP.

 

Calculation of psi is easy enough, but time consuming if you have a complex structure, unless you use a thermal modelling application that will do the analysis for you.  Even then there are limitations.  For example, some models use 2D flow approximations to true 3D flow, to ease the time taken to run the model.  This is reasonable for a typical form of construction, but falls over a bit when looking at methods where any thermal bridging is by long and relatively slender members surrounded with insulation. 

 

Some time ago I did take the time to do a proper 3D model of a 1m² square in the centre of a 300mm Larsen truss type wall, with two noggins included.  I spent a lot of time doing it for no useful purpose, as the heat transmitted through the noggins and the thinner insulation between the twin studs was so small that it was lower than the inaccuracies in some of the other assumptions made in the standard method.  Rightly or wrongly I took the view that when you were dealing with 1% to 2% variations it wasn't worth the disproportionate effort of calculation involved.  I don't think this is unreasonable, as it's similar to the way BS ISO 6946 works; that also makes some assumptions and gives approximate calculation methods that are simplified to the point where the absolute errors they introduce are down in the noise.

Edited February 5, 2017 by JSHarris

[NSS]

Wow, lots of BS involved then 

[Jeremy Harris]

15 minutes ago, NSS said:

Wow, lots of BS involved then 

 

I think that, at the moment at least, they are probably really more ISO (DIN) than BS, given the subsidiary ranking BS have within our current rule-making arrangements!

[Russell griffiths]

Bugger me I only want to build a wall, 

[ADLIan]

Good point!

 

Phone call or email to your insulation manufacturer of choice tomorrow outlining your proposed wall and you'll get the number crunching done in accordance with the BS and BRE 443 which is all you need to keep your BCO (and SAP assessor) happy. All of the mainstream insulation manufacturers will provide accurate calculations along with detailed breakdown of the numbers used.

 

Ian

 

[Jeremy Harris]

34 minutes ago, ADLIan said:

Good point!

 

Phone call or email to your insulation manufacturer of choice tomorrow outlining your proposed wall and you'll get the number crunching done in accordance with the BS and BRE 443 which is all you need to keep your BCO (and SAP assessor) happy. All of the mainstream insulation manufacturers will provide accurate calculations along with detailed breakdown of the numbers used.

 

Ian

 

 

Ian, I agree, but during the early stages of trying to get your head around which supplier to use, which build method, etc, it's useful to at least have some understanding as to how the basics of insulation work, before going to the extent of seeking proper values using the BS and BRE 443.

 

As an example, when we first started planning our build we knew, in broad terms, what we could afford and what we wanted to achieve in terms of performance.  I also had a basic set of drawings of the house, with no construction detail.  I went around at least a dozen frame suppliers to get price and performance data, and found it very, very hard to be able to compare any of them.  The final straw for me was visiting a small development being built by one supplier and seeing first hand that the specification they had quoted wasn't being followed in practice by their workers (missing insulation, no cavity closers around windows, problems even getting through the BR air test etc).  It was a thoroughly demoralising time.

 

It was then that I decided I needed to try and understand the relative importance, to us, of each element.  Doing some basic research (and a lot came from the BRE) I came up with overall U values that would meet our heat loss target, but then learned about decrement delay, and the impact this has on practical comfort level.  I ended up going around the whole loop again, and approached a sub-set of suppliers as a "slightly more intelligent" customer, with the intention of trying to find out, as best I could, who really understood what they were offering and who didn't.  If I'm honest, I was very disappointed with the majority of the suppliers I took the time to drive fairly long distances to talk to.  I ended up with two potential suppliers, both offering similar overall thermal performance in terms of heat loss, but one offering a very much higher decrement delay solution.

 

The only way I got to that point was by a hard slog, and digging through mountains of misleading information, including some from insulation suppliers themselves (try ringing one of them up and asking for the specific heat capacity of their insulation, for example!). Only a handful of insulation manufacturers could provide specific heat capacity or decrement delay factors for their materials.

 

In the end we chose our supplier on the basis that they met our performance spec, offered a lower risk build, by including the insulated foundation system, the fact they had a high decrement delay build system and that their price was OK.  I'd be the first to admit that our requirements were just that, our requirements.  Everyone doing a self-build will have differing needs, and differing priorities, so what worked for us may well not work for someone else.

 

Finally, I'd just like to reiterate that if I offer something in the way of advice, it should be read in the context of my experience, which is designing and building ONE very low energy passive house.  I'm not a professional in the building industry, I had only a DIY level of understanding when we decided to self build around 9 years ago now, and I'm entirely self-taught over that 9 year period when it comes to specific building-related stuff.  The only slight advantages I had was having a lot of time to study (I've been retired for 6 years now) and having a reasonably good understanding of physics and mathematics, and I have to say that often the latter has created a bit of controversy, particularly when some suppliers don't seem to understand basic physics and quote data that is highly questionable!

Edited February 5, 2017 by JSHarris
typos

[TerryE]

1 hour ago, Russell griffiths said:

Bugger me I only want to build a wall, 

 

Thanks for bringing us back to the subject.  In your original post, you said that you were considering two methods of wall construction both timber frame both with external timber cladding: the single and double stud wall constructions.  The double stud wall construction technical, usually using a Larsen Strut design has only come into popular use after BRE 433 was first developed and even the 2006 edition which is the one that I have doesn't even discuss it.

 

So what this boils down to is do you want a number to plug into a SAP calculation to keep your building inspector happy in which case take the advice that Martin (@Sensus) and Ian (@ADLIan) gave and go with that.

 

However, Jeremy and I made the assumption from your OP that you actually want to understand the difference between the benefits of the "double stud wall" construction.  Perhaps addressing your original Q was a mistake on our part.  But at a high level, my summary is that:

•  A single wall construction will typically have a U value of ~ 0.25 and a decrement delay factor of <3 hrs.  You are also critically dependent on quality control during manufacture since poorly fitted insulation will allow air circulation within the panels with a severe degrading of the actual U-value achieved.  There is a some risk of material cold bridging in the frame
• A Larsen strut construction will typically have a U-value of ~0.12 and if filled with blown cellolisic filler have a decrement delay factor of much > 24hrs. The blown filling technique is far less prone to voids and consequential bridging.

The first approach will give you a warm house that you will still need to heat with a conventional central heating system on all floors.  The latter (if you get the other system components right) will give you a house that you will only need to heat a couple of months a year and will essentially act as a single zone, so no central heating system is really needed, though you do need some method adding say up to 1 kW during the mid-winter months.

 

Jeremy and I both have a twinwall house so we are talking personally about our experiences. Martin is professionally an architect, Ian is a SAP assessor.    Perhaps they can use their expertise also to address your original Q from their perspectives: what are the pros and cons of single wall vs twinwall construction?

[Jeremy Harris]

6 minutes ago, TerryE said:

 

Thanks for bringing us back to the subject.  In your original post, you said that you were considering two methods of wall construction both timber frame both with external timber cladding: the single and double stud wall constructions.  The double stud wall construction technical, usually using a Larsen Strut design has only come into popular use after BRE 433 was first developed and even the 2006 edition which is the one that I have doesn't even discuss it.

 

So what this boils down to is do you want a number to plug into a SAP calculation to keep your building inspector happy in which case take the advice that Martin (@Sensus) and Ian (@ADLIan) give and go with that.

 

However, Jeremy and I made the assumption from your OP you actually want to understand the difference between the benefits of the "double stud wall" construction.  Perhaps addressing your original Q was a mistake on our part.  But at a high level, my summary is that:

•  A single wall construction will typically have a U value of ~ 0.25 and a decrement delay factor of <3 hrs.  You are also critically dependent on quality control during manufacture since poorly fitted insulation will allow air circulation within the panels with a severe degrading of the actual U-value achieved.  There is a some risk of material cold bridging in the frame
• A Larsen strut construction will typically have a U-value of ~0.12 and if filled with blown cellolisic filler have a decrement delay factor of much > 24hrs. The blown filling technique is far less prone to voids and consequential bridging.

The first approach will give you a warm house that you will still need to heat with a conventional central heating system on all floors.  The latter (if you get the other system components right) will give you a house that you will only need to heat a couple of months a year and will essentially act as a single zone, so no central heating system is really needed, though you do need some method adding say up to 1 kW during the mid-winter months.

 

Jeremy and I both have a twinwall house so we are talking personally about our experiences. Martin is professionally an architect, Ian is a SAP assessor.    Perhaps they can use their expertise also to address your original Q from their perspectives: what are the pros and cons of single wall vs twinwall construction?

 

Terry, you are exactly right. 

 

I did look at the question from MY perspective, and that is different from the way that Martin (Sensus) looked at it, different again from the way Ian (ADIan) looked at it and definitely more aligned with the way you looked at it.

 

Perhaps we all need to highlight where we are coming from when answering a question like this, or better still, ask the OP why they are asking the question - is it to gain a bit more understanding, or is it to satisfy building control, for example.

 

 

Edited February 5, 2017 by JSHarris

[Russell griffiths]

51 minutes ago, JSHarris said:

.  The final straw for me was visiting a small development being built by one supplier and seeing first hand that the specification they had quoted wasn't being followed in practice by their workers (missing insulation, no cavity closers around windows, problems even getting through the BR air test etc).  It was a thoroughly demoralising time.

Js has hit the nail on the head. 

The reason for all the questions is I will probably end up building the frame myself or at least buying a standard frame and improving on it. 

Last week I was on a site where two houses are going up,I decided to be nosey and go and have a poke about, from 200m away they looked like a fairly cheap frame 150mm studs with some Kingspan stuffed in, well on closer inspection I was horrified with the build quality, I didn't say anything and chatted with the lads putting it up, I asked a few questions and told them I was about to submit my planning and was just doing some research. 

They both chuckled and said they wouldn't buy one of these, they're crap. 

When you are dealing with a company with Swedish links in its name you would think you were buying a quality product, how wrong you are, be careful out there. 

Just as an add on, both these houses have a build cost of over £500,000 that does not include land price. 

I was concerned so much I have even thought of trying to find the purchaser to warn them. 

[NSS]

29 minutes ago, TerryE said:

 

•  A single wall construction will typically have a U value of ~ 0.25 and a decrement delay factor of <3 hrs.  You are also critically dependent on quality control during manufacture since poorly fitted insulation will allow air circulation within the panels with a severe degrading of the actual U-value achieved.  There is a some risk of material cold bridging in the frame
• A Larsen strut construction will typically have a U-value of ~0.12 and if filled with blown cellolisic filler have a decrement delay factor of much > 24hrs. The blown filling technique is far less prone to voids and consequential bridging.

The first approach will give you a warm house that you will still need to heat with a conventional central heating system on all floors.  The latter (if you get the other system components right) will give you a house that you will only need to heat a couple of months a year and will essentially act as a single zone, so no central heating system is really needed, though you do need some method adding say up to 1 kW during the mid-winter months.

 

Jeremy and I both have a twinwall house so we are talking personally about our experiences. Martin is professionally an architect, Ian is a SAP assessor.    Perhaps they can use their expertise also to address your original Q from their perspectives: what are the pros and cons of single wall vs twinwall construction?

 

Sorry Terry, but can you qualify what you mean by a "typical" single wall construction? The way you have worded this comparison gives the impression that using the system you and JSH employed produces a house that will be twice as good on u values and at least 8 times better on decrement delay than a single wall system. Some reading this forum might get the impression that you are saying any single wall construction will as poor as you suggest.

[TerryE]

@NSS, Neil,  you can definitely get a single wall construction up to the same standard as a twinwall in terms of U-value, typically by adding extra layers of insulation such as a plasterboard/RUR composite over the service cavity.  I haven't done the numbers but a 50mm PUR or PIR layer would help a lot.  The quality issue is still problematic particularly if the inner skin has been incorporated in the factory.  At least if this is done on site, the home owner or project manager can inspect the fit of the internal block insulation and where necessary use injected foam to close up fitting gaps.  (A gap of 5mm top and bottom can more than double the effective U-value.)

 

As to the DDF, this is more down to the bulk properties of the main fill material.  PUR and PIR have excellent the thermal conductivity, but also very low the heat capacity, and its the ratio that largely dictates the DD. Any wall which uses these as its main filler (or any other low thermal capacity insulation) will usually end up with a low DD.  See this article which describes the concept and issue:  Greenspec: Decrement delay & Thermal buffering.

 

[Jeremy Harris]

5 hours ago, Russell griffiths said:

Js has hit the nail on the head. 

The reason for all the questions is I will probably end up building the frame myself or at least buying a standard frame and improving on it. 

Last week I was on a site where two houses are going up,I decided to be nosey and go and have a poke about, from 200m away they looked like a fairly cheap frame 150mm studs with some Kingspan stuffed in, well on closer inspection I was horrified with the build quality, I didn't say anything and chatted with the lads putting it up, I asked a few questions and told them I was about to submit my planning and was just doing some research. 

They both chuckled and said they wouldn't buy one of these, they're crap. 

When you are dealing with a company with Swedish links in its name you would think you were buying a quality product, how wrong you are, be careful out there. 

Just as an add on, both these houses have a build cost of over £500,000 that does not include land price. 

I was concerned so much I have even thought of trying to find the purchaser to warn them. 

 

 

After our experience back then, I've deliberately been a bit nosey around building sites, and it seems pretty common for construction standards to be pretty lax.  I think I know the main reason, it's to do with the amount of money some of the big builders are prepared to pay tradespeople.  They expect to pay labourers rates for skilled trades, and end up with the bottom of the pile people who couldn't pass muster working for themselves or a more discerning employer. 

 

I've gone so far as to walk around a local part-complete estate nearby, that was only approved on the basis that the houses be built to the old Code For Sustainable Homes Level 4.  That implies inspection of every house, unlike the usual system where only one house of a particular type is inspected and the rest are assumed to be the same.  I walked around with a thermal imaging camera, on a cold evening before Christmas.  The results were shocking.  Every single completed house was leaking heat like a sieve, there was missing insulation evident all over the place, including one house where the entire first floor wall at the front had no wall insulation at all.  Thermal bridges were all over the place, some almost certainly air leakage around door and window apertures.

 

I discussed what I'd seen when out and about (before I walked around with the thermal camera) with our building inspector, who I'd got to know reasonably well during our build.  He reckoned that probably around 60% of new homes didn't comply with Part L of the regs, and, from the tone of his voice, I got the impression that he was quite concerned about it.  There's nothing that can be done by building control, whether LABC or a private company, as big builders are allowed to build the majority of their houses without any inspections at all.

 

It seems to hit the news now and again, almost always when a newly moved in resident realises that the heating bills are way higher than they thought they should be.  I saw one investigative TV programme some time ago that followed such a story, and one or two unhappy new residents have posted their thoughts around the web.  However, the bottom line is that the majority of people who move into a new house just won't bother to complain about, or probably don't even notice, the poor thermal performance.  As someone said to me when I was having a bit of a rant about it a couple of years ago, people buying new houses are far more influenced by the kitchen and bathroom bling than they are by the Energy Performance Certificate.

Edited February 5, 2017 by JSHarris
funny typo corrected, "mew" when I meant "new"......

[Ferdinand]

2 hours ago, Russell griffiths said:

Bugger me I only want to build a wall, 

 

Ah. The immortal words of Emperor Hadrian President Trump :-) .

 

To add something useful @Russell griffiths, I think that a "typical" wall that you asked about will be referring to one built to meet basic Building Regs' standard or a little better on a "typical" new build house. Say with the u-value quoted of 0.25, whereas people here tend to build walls to a u-value significantly lower than that.

 

One of the design tools that should help you decide what is appropriate could be to model your long term etc costs over periods of 10 or 25 years against the extra cost of the higher-spec wall. Or you could use your intended occupation period if you know it.

 

Others (usually including me) would take a less pragmatic position and build it to the higher spec anyway. Many here are planning to stay for 20 years or often forever. I would argue that at some point the value of a high spec (or not) house will reflect the excess 20k in bills which will be paid over the next 15 years.

 

Hope that helps.

 

Ferdinand