The Happy Medium

[Gorlando]

As the title suggests I'm trying to work out where the happy medium in relation to the target u value/s and air tightness for my new build.

 

We're about to have our initial design meet with our designer and I want to have some u values in mind to specify. I very much want to adopt a fabric first approach using block and brick and have already had thoughts around specifying thin joint aircrete for the inner skin and a tetris insulated floor.

 

My question is, at what u values for the different elements (walls, floor, roof, windows etc) do you get the most bang for your buck? 

 

I know this could be quite subjective and is dependant on my house size and budget, but I want to target that sweet spot before costs get silly for minimal improvement over and above that spot.

 

Cost effective is key. 

 

Further details on request.

 

Thanks in advance

 

 

Edited March 17, 2017 by Gorlando

[joe90]

Well, I can't answer your question because I don't know what U value I am building too!!!  But, like you I want value for my buck. I am building in a part of the country with a mild climate, using block and brick and 200 mm cavity with dritherm 37 full fill. I will do my best to make it airtight. Currently getting prices for windows and my local joiner ( who has a glazing design expert on call) can get double glazing within 0.1 U value of other triple glazing I have been quoted for. 

 

In in my opinion my build is cost effective and value for money ( I am a Luddite).

[Calvinmiddle]

My view is that you going about this the wrong way.

I'd pick a u value and then see which construction is the cheapest.

After have a factory built timber frame there is no way I would go for brick and block.  Timber frame quicker, more QA and cheaper (for me anyway)

if you want solid external walls then you can bills a brick or stone skin easy.  If you want solid internal walls you may be surprised how solid they are.  Could can also double skin the plasterboard or even build solid internal walls of you really want to.

Evertone will have their own bias on the best method method, but try and keep and open mine and get your targets first - then see how you can get there

[TerryE]

30 minutes ago, Gorlando said:

As the title suggests I'm trying to work out where the happy medium in relation to the target u value/s and air tightness for my new build. ... Cost effective is key. 

 

We've got a passive-class house which cost no more than a conventional build.  The stone skin that we had to put on to keep the planners happy cost as much as the slab, the structural frame including warm roof, it's insulation and airtightness testing combined.  So I would challenge your implied assumption that improved energy efficient = more expensive.

 

What is true is that most of our small builders and major construction firms seem incapable of delivering on the quality and attention to detail that is needed to construct a passive-class house, so going down this route my be more difficult / stressful, but it's not more expensive as such. If builders and their trades cut corners or are sloppy in their work, then you don't see any cost savings because of this do you?  Though you do have to suffer the through life consequences.   

[Jeremy Harris]

The same applied for our build; we have a "negative energy" house (it makes us more money for exported energy than it uses over a year) and yet the fabric of the house didn't cost any more than a conventional build would have, in fact one architect who asked to see the cost breakdown got back to me with a comment that he thought the building fabric cost was perhaps 10% lower than he expected for our area.

Edited March 17, 2017 by JSHarris

[Declan52]

The simple answer is build what you can build to suit your budget. Price every option up from block to timber frame and work it all out. Each method has its pros and cons and it's only you who can make that choice. 

Once you decide on the method then it's not that hard to work out how much it costs to get each u value. @JSHarris has done a good calculator where you can play about with different u values for everything from the walls to windows and see how much better of you will be if you go 0.1,0.2, etc lower. 

No matter what method you go all the u values you will work out will be how good it will be in a lab, how close you get to that will depend on how much care and attention is paid to all the details you need to get correct during the build. No point in paying for 200mm insulation to be put in a cavity or between studs if it's installed badly. 

 

Is there a specific reason for going for a suspended beam and block floor??? Poor ground conditions for example.

[Crofter]

A lot of the best ways of improving bang for buck come from the basic design and layout. Mark Brinkley explains this well in the   Housebuilders Bible. Looking specifically at energy performance, you want to tend towards a square footprint and avoid complicating the basic box shape. Don't overdo the glazed area (even good windows are far worse insulated than poor walls, and obviously cost much more to boot); consolidate many smaller windows into individual larger units. Don't have a chimney. All of these factors improve thermal performance yet actually lower build cost.

[MikeSharp01]

3 hours ago, JSHarris said:

The same applied for our build; we have a "negative energy" house (it makes us more money for exported energy than it uses over a year)

Eh up Jeremy.  I think @Gorlando needs to know that while your house has amazing insulation and very cost effective at that, the negative energy aspect is a function of how much Solar you can harvest - and too some extent the fitting of ASHP, its COP coupled to the effectiveness of the various control systems in the home. The size of the solar array - in your case more that the 4Kw of a bog standard, well the most common size out there, system helps you a bit - have you, I wonder, ever worked out how big it would need to be be be a zero energy (not negative) home.  Clearly size of the array makes it possible to bring any house into negative energy but the essential point, which you have achieved and pointed out is that the insulation / air tightness engineering is key at the outset and that this should cost no more than a normal home to build - its just attention to detail that makes it possible.

[Oz07]

5 hours ago, joe90 said:

Well, I can't answer your question because I don't know what U value I am building too!!!  But, like you I want value for my buck. I am building in a part of the country with a mild climate, using block and brick and 200 mm cavity with dritherm 37 full fill. I will do my best to make it airtight. Currently getting prices for windows and my local joiner ( who has a glazing design expert on call) can get double glazing within 0.1 U value of other triple glazing I have been quoted for. 

 

In in my opinion my build is cost effective and value for money ( I am a Luddite).

Out of interest is the timescale for payback no good on swapping full fill 37 with dritherm 32? What difference in u value would you get and how long would it take to pay back?

[Gorlando]

Thanks for all the replies.

 

I see the point of what you are all saying. I definitely need to have a play around with the u value calculator and then cost a few things out etc. 

 

A main contractor will be completing foundations, slab and erect the weather tight shell. I will insulate it and then finish the rest/organise subs where needed so I can control a lot of the installation. Our intended builder/contractor is a good guy I know who will take pride in it I feel.

 

I'm set on brick and block that's final, timber frame just isn't for me. 

[Declan52]

Then go for a cavity that is a min of 150mm wide and get the beads blown in. Don't let the builder put cavity insulation in as the beads will give a better real world result. Also means you save some money as the beads don't have to go in till  just before it's plastered so helps cash flow at the start.

[Ian]

I thought I'd play 'devil's advocate' and give you an alternative viewpoint.

 

I've just completed a new build holiday home bungalow in N Wales of 71m2 (internal area).

 

After looking closely at the economics of building something to Passivhaus standards of insulation I decided instead to keep things very simple and instead designed it to comply with minimum building regs standards.

 

We exceeded the minimum building regs standards by a small margin and, as built, our predicted energy demand is 43 kWh/m2/yr compared to the Passivhaus standard of 15kWh/m2/yr.

 

We're using bulk LPG (which is approx double the price per kWh compared to mains gas) yet the predicted annual cost to heat the house works out at £135/year compared to £45/yr if it were to Passivhaus standards.

 

Our annual space heating demand is predicted at 3,012 kWh

 

£90/year is not a lot of difference and it would be circa £45 if we had mains gas available.

 

The building is very small to keep costs low and this size issue was a factor in the decision to go with minimum building regs compliance. For example it has meant that I was able to keep external wall thickness to just 270mm maximising the usable internal area of the building (140mm timber stick frame with external cedar weather boarding).

 

Just a thought for you to consider.

 

Ian

Edited March 17, 2017 by Ian

[MrP]

Traditional construction can be built to very high even passivhaus standards with a little care, the right spec and not too much additional expense. Google the Denby Dale or Golcar passivhaus and most of the construction details, thermal values and costs are online.

[A_L]

2 hours ago, Oz07 said:

Out of interest is the timescale for payback no good on swapping full fill 37 with dritherm 32? What difference in u value would you get and how long would it take to pay back?

 

Taking a wall of 105mm brick/200mm cavity/100mm lightweight block (lambda 0.11) wall gives U-values of 0.151/0.134 W/m².K for dritherm 37/dritherm 32 full cavity fill respectively. The difference of 0.017 represents an energy saving of very much less than 1 kWh/m²/annum in a central England climate (probably around 0.4kWh/m²/annum). I do not have costs for dritherm 32 but it is rarely worthwhile paying for the premium versions of any insulation.

[MikeSharp01]

1 hour ago, Ian said:

I thought I'd play 'devil's advocate' and give you an alternative viewpoint.

I can see Ian's point of view here but the passive house concept is more than just insulation and energy costs its about comfort and to some extent ecology. The obligatory MVHR, recovering as much heat as possible. The drive for air tightness so there are no draughts and the insulation does not have to work against airflows within it allowing the MVHR to be fully effective. In the longer run the effect on the planet must also be a factor. Finally Ian has also used the max energy consumption a passive house can have while many will be well below this thus making a greater difference. In the end its down to your, @Gorlando, balance, hence your post, but I think cost is not really in there as you can build passive very close to non passive costs and the return on investment make up the difference in a few years. PS Not sure why I am such an advocate...

[TerryE]

Ian, Crofter and others have made some very good points.  A lot of the value for money discussion is about overall balance.  In our case having a traditional "Cotswold stone" exterior (or the Northampton equivalent which looks the same and costs as much) was the dominant cost.  Making the house passive class was small beer in comparison.

 

Keeping the design simple (four walls, singe pitched roof; sensible windows sizes;; not fancy design statements; putting all wet services around a central core) will keep the costs down.   Only add fancy stuff if it is really going to add value that you want.  For example,  there's no point having a wall of glass to overlook a local housing estate, and giving the neighbours a full show of your and your wife's bits and pieces when you want to get out of bed to have a pee.

 

Ditto avoiding architects, project managers, quantity surveyors -- these are great if you are rich and have a fancy build, but you could easily find yourself spending out £50K+ on stuff you could do yourself.  Keep the design simple; minimise the number of major sub-contractors; pick a local builder with a good local rep who wants to keep it.  Have a clear idea of what you and your partner want to do yourselves and what you need to pay hard cash for a "suitable qualified person" to do is a good point to consider.  Some specialist stuff, you have to subcontract.  IIRC you are a sparkie.  If you have your tickets then you can do all this yourself, and if you haven't then you can still probably do it all yourself, but get a mate to certify it. But what about woodwork, ... what do you want to take on and what do you want to pay for?

 

There's no point of going from a wall U-value of 1.6 to 1.2 unless you have first sorted out the MVHR system because by 1.6, your air leakage is dominating your overall efficiency.

 

I personally thing that beam and block floors are crazy: why stick an air void under the house which is only going to kill the thermal performance of the floor?  Concrete isn't known to suffer from dry rot.  But a lot of this is down to you choosing the right local builder, and staying within his (or her) comfort zone.  So if you do go for beam and block; then keep it simple; put the beams low; then 200mm EPS min and a single 100mm concrete layer with the UFH embedded in it.  Use the continental approach of casting the whole lot as a single slab before you go above FFL.

 

If you are going to use a blockwork skin, then read up on Tony's House joist socks, because air sealing the joists in a conventional build is a major pain unless you address in the build -- at which point it becomes trivial.  Think about how you are going to do thermally efficient gap closing.

 

Throughout the news and on this site, you will see the consequences of crap quality control in using block thermal PUR insulation, or the equiv.  As @Declan52 says, a blown insulation such as cellulosic filler or balled EPS avoids all of these risks.

 

Lastly design the house to your own specification, then make sure that it is actually built to it, and then take advantage of this.   It really isn't that hard to get the worst winter months' heating requirement down under 2 kW.  But once you are down at this level, then why spend £10-15K on some complex central heating system?  As I said, stick some UFH loops in your slab and use a small heater / ASHP / combi + buffer.  You might need a towel rail or two heated but if the house is at this spec then there's no point in putting in upstairs heating.

 

 

 

[Oz07]

6 hours ago, TerryE said:

Ian, Crofter and others have made some very good points.  A lot of the value for money discussion is about overall balance.  In our case having a traditional "Cotswold stone" exterior (or the Northampton equivalent which looks the same and costs as much) was the dominant cost.  Making the house passive class was small beer in comparison.

 

Keeping the design simple (four walls, singe pitched roof; sensible windows sizes;; not fancy design statements; putting all wet services around a central core) will keep the costs down.   Only add fancy stuff if it is really going to add value that you want.  For example,  there's no point having a wall of glass to overlook a local housing estate, and giving the neighbours a full show of your and your wife's bits and pieces when you want to get out of bed to have a pee.

 

Ditto avoiding architects, project managers, quantity surveyors -- these are great if you are rich and have a fancy build, but you could easily find yourself spending out £50K+ on stuff you could do yourself.  Keep the design simple; minimise the number of major sub-contractors; pick a local builder with a good local rep who wants to keep it.  Have a clear idea of what you and your partner want to do yourselves and what you need to pay hard cash for a "suitable qualified person" to do is a good point to consider.  Some specialist stuff, you have to subcontract.  IIRC you are a sparkie.  If you have your tickets then you can do all this yourself, and if you haven't then you can still probably do it all yourself, but get a mate to certify it. But what about woodwork, ... what do you want to take on and what do you want to pay for?

 

There's no point of going from a wall U-value of 1.6 to 1.2 unless you have first sorted out the MVHR system because by 1.6, your air leakage is dominating your overall efficiency.

 

I personally thing that beam and block floors are crazy: why stick an air void under the house which is only going to kill the thermal performance of the floor?  Concrete isn't known to suffer from dry rot.  But a lot of this is down to you choosing the right local builder, and staying within his (or her) comfort zone.  So if you do go for beam and block; then keep it simple; put the beams low; then 200mm EPS min and a single 100mm concrete layer with the UFH embedded in it.  Use the continental approach of casting the whole lot as a single slab before you go above FFL.

 

If you are going to use a blockwork skin, then read up on Tony's House joist socks, because air sealing the joists in a conventional build is a major pain unless you address in the build -- at which point it becomes trivial.  Think about how you are going to do thermally efficient gap closing.

 

Throughout the news and on this site, you will see the consequences of crap quality control in using block thermal PUR insulation, or the equiv.  As @Declan52 says, a blown insulation such as cellulosic filler or balled EPS avoids all of these risks.

 

Lastly design the house to your own specification, then make sure that it is actually built to it, and then take advantage of this.   It really isn't that hard to get the worst winter months' heating requirement down under 2 kW.  But once you are down at this level, then why spend £10-15K on some complex central heating system?  As I said, stick some UFH loops in your slab and use a small heater / ASHP / combi + buffer.  You might need a towel rail or two heated but if the house is at this spec then there's no point in putting in upstairs heating.

 

 

 

To be fair I had blown in dritherm on last house I built. This was after bad experiences of brickies installing poorly. However I'm not sure how well the blown in job was. If going down glass fibre route you just need to make sure subbies are thorough. 

 

Eps beads are the way forward though as you say. Better flow and better performance 

[Ferdinand]

@Gorlando

 

My two comments:

 

1 - To get some idea of balance between spending money up front on better fabric, and energy costs that will save you, I suggest doing cost models of your house including the first 10 and 25 years of energy bills. There will be assumptions, but you will get a feel for the cost/benefit.

 

2 - Think about extra floor area from better insulation. It is true that EPS beads or pumped cellulose will completely fill your cavity, but a more effective insulant will make your walls 50-100mm thinner for the same performance and give you more space .. you will be surprised how much. And you can mulitply that extra by perhaps £3k per sqm to work out the increase in value, everything else being equal.

 

I am thinking of materials such as Kingspan Ecobead Platinum or Celotex sheet.

 

Ferdinand

 

(Sorry - the thread title makes me think of Doris Stokes' Sherry Consumption.

 

Edited March 18, 2017 by Ferdinand

[Oz07]

Cmon Ferdinand if floor area was that crucial the big boys wouldn't be doing dot n dab!

 

50mm round a perimeter adds up but I doubt the internal area/value is going to stack up exactly like that. Especially with how the British buy. Bedrooms as opposed to floor space 

[Ferdinand]

21 minutes ago, Oz07 said:

Cmon Ferdinand if floor area was that crucial the big boys wouldn't be doing dot n dab!

 

50mm round a perimeter adds up but I doubt the internal area/value is going to stack up exactly like that. Especially with how the British buy. Bedrooms as opposed to floor space 

 

I don't know the firm answer to that; I have not had a conversation with a relevant professional yet.

 

But on a 0.1 u value wall the difference in thickness between EPS and PIR (other things being equal) is over 100mm. 2 sides of a room would make an extra 200mm inside - which seems significant to me.

 

I have just *not* bought a terraced house because the necessary extra insulation to be added to make it legal to rent out in a few years would turn a just-double bedroom into a single.

 

I would love to build 2 semi-detached houses side by side with the different types for comparison. It is just the kind of thing I would do, too !

 

But let's not derail this thread - was just throwing in a concept for the OP to consider.

 

F

Edited March 18, 2017 by Ferdinand

[SteamyTea]

Floor area is an interesting concept.  Just looking at my kitchen as I type this.

I have 600mm wide units, 3 in a row.  Then a 50mm gap to the end of the wall by the door.

Seems to me that building the walls the correct length to fit in with standard sizes can save money.

 

[PeterW]

House design pays dividends here and it's fair to say that the "tea cosy" effect is key. 

 

Our design is a big dormer based Build due to it being a conversion and extension. Walls account for much less than you think in this sort of build so I've focused on the roof (190mm Crown 32  lined with 40mm Kooltherm) and the floor with 125mm of PIR plus 25mm EPS. Walls will be 150mm blown graphite EPS as part of the existing building will also be done and I don't want to be mucking about with batts etc. Its also lowered my bricklaying prices and I can also see how clean the cavity is being kept.

 

I've also taken EPS down the cavity below the floor - BC was fine with this but instead of having a concrete filled cavity that leaks heat from the UFH slab to the ground, I've got 100mm EPS that's secured in with a mortar back fill and the benefit of that will be significant I expect - at a cost of £100 which is only £10 more than the cost of a mortar backfill. 

[Gone West]

Our starting point was that we decided we didn't want to have a heating system. We also wanted an portal timber I-beam frame, so it was a case of working out U factors for the walls and roof and playing around with wall thicknesses, insulation types and glazing in the PHPP. After many months of work we came up with a max U factor of 0.1 for walls and roof would allow us not to need a conventional heating system for our layout. It is very dependent on the climate and micro-climate where you intend to build. Even where you obtain that climate data from is something that needs careful consideration.

[Gorlando]

Some very interesting lines for me to investigate. The EPS bead fill is appealing to me as it's another fail safe in that I don't have to be watching and worrying about the quality of the wall insulation intallation.

 

This will help also, in regards to my shift pattern, as I will be away from site for 4 consecutive days out of 8 so will not be there to oversee the superstructure build at times. Like I said I do trust my builderbut the less risk attatched to sloppiness the better.

 

In terms of worker control post shell. At present i'm only planning on hiring a plasterer and subies for speacilist commisioning (RHI eqip, MVHR) As said previous, i'm a spark and dad and brother are Gas CH Engineers. Although there's no gas service to the site, they can still do the majority of the pipe work.  Anything else I feel pretty confident we can get it done together.

 

Apart from using a block and brick building method, nothing else is set in stone. Whatever gets me the values I need at the right price is very much on the table.

 

And to summarise what you all seem to agree on is that passive house standards can be achieved for the same or only slightly more thay a building regs house.

 

Correct me if i'm wrong!

 

Once again thanks for all the replies. Really is proving to be a great community. Hopefully this thread will help others who are on a similar thought trail as I was....

 

 

[joe90]

I wanted to use beads but found it difficult to find a supplier for a decent price. My builders are excellent and as fussy and as tidy as I am. They fully take on board the passive ethos and I cannot fault their work, perhaps I am lucky. Re the comment above about the difference between 37 and 32 I could not see me getting a reasonable return for the extra money ( I am 62 Already ?)

Edited March 18, 2017 by joe90
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