Final pre start meeting - Groundworks/Foundation design

[Lots2learn]

Hi All,

Happy New Year to you all!

I will shortly be having the final meeting with the groundworks team before committing to the start.  I attach the designs for the foundations from the architect/SE.    I am particularly interested in ways of improving thermal performance and of course cost. As a reminder it will be a T/F house with renderboard cladding, MVHR and UFH.  Any thoughts or concerns from you band of brothers/sisters?

 

[TerryE]

Hi, my main comment is that you've really started to ask some pretty fundamental Qs at too late a point in your project.  You seem to have a design that includes a reasonable thermal spec as designed goal, but I am not sure that you or your architect how looked at this from a total system perspective, and at this stage any fundamental rebalancing could involve costly rework so I suspect that you will have little wish to do this.

 

There's over 8,000 topics/threads on the site, most with 10s many with 100s and one with 1000+ posts on all sorts of detailed aspects of the why, hows and why nots of building your own home.  There is a lot of gold to be mined if you are willing to use the forum search facilities, do the research of existing knowledgebase and ask the right targeted questions.  Like any forum there is a spectrum of users giving advice and opinion here.  At one end of the spectrum you've got people that clearly are "shooting from the hip" and not doing the necessary research (or hiring competent experts who are able to do this on their behalf), and their blogs and posts are horrifying to read.  At the other end you've got individuals who have come through this process with a house that they love; that performs to their expectation and that was build in a well controlled manner.  I am not going to give examples of either or the middle ground, but I do suggest that you research some of the blogs and frequent posters to rank the value of their advice in your own assessment, and weight this advice accordingly.

 

Anyone who builds a house has to deal with the same issues: planning and other external constraints; the feasibility and practicality of their design and its build process; the vagaries and quality of tradesmen and suppliers; financial management of the entire process; and the integration of all of this to achieve an end result that they wanted to achieve.  I suggest that the main difference between the two ends of this user-spectrum is the degree to which individual self-builders approach this process.  IMO, the best way to solve a problem is to avoid it in the first place by design.  The next best way is have sufficient audit and quality checks in place to detect each issue as absolutely early as possible in the life-cycle and at a point where you still have some freedom to identify options and workarounds that you can still implement and at a modest cost impact.  The worst is to be left with a fait accompli that will be costly to mitigate and will constrain and annoy you for the rest of your time in the house.

 

As I write this, I am sitting in my home office / 2nd guest bedroom on the 1st floor of my new-build that we have now lived in for the last year.  My wife and I are unreservedly delighted with our new house.  I am sitting in my shirt sleeves in a room at about 22°C (a personal preference), but we don't have any heating on the 1st or 2nd floors; I do have UFH throughout the ground-floor, but this is currently run off a 3kW immersion heater element that comes on for about 8 hrs a day (mostly E7 cheap rate) at current outside temperatures.  OK, achieving the spec to do this added maybe £10K or so to my build costs (if that) but we saved as much in not having to have a gas supply, a conventional CH system and radiators everywhere, and the annual maintenance costs for the same.

 

In terms of thermal design, IMO you need to start will an intimate understanding of the macroscopic thermal performance of the house and what makes up the pie chart of heat losses, because this impacts so much one the costs and characteristics of the build.  @JSHarris and I amongst others have blogged and posted copious details on this. 

• It seems that your design is out of balance, e.g. your floor has too low a U-value.  Are you planning to use MVHR, what is your fenestration design goals, etc.
• With these sorts of U-values, you are going to have a pretty conventional house in terms of running costs and heating solutions.  Is this a deliberate decision?
• Why have a garage?  You will have less parking capacity and more costs with one.  Why not drop the garage and put in an electric gate to secure the property instead?
• Your floor profiles don't allow UFH unless you have in-slab UFH.  This is entirely doable (and we have this), but IMO most builders are not competent to do this to an acceptable standard.
• The L-shaped design has a large surface area for the enclosed floor space.  I can see that your plot geometry really makes it impractical to do otherwise and I assume that you are locked into a approved plan, but this larger surface area exacerbates U-value impacts.

There are other downstream comments that I could make, but I'll defer them for now.  Time to do some reading of blogs and topics covering some of these points, I think.

 

 

 

Edited January 3, 2019 by TerryE
Missed an important not

[Bitpipe]

+1 on Terry's comment - however still time to make changes if you feel the need.

 

After insulation, air tightness is the second major source of heat loss and while not expensive to get a higher than BCO standard from a materials point of view, it needs to be designed in (and issues designed out) and needs a decent attention to detail during the build. MVHR is also required as the house will not naturally ventilate (in winter at least but also applicable in summer to a degree).

 

Equally major consideration is solar gain as in a well insulated airtight home, overheating is a significant risk.

 

Then you need to look at your space and DHW heating requirements and decide what system works best for your location and utilities.

 

Fundamentally, your house should be thought of and designed as a system and there should be some degree of analysis and modelling to understand how all the sub systems interact, otherwise you risk having a well built and pretty but poorly performing house that is too hot / too cold / too expensive to run etc..

[Dudda]

I don’t know where to start as I don’t know what your budget is, the timeframe or what’s your relationship with your architect and or builder.

 

The drawings show your house is built to minimum regulations. For example it’s showing 75mm of insulation in the floor. That might just allow you to get sign off and pass regulations but it’s not near enough in my mind especially for UFH. You’ve a big concrete lintel under the door for another example. That’s a huge cold bridge. Easy to fix but will cost more. This could be all you can afford though. The architect might be taking this into account and know space is more important to you than lower running costs. You need to discuss this with your architect BEFORE you sit down with the contractor as he’ll start seeing money signs everywhere. Talk to them about levels of insulation, airtightness, what the contractor has signed up for (eg does it state in the contract the builder has to reach a certain level of airtightness) what are the running costs and savings to be made long term, etc. Your paying them for a service and you need them to explain in detail to you the insulation they’re putting in and how much more it would cost to increase this.

[ProDave]

A few observations.

 

I see no mention of a service void. How are you going to run cables around the outside walls to switches and sockets without either burying the cables in insulation (will need derating / much larger cables) or compromising the air tightness or insulation of the building?  Ideally you want a completely sealed and insulated building, inside which you create a 25mm deep battened service void before applying ordinary plasterboard.  I am going to see a client next week to try and thrash out a plan of how exactly he expects me to wire the extension he is building and hopefully convince him to incorporate a service void.

 

You have a rendered finish on a render board. That adds nothing to the insulation.  Consider using as we have a wood fibre external cladding board and render onto that, this adds more insulation to the building.

 

Kingspan type insulation while a good insulator has a poor decrement delay which means the building will cool down quicker. Other types of insulation have a longer decrement delay so the building stays warm longer when the heating goes off. But you will need more thickness so probably again too late to change that.

 

But it may be too late in the day to make changes like that?

[Mr Punter]

I assume you are doing beam and block because you are on shrinkable soil and close to trees and you were not able to do an insulated raft.  If not, look at this option.  Noted also that the garage floor is ground bearing and that no clayshield is specified.

 

Have you chosen the timber frame supplier yet?

 

Whatever, increase the ground floor insulation and allow for ufh on the ground floor.

 

MVHR is not a big ticket item if you DIY and worth doing if you have sufficient airtightness.

[TerryE]

As @Dudda discusses, have you though of using one of the insulated block + beam solutions instead of trad block and beam.  This would improve the U-value of the floor slab significantly.  Also UFH is so much easier IMO because it leaves your heating source options open and avoids all of those rads everywhere, though you might need to augment the 1st story with with some vertical low temp rads.

 

Our airtightness measured is 10× better than your design goal.  Improving the air tightness makes a big difference with a decent MVHR. I see no reason why you can't achieve factors improvement with a TF house.  This would have small or no cost impact, as this doesn't involve fundamental design changes but more quality checking and attention to detail in the construction phase, and doing all of the required taping up before or during 1st fix.  This will make a big difference on winter nights even with gentle winds.

 

We have a blow cellulose Larson strut twinwall which avoids many thermal bridging issues by design and implementation.   The as-built performance of a single-wall+ slab PUR critically depends on the fit and completeness of the PUR panels to prevent gaps and internal air circulation in the panels.  Again, this is a case of you inspecting all panels closely and using however much foam gap filler is required. 

 

As @ProDave suggests, trying to cut your wiring into the back of the Kingspan insulated PBoard is going to be a total PITA.  Most single frame + insulated PBoard profiles that I've seen set the insulation 10mm or so back from the CLS front to leave a wiring gap, but yours doesn't.

 

I did ask about the garage for a reason.  If you use it for storage and not for a car as most people do, then you only have off-street parking for one car.  Garaging a car adds little or nothing to its life.

 

Even if we agree that the basic TF design is now locked in, you've got a mountains for design issues to resolve before TF erection and ditto before 1st fix.  It's a pity that you didn't find the site a few years ago. 

 

Edited January 3, 2019 by TerryE

[ProDave]

27 minutes ago, TerryE said:

As @ProDave suggests, trying to cut your wiring into the back of the Kingspan insulated PBoard is going to be a total PITA.  Most single frame + insulated PBoard profiles that I've seen set the insulation 10mm or so back from the CLS front to leave a wiring gap, but yours doesn't.

 

But that still leaves the wiring between insulation layers, it just saves you having to cut channels for it. So the insulation and derating factors still apply.

[Adam2]

Could blown in EPS beads be a useful and low impact (to overall design) measure to improve foundation/B&B insulation? I believe I saw this mentioned as a possibility on some other thread a while back but have no info on conformance to building regs or costs

[TerryE]

16 minutes ago, Adam2 said:

Could blown in EPS beads be a useful and low impact (to overall design)

 

EPS has a lower thermal performance than PUR: you need deeper panels to get the same performance.

[Ferdinand]

On 02/01/2019 at 17:28, Lots2learn said:

I will shortly be having the final meeting with the groundworks team before committing to the start.  I attach the designs for the foundations from the architect/SE.    I am particularly interested in ways of improving thermal performance and of course cost. As a reminder it will be a T/F house with renderboard cladding, MVHR and UFH.  Any thoughts or concerns from you band of brothers/sisters?

 

Your spec is ... as highlighted ... roughly Building Regs basic.

 

That concerns me given your original aspiration to be Near PassiveHaus spec.

 

 

Are you actually getting what you think you are getting?

 

If you are not, then you need to stop now and reoriente / check your expectations, as every project stage will reduce your scope for correction, and related costs will increase by several time at each stage. Without running any numbers, I would estimate that this design will have you with energy bills of more like £1200-£1500 than £400-500 per annum. Not a problem if you are expecting it, but a surprise if not. Estimate or  measure 10 times, build once.

 

Do not feel got at ... all of us here have made various choices that could have been done differently to save 4 or 5 figure sums, and tried to learn from them. For self-Builders we usually only do it once, so need to learn and apply the lessons from others before we make any mistakes, as we do not get the chance to learn from our own mistakes or sub-optimal decisions.

 

Wishing you a good New Year and the very best of luck.

 

Ferdinand

 

Edited January 3, 2019 by Ferdinand

[Jeremy Harris]

3 hours ago, Dudda said:

I don’t know where to start as I don’t know what your budget is, the timeframe or what’s your relationship with your architect and or builder.

 

The drawings show your house is built to minimum regulations. For example it’s showing 75mm of insulation in the floor. That might just allow you to get sign off and pass regulations but it’s not near enough in my mind especially for UFH. You’ve a big concrete lintel under the door for another example. That’s a huge cold bridge. Easy to fix but will cost more. This could be all you can afford though. The architect might be taking this into account and know space is more important to you than lower running costs. You need to discuss this with your architect BEFORE you sit down with the contractor as he’ll start seeing money signs everywhere. Talk to them about levels of insulation, airtightness, what the contractor has signed up for (eg does it state in the contract the builder has to reach a certain level of airtightness) what are the running costs and savings to be made long term, etc. Your paying them for a service and you need them to explain in detail to you the insulation they’re putting in and how much more it would cost to increase this.

 

So glad you said this, as I'd been looking at the drawings and thinking exactly the same, but didn't want to be overly critical.

 

It seems to me that there are simpler and cheaper ways to achieve much better thermal performance without all the thermal bridges, if it's not too late to change the design.

 

@Lots2learn, as an example of a very simple foundation that has no thermal bridging and very much better thermal performance, have you looked at using a passive slab?  Quick and easy to lay for a lot of soil conditions, inherently good in terms of thermal performance (ours has 300mm of EPS under the slab and 200mm of EPS around the edges of it) and there can be significant saving in ground works cost, too, which offsets the slightly higher cost of the passive slab itself.

[Declan52]

Not the end of the world yet.  If your founds have to done this way then so be it.  You will need to increase the last course to give you more depth  for floor insulation.  Not that expensive a step but if you want your ufh to work it needs done.  

The rest is just details.  You can go back to whoever is doing your frame and ask about different options for increasing the wall depth and for using other types of insulation.  Obviously this will affect the budget.  

Things like factoring in a service cavity is an easy remedy.  

The  timber frame company won't start making the frame until the founds are done and they measure up so you still have time to make changes.  Most important thing  to remember is to build the house to suit your budget.  If you can't stretch the budget to make any changes then fine it is what it is.  

[ProDave]

I thought I was being critical, but if you are expecting near passive house performance, then you need to call a halt immediately and have a very serious discussion with your architect. Either he does not understand what a passive house is, or he is seriously misleading you.

[Lots2learn]

I am very grateful for the replies and they do confirm my concerns about the thermal efficiency.

The T/F company have stated that the thermal efficiency of their product is 0.13 with a 30mm internal board fitted. 

The internorm triple glazed windows will be an average of about 0.6.  The roof will be 0.13.  The weak area in my view is the floor which at 0.18 is below what I want. 

The site is tricky in a conservation area with grade I and II listed buildings around it.  PP has taken over 4 years.  The ridge height has been very limiting and the architect has done a very good job of providing us the adaptable and size of space we specified but it maybe that this has come at the expense of thicker walls and deeper roof insulation giving better thermal performance.  That is a choice we have made.

To answer some of the points raised.

The garage is an important part of the adaptability as this will have services and a layout to enable it to be converted to a ground floor bedroom/en suite to allow us to remain in the property once mobility becomes an issue.

UFH and MVHR is specified although I need to confirm how the UFH will be installed.  Solar gain is mitigated on the ground floor by a 2 meter overhang on the South south westerly elevation with built in blinds in the outerleaf of the upstairs windows.

A 38mm service void is specified on the internal face of the perimeter walls although for some reason this is not shown on the drawings I posted.

The render board is vented so is it worthwhile using material that has thermal property?  Does this wall have to be vented to deal with condensation to the T/F?

The B&B floor is to cope with the 1.3 meter deep clay sitting on chalk which until 2 years ago had large leylandi and beach trees around the perimeter.  There is also an ash tree in the neighbours garden about 8 meters from the SE corner of the property.  I think that was the reason for not using a slab.  I will however ask the question.

I like the idea of using insulated B&B material.  If any of you have a manufacturer for this I would be grateful.

Budget wise we have a fair amount, roughly £2000/M, however the planning process and legal fees regarding a covenant have already gobbled up nearly £60k.

This is going to be our "cart us out in a coffin" house so we want to get it right.  In my view I want a well designed home which can adapt with us as we grow old.  Efficiency comes a close second.

Any more suggestions would be gratefully received, thank you all for your time.

[Bitpipe]

If its any comfort, our house was originally designed by the architect, and PP achieved, with no thought at all given to thermal efficiency, airtightness etc.. not until I discovered this site's predecessor. 

 

We were able to introduce near passive standards - slight reduction in internal area due to thicker walls but nothing really noticeable. If ridge height is an issue (as it was for ours) then you can look at going down into the ground a little if your sewer invert levels allow.

 

If this really is your forever house then make sure you're creating an efficient space as running costs and comfort will be a big consideration in years to come.

 

[epsilonGreedy]

21 minutes ago, Lots2learn said:

I Any more suggestions would be gratefully received, thank you all for your time.

 

 

It is reassuring to read that you know what constraints have influenced the current design. You say the architect is working to a ridge height limit hence the poor floor u-value, what internal ceiling heights will you have?

[Lots2learn]

Internal heights are 2.55 ground level and 2.4 + first floor - some of the first floor rooms have vaulted ceilings.

Digging down could help but there is a nearby stream which we have to be wary of and the rainwater runoff will be dealt with through a soak away so this might be an issue.

I've just been looking at Tetris insulated Block and Beam which could be a useful way to increase the ground floor insulation without adding depth. 

Several of you have pointed out that as far as the drawings are concerned they do not allow for UFH but my understanding is that the UFH is layed in the 75mm screed which is shown in the drawings so I am not too sure how this is an issue.

[Jeremy Harris]

43 minutes ago, Lots2learn said:

Internal heights are 2.55 ground level and 2.4 + first floor - some of the first floor rooms have vaulted ceilings.

Digging down could help but there is a nearby stream which we have to be wary of and the rainwater runoff will be dealt with through a soak away so this might be an issue.

I've just been looking at Tetris insulated Block and Beam which could be a useful way to increase the ground floor insulation without adding depth. 

Several of you have pointed out that as far as the drawings are concerned they do not allow for UFH but my understanding is that the UFH is layed in the 75mm screed which is shown in the drawings so I am not too sure how this is an issue.

 

UFH heat loss downwards can be high unless there is a decent level of insulation underneath, as UFH increases the heat loss rate through the floor when compared to other forms of heating.  As an example, we have 300mm of EPS insulation under our 100mm thick floor slab (which has UFH pipes embedded in it) and we still lose around 8.5% of the heat that we pay for down through the insulation into the underlying ground.  We live with the heat loss, but I'd not want to throw away more paid-for heat energy by having any less insulation under the floor.

[ProDave]

The biggest issue does seem to be the low level of floor insulation.

 

Obviously your ridge height is fixed and thus by definition your finished ground floor level.

 

So surely all this means is lowering the foundations and ground floor structure, to create room to add more insulation and end up at the same finished floor level.

 

The details need to be agreed, drawing updated and foundations built accordingly so I still suggest you put the brakes on the project until that is all sorted out.

[dpmiller]

Lose 150mm of the GF ceiling height?

[Ferdinand]

@LearningCurve

 

One option which may help and has not been mentioned is to use what we call Skirt Insulation ... that is vertical insulation down the outside of the foundations.

 

That is used for renovating and improving existing properties where the floor cannot be lifted as part of some External Wall Insulation projects. You run say one foot of EPS insulation along the outside of your house wall below ground level to the bottom of your foundations in a trench with a drain at the bottom ... a French Drain ... to keep it dry, to lengthen the perimeter heat loss path so your floor loses less heat.

 

Over time the earth under your house warms up to higher than its normal temperature.

 

The constraint here will be whether your ground conditions somehow inhibit that tactic.

 

Ferdinand

Edited January 4, 2019 by Ferdinand

[Dudda]

12 hours ago, Lots2learn said:

The garage is an important part of the adaptability as this will have services and a layout to enable it to be converted to a ground floor bedroom/en suite to allow us to remain in the property once mobility becomes an issue.

Then you need to insulate the floor of the garage. Leaving 150mm will only allow the same 75mm of insulation in the floor later.

 

13 hours ago, Lots2learn said:

The render board is vented so is it worthwhile using material that has thermal property?  Does this wall have to be vented to deal with condensation to the T/F?

Yes you have to have it ventilated. The silicone render your using doesn't allow air or moisture to pass through it. If you put insulation in that void the timber frame won't be able to dry out and you'll have serious issues. The alternative is to fit wood fibre insulation and a breathable render. As both of these allow moisture to pass through you should be ok but it's a much more expensive option to just increasing the insulation you have internally.

 

The UFH will easily fit in the 75mm sand and cement screed so that's not an issue. As others have mentioned the insulated B&B floor is a good idea to overcome your floor problem of losing heat.

[TerryE]

6 hours ago, Dudda said:

The UFH will easily fit in the 75mm sand and cement screed so that's not an issue.

 

Technically, yes but safer if you have 100mm.  Have a look at the blog posts a number of us such as Jeremy and myself on warm-slab construction which uses this approach.  The risk is that you really need a slab crew that are used to working with embedded UFH -- which isn't the case for most jobbing builders.  The devil is in the detail here and I've seen recent examples posted on the forum where members have shown how their builders made of total mess of doing this. 

 

The slab needs to be reinforced with mesh rebar. A typical method is to snappy the UFH runs directly to rebar to place it. The height of the rebar mesh needs to set correctly with correctly centred spacers.  You need the correct structural grade EPS/PUR otherwise the spacers get punched down into the EPS during pour especially if the concrete is barrowed rather than directly pumped.  If the rebar buckles then you might find the UFH breaching the surface and getting damaged during power floating.  If the crew know what they are doing this is all pretty straightforward, but a lot can go wrong if they don't. 

 

It is essential that the pour and the slab as a whole is checked and to spec before the TF is scheduled.  Whoever you contract to lay the slab needs to have the contractual obligation to guaranty to lay the UFH within it to spec.  I know of at least one member who ended up getting his slab broken up, removed and relaid because of quality issues.  Should you have such quality failures, then you want to ensure that you aren't financially liable.  The slab perimeter should be level to 5mm or better at the perimeter otherwise the TF crew will have problems truing the sole plate for the TF. 

 

In my own experience as a PM (I was IT and my main job was as a tech manager / CTO , but I did enough PM to be aware of the main risks)  I always found that interfaces between subs was this biggest black hole on any project: both subs claiming that their bit worked and the other was at fault.  Here we have a three-way potential nightmare: the slab,  the heating system and the TF.  I mitigated this risk myself by choosing a single specialist supplier to cover all three.  My other main subcontractor was my builder who did all of the groundworks, drainage and the external stone skin and slate roof.  

[Dudda]

52 minutes ago, TerryE said:

Technically, yes but safer if you have 100mm.  Have a look at the blog posts a number of us such as Jeremy and myself on warm-slab construction which uses this approach.  The risk is that you really need a slab crew that are used to working with embedded UFH -- which isn't the case for most jobbing builders.  The devil is in the detail here and I've seen recent examples posted on the forum where members have shown how their builders made of total mess of doing this. 

 

The slab needs to be reinforced with mesh rebar. A typical method is to snappy the UFH runs directly to rebar to place it. The height of the rebar mesh needs to set correctly with correctly centred spacers.  You need the correct structural grade EPS/PUR otherwise the spacers get punched down into the EPS during pour especially if the concrete is barrowed rather than directly pumped.  If the rebar buckles then you might find the UFH breaching the surface and getting damaged during power floating........

I know with Jeremy's slab (not sure of yours) was a structural concrete slab which the timber frame was built off. This is one is completely different as it's just a finishing sand and cement screed on top of a block and beam structure. It's put in after the timber frame is up. See the detail showing internal load bearning walls above. It could probably be a liquid screed with the fiber mesh reinforcement but the SE would need to confirm that.