New Build - Choosing Insulation: IWI, CWI, EWI

[Gaf]

Getting construction drawings completing for a one-off 218m2 (2,347 sqft) house in Co. Louth in Ireland.

 

We've gotten a lot of advice about the insulation to go with and we're feeling a bit stuck with what to choose. The building standards require the house to meet an "A2" rating, which translates to household energy consumption of 25-50 kWh per square meter. The spec of the house will include an air-to-water heat pump, mechanical ventilation and heat recovery (MVHR), concrete first floors, underfloor heating on ground and first floor, solid block inner walls on ground and first floor (no stud walls), and full external render (natural stone will surround the main entrance as a feature but no-where else).

 

The different advice about insulation seems to focus around the dew point leading to internal problems (condensation, mould) as well as external problems (interstitial condensation causing damage to brick work).

 

The advice we've gotten is:

1. Architect: Single leaf (215mm / 225mm) blocks laid flat with external wall insulation (EWI). Advising this to push dew point out of the house and avoid all dew point issues.
2. One Building Surveyor: Double leaf, with rigid cavity insulation to leave a partial cavity air gap and possible internal wall insulation (IWI) if needed.
3. One Building Surveyor, Two Building Energy Rating (BER) Assessors, and Three Recommended Builders: Double leaf, pumped platinum cavity bead insulation, with IWI.

 

Those suggesting #3 above all advised the same thicknesses of insulation and that if we stick to those thicknesses we won't have condensation and mould problems. They suggested 150mm pumped cavity bead with a maximum of 50mm IWI. They've said that it's too much IWI that causes internal condensation and mould, so keep it to 50mm at an absolute maximum. The IWI advised will be 62.5mm technically as they all advised pre-fabricated insulated plasterboard, which is 50mm insulation and 12.5mm plasterboard. They all said that EWI is almost exclusively a retrofit option and is the most expensive by a long shot (quoted €30k extra for the build to go with EWI).

 

Across the board, the interstitial condensation seemed to be less of a guarantee in terms of it not happening but all said it's really unlikely to occur, particularly if we ensure the correct render thickness.

 

We've read that driving rain can be a factor in the insulation choice. We have family who built a similar sized house approximately 500 yards away from our site using #3 and they have had no mould or condensation issues. Their house spec is almost identical to ours, except they went with half natural stone on the whole house with half render, and they have oil-fired central heating. Their house is far more exposed to the elements than ours; theirs is on top of a hill and has no natural protection from the elements (no trees, little hedges, etc.) and they can't sit outside their house too often because of the strong winds. Ours will be built 'into' the side of a hill and has trees that reach above the house 50 yards away to the south, single houses to the east/west, and a hill to north. Standing on our site is almost always very calm in terms of wind because of these wind barriers, which makes us feel somewhat reassured we won't have driving rain issues.

 

We're leaning towards #3 because it's been advised by almost everyone we've spoken to, we have family who did this method and have no issues, and it's the approach the recommended builders are very familiar with (one built the other family home) so we feel confident in their abilities to build it to a high standard. Our architect is the primary driver of not going with this method and said it's a completely outdated building method; but we're still leaning to #3.

 

Sorry for the long post. We would really value any advice on this in case we have missed anything in our choice of insulation!

 

[JohnMo]

I think all will work, with different advantages and disadvantages.  You seem to be going concrete blocks everywhere, this will add thermal storage and keep the house temperature stable summer and winter once everything is heated up. Assume you chose this build method for a reason, is this it, or other reasons?

 

Adding IWI blocks the external walls away from the house internal environment. So not seeing any advantage to that. IWI converts the external walls to thermally light for thermally heavy.

 

Not really seeing why external insulation is adding such a premium to the costs (especially as its being rendered, sounds like it's outside their comfort zone, so quoting so you don't go that direction.

 

Cavity with solid boards is difficult to do well, easy to do badly, once blocked up you have no idea if good or bad. Dismiss that one. Big cavity with mineral wool type insulation yes.

 

Insulated plasterboad, sounds like a retrofit or you really messed up the U value calculation after thought. Not a fan on a new build.

 

25 minutes ago, Gaf said:

Across the board, the interstitial condensation seemed to be less of a guarantee in terms of it not happening but all said it's really unlikely to occur, particularly if we ensure the correct render thickness.

Quotes like this always worry me, as the external render has no bearing on interstitial condensation. They either have done or haven't done an interstitial condensation calculation and do or don't know the correct answer. 

 

Amongst all these decisions you need to consider your airtightness strategy also.

[Alan Ambrose]

FYI there's a current thread on condensation risk here with a lot of detailed info. In short - it's easy enough to model the options but a little more difficult to interpret the results for moisture. The heat results are fairly accurate though - except for reductions for any unmodelled thermal bridges that you anticipate. If you're mischievous, you could ask any of the 'advisors' whether they've modelled the option they're recommending - what u-value does it give etc 

 

[SimonD]

I agree with @JohnMo, avoid the IWI options and rigid cavity wall insulation. I do like the suggestion made by your architect in terms of EWI but if there are concerns about exposure, I would add battens and void outside the ewi and use render boards/render on the outside to finish. That way you reduce risks of failed thin coat render, which is known to happen and it can increase your render product choices too.

[Gaf]

@JohnMo Cheers for that. We are going concrete blocks everywhere. initially this was advice from family who built and they noted the significant sound proofing this offered. But as we spoke to architect/builders they noted the thermal benefits you mention. I think you're spot on with EWI being outside the builders comfort zone; one in fairness said he had no experience with it and would have to hire another company to do it for him. One building surveyor expressed concern about going EWI as the proposal was to go with single leaf 215mm flat laid blockwork, which he thought could cause structural issues. Great to rule out the solid boards (at least that narrows things). The insulated plasterboard seems to be common here, at least across builders, BER assessors, and the building surveyors.

 

We've gotten a U-value calculated by Kingspan of 0.16W/m2K. This is using the below (internal to external):

1. Plaster skim coating
2. Kingspan Kooltherm K118 37.5mm (inclusive of 12.5mm plasterboard)
3. Inner leaf blockwork 100mm (2000 kg/m3)
4. Kingspan Ecobead Platinum 150mm cavity
5. External lead blockwork 100mm (2000 kg/m3)
6. Cememt Render 20mm

A condensation risk assessment by Kingspan indicated the "Risk level if 1 in 20 years". I'm not fully sure but I think this translates to a 5% chance of condensation problems occurring in a 20 year period. I've no sense of whether this is good, bad, or 'satisfactory' in terms of what the typical probability of condensation problems is. The analysis is attached. I'm not 100% sure but I think I read that if the two lines in the cross section meet within the wall, then that's bad new. It looks like the two lines are very very close to meeting, which may warrant another look at the insulation as set out.

 

In terms of the render comment and interstitial condensation, I think I've actually got that wrong. I think one BER assessor commented that the render should include a barrier to reduce driving rain entering the block work. This may also be inaccurate.

 

The architect has provided quite a lot of information for the tender process about air tightness. We're having to assume they know their stuff.

 

@SimonD It's a difficult one for us, as the builders who have come recommended have said they have no experience using EWI on new builds. We're conscious that we want to go with a builder who is both confident and competent in their building approach. But we do take your point on the IWI and rigid cavity.

 

 

Edited November 22, 2023 by Gaf

[Conor]

For those asking about the block build method, housebuilders in Ireland have a historical obsession with concrete blocks. Some builders don't speak any other langue other than concrete.

 

From memory of a PHPlus magazine article, 215mm block with rendered EWI is one of the most carbon intense building methods. That, coupled with the all concrete interior structure means that embedded carbon in the build will be as about as high as it can be. We went ICF, which is just as bad, but I did not consider this as part of our process. Same article was arguing the embedded carbon and other impacts during a build should somehow be taken in to account in the final SAP rating. 

 

We have metal stud walls in our house, super quiet. No need for concrete walls internally, and makes running of services and lask minute tweaks so much easier.

 

Anyway, keep things simple, adding a second method of insualtion, whether internal or external, will bring significant costs over a single method. My vote would be 200mm cavity with blown insualtion. Think that's what @joe90 has.

[joe90]

1 hour ago, Conor said:

My vote would be 200mm cavity with blown insualtion. Think that's what @joe90 has.

Nearly, 150mm cavity full filled with batts. My build was in a very mild (north Devon) area. Went brick and block as I like brick and virtually no maintenance, block inner as I like solid and heavy vs lightweight building. It certainly worked fir me.

[Gaf]

@Conor So true! We did explore timber frame but couldn't get a builder recommendation who would take on the remaining works after the timber structure was in place. Everyone we were linked with went self-build and we have a preference for a contractor.

 

It has been suggested that we could go with first floor stud walls with 50mm isover acoustic insulation instead of solid blocks.

 

@joe90 Stupid question, but are the full fill batts different to the cavity with solid boards, or is that the same thing?

Edited November 23, 2023 by Gaf

[joe90]

35 minutes ago, Gaf said:

Stupid question,

No such thing, stupid is not asking🤷‍♂️, the advantage IMO with batts are that they are a bit squidgy so can be stuffed into gaps, boards are solid and require taping or foaming to make them effective. Also I don’t like the idea of a “plastic” insulation (Grenfell!!,)There is an argument against full fill without an air gap but mine had a bba certificate fir full fill and even when a wall got soaking wet through none if it migrated into the insulation.

[JohnMo]

12 hours ago, Gaf said:

It looks like the two lines are very very close to meeting, which may warrant another look at the insulation as set out.

Think that is one of the issues with internal and cavity insulation together. If the external wall temperature has been set at -3 as the lowest temp, you would be producing condensation, but as the outside temp increased this would dry out. But not sure you pass at a lower temp? \someone more knowledgably will step in I am sure

[HughF]

We’ve just done an extension in an exposed part of Somerset (high wind area) and we’ve gone celcon/concrete with opc render. 150 cavity full fill with rockwool.

 

Upgrading the older parts of the house (50mm cavity) with iwi.

 

Seems to be quite ok, and everything required was off the shelf at the merchants, which is something to consider.

Edited November 24, 2023 by HughF

[SimonD]

On 22/11/2023 at 23:43, Gaf said:

@SimonD It's a difficult one for us, as the builders who have come recommended have said they have no experience using EWI on new builds. We're conscious that we want to go with a builder who is both confident and competent in their building approach. But we do take your point on the IWI and rigid cavity.

 

I agree it's a difficult one. The question is why has your architect tried to steer you down a route that isn't supported by your local building community (I kind of know the answer to this as I've been in a similar position thanks to the architect's design 🙄). Does your architect know someone who does have the relevant knowledge?

[Gaf]

@JohnMo I called Kingspan about the proposed insulation and their condensation risk assessment. They said that the dew point and temperature will always come close when using the cavity bead insulation (said it wasn't related to the IWI). They said their calculations indicate zero risk of condensation. Only after talking with them I forgot to check if that's related to both internal and interstitial - plan to give them a bell on Monday.

 

@HughF I think from all my reading here and elsewhere, it looks like the insulation choice often comes down to an element of personal preference on both the homeowner and the builders side. There are so many noted preferences we have found ourselves with choice paralysis!

 

@SimonD Absolutely take your point on this. Two builders we spoke with made comments of this nature, but who knows, we're choosing not to think too much along these lines. We have recommended builders from family and locals who built, and are very happy with the work done, so we're going our own way with choosing a builder. We did note the architect's pre-tender estimate for the house build was €96k higher than the pre-tender estimates from all builders and the building surveyors.

 

- - -

 

From everything so far, combining CWI and IWI seems like a no-no from the posts here. We're a little reticent about it but with Kingspan doing their analysis indicating no condensation concerns we're likely to go with this as it's what our family had done, it's what the builders have said they do for pretty much everyone, it's what the BER assessors say they see a lot, and it's what our building surveyor said it also done a lot. Maybe it's a local thing! Thanks all for your input.

Edited November 25, 2023 by Gaf