General: IWI - more thoughts on thickness of PIR

1 to 16 of 16

- CommentAuthorJeff B
- CommentTimeOct 7th 2022
I have requested revised U value and condensation risk calculations from Celotex. The previous calculations were done assuming a total of 75mm of insulation. I revised the plan to use just 50mm of insulation - please see attached schematic. The new design is basically the same as before but with the omission of 25mm Celotex between the battens i.e. leaving a void between the battens.

I was surprised that despite reducing the depth of insulation by a third the change in U value is only 10% (from 0.20 to 0.22).

I would appreciate any thoughts on whether it is worth spending the extra Ã‚Â£'s on 25mm Celotex (and the extra amount of fiddly work required) for such a small change in U value? I suppose the implications of "overinsulating"
the walls with possible attendant condensation issues are also on my mind, especially any adverse effects on the Rockwool loose fibre insulation inside the cavity.

Thanks.
Jeff
- IWI schematic 7 Oct 2022.pdf
- CommentAuthorJeff B
- CommentTimeOct 7th 2022 edited
Chiselling out of walls to allow extension of cables successful! Render was rather soft and therefore a straightforward job. Anyone need any WAGO connectors!?

Sorry just realised - wrong thread!
- CommentAuthorWillInAberdeen
- CommentTimeOct 7th 2022 edited
Jeff, the 25mm layer is bridged by the timber battens, which have something like 5-10x more thermally conductivity than the insulation. You also have 50mm wool in the cavity, so overall the 25mm layer is less effective than if it were unbridged, and is much less than a third of the total insulation value.

Whether it is worthwhile is down to your views on energy saving, regs compliance, cost/benefit, future energy prices, and whether the embodied carbon in the insulation will ever pay back before heating is decarbonised... So pretty tough to figure out!

On a practical level, what are the battens intended for? If you are not running cables through them, then just glue the plasterboard and insulation sandwich to the wall. If you did want battens to create a wiring space, which some people do, then could be worth filling the unused sections with insulation to stop air circulating behind the plasterboard (and possible future hiding places for drafts, wasps, fires, mice etc)

Cavity walls are intended by design to collect moisture and drain it away, that's what they are for and why they are never filled with biodegradable insulation. If you do a condensation calc for a standard filled cavity wall without IWI, it will flag up condensation, yet millions of houses have CWI without trouble. I wouldn't worry about Ã‚Â±0.02 on the U value making any difference, particularly as you are using foil insulation, VCL and foil plasterboard as well
- CommentAuthorJeff B
- CommentTimeOct 7th 2022
Will - thanks for your comments. The battens are just there to screw the plasterboards onto. I guess I am just old-school in that if I have an option I would always opt for a mechanical fixing rather than rely on adhesives, which is probably nuts I know, with modern adhesives.

I have had some wobbly moments thinking about the effect of the IWI on the temperature of the inner skin of blocks in the cavity wall. In the current situation there is heat loss from the room but when the inner wall is well insulated then this effect is nullified - wouldn't this tend to exacerbate condensation within the cavity under wet winter conditions? Hence my rethinking about the additional 25mm of PIR (apart from the cost and labour time considerations). Should there be a halfway house situation where some heat loss should be permitted to avoid problems?

I should add that the calculations provided by Celotex predict no condensation risk with 75mm IWI (see attached). Maybe I am just being paranoid!
- Interstitial condensation risk analysis 19 Feb 2020 75mm Celotex.pdf
- CommentAuthorWillInAberdeen
- CommentTimeOct 7th 2022
Hi Jeff, if you think about how a dehumidifier works, there's a cold bit that draws humidity away from the warmer places. It's the same in the cavity wall - the inner leaf is a little warmer than the outer in winter (especially with CWI) so any humidity is drawn towards the outer leaf which is the 'dehumidifier' for the wall. Then, it drains down the cavity, or is ventilated away next time there is a dry day. The condensation is a minor contribution to humidity in the wall, which mostly is from driven rain.

If you have a gable wall that sticks up above ceiling level in the loft, that section has been essentially unheated since the house was built, but with no ill effects I expect?!?
- CommentAuthorJeff B
- CommentTimeOct 8th 2022
Will - thanks for your comments and the reassurance! Yes there is an unheated gable end and AFAIK no problems with the CWI. I just know that if Rockwool blown fibre gets wet it gets really soggy!

Yesterday I bought some so-called vapour barrier polyethylene sheet:

https://www.screwfix.com/p/capital-valley-plastics-ltd-vapour-barrier-green-300ga-20-x-2-5m/12869

When I opened the packing I found that this stuff is incredibly thin (300G as opposed to 1000G for the usual DPM) a bit like the stuff I would use to cover furniture when decorating. However do you think that this should be adequate enough as it is being used in conjunction with foil backed plasterboard and foil faced Celotex boards with all joints sealed with self-adhesive foil tape?
- CommentAuthordjh
- CommentTimeOct 8th 2022 edited
Posted By: Jeff BYesterday I bought some so-called vapour barrier polyethylene sheet:

https://www.screwfix.com/p/capital-valley-plastics-ltd-vapour-barrier-green-300ga-20-x-2-5m/12869

When I opened the packing I found that this stuff is incredibly thin (300G as opposed to 1000G for the usual DPM) a bit like the stuff I would use to cover furniture when decorating. However do you think that this should be adequate enough as it is being used in conjunction with foil backed plasterboard and foil faced Celotex boards with all joints sealed with self-adhesive foil tape?
Yes, ordinary poly vapour barrier is remarkably thin and delicate, compared with a DPM that is made to be robust and withstand damage. It works perfectly well in the proper place. You shouldn't need both a poly vapour barrier and a foil one. In both cases all taping should be done using pukka airtightness tapes and not general foil tapes.
- CommentAuthorJeff B
- CommentTimeOct 8th 2022 edited
<blockquote><cite>Posted By: djh</cite><blockquote><cite>Posted By: Jeff B</cite>Yesterday I bought some so-called vapour barrier polyethylene sheet:

https://www.screwfix.com/p/capital-valley-plastics-ltd-vapour-barrier-green-300ga-20-x-2-5m/12869

When I opened the packing I found that this stuff is incredibly thin (300G as opposed to 1000G for the usual DPM) a bit like the stuff I would use to cover furniture when decorating. However do you think that this should be adequate enough as it is being used in conjunction with foil backed plasterboard and foil faced Celotex boards with all joints sealed with self-adhesive foil tape?

</blockquote>Yes, ordinary poly vapour barrier is remarkably thin and delicate, compared with a DPM that is made to be robust and withstand damage. It works perfectly well in the proper place. You shouldn't need both a poly vapour barrier and a foil one. In both cases all taping should be done using pukka airtightness tapes and not general foil tapes.</blockquote>

OK, thanks for the info. Yes, I realise it's probably OTT to do both but it's a one-off!

I have ordered airtight tape from Afixit.
- CommentAuthorwookey
- CommentTimeOct 9th 2022 edited
I put 100mm PUR on the inside of a wall like yours (but no render). It's been fine for 12 years now. I was careful to seal the backs and sides and joints so there was no/minimal circulating air getting behind. So I wouldn't worry that you are putting on too much. As you say the calcs discourage you from going above 80, but unless you are exposed and getting wind-driven rain so need a bit of extra wiggle-room it should be fine.
http://wookware.org/house/retrofit/pics/html/005-IWI-around-joist.jpeg.html
- CommentAuthorJeff B
- CommentTimeOct 9th 2022 edited
Posted By: wookeyI put 100mm PUR on the inside of a wall like yours (but no render). It's been fine for 12 years now. I was careful to seal the backs and sides and joints so there was no/minimal circulating air getting behind. So I wouldn't worry that you are putting on too much. As you say the calcs discourage you from going above 80, but unless you are exposed and getting wind-driven rain so need a bit of extra wiggle-room it should be fine.
http://wookware.org/house/retrofit/pics/html/005-IWI-around-joist.jpeg.html

Wookey - thanks for your input. When you say "a wall like yours", do you mean a cavity wall with CWI and not rendered externally in your case? Mine is rendered and painted with a Pliolite resin paint as the one wall is west facing which is the direction from which the wind and rain comes.

I can say I am being obsessive about vapour control:

All Celotex joints are taped over with aluminium foil tape and the edges at the top, bottom and the extreme edges of the sheets at the internal corners are sealed with hybrid mastic (e.g. CT1 or Screwfix's X8). The 50mm sheets will be tightened firmly against the walls when the battens are screwed to the walls. The 25mm sheets will be placed between the battens and both the battens and the joins/edges will be sealed with more foil tape.

There will be a VPC sheet over the whole lot with all joins sealed with airtight tape and then covered with foil backed plasterboard which will be skimmed and painted with vinyl emulsion. The VPC will inevitably be penetrated with plasterboard screws (when screwing into the 25 x 50mm battens) and where the 13amp ring main cables come through. My plan is to try to inject some silicone mastic into the holes in the plasterboard and hopefully the screws when screwed in will push this in and make a seal. I have come across a putty like material called R391 which can be used to seal around the cables - just need to see if our local electrical wholesaler has some.

However my main concern was more about condensation within the cavity because of the reduction in heat loss through the wall but from what you and others have said this should not be a worry.
- CommentAuthorJeff B
- CommentTimeOct 11th 2022
In case anyone is interested, during my IWI project I have come across two useful products which you may or may not already be familiar with:

Soudal VapourSeal airtight acrylic mastic. Can be used in conjunction with vapour barriers to provide an airtight seal.
https://www.soudal.co.uk/pro/products/adhesives/technical-adhesives/vapourseal

R391 - a putty-like compound which can be used to seal cable entry points and is safe to be used in contact with electrical cables.
https://www.bicon-uk.com/BICON-Compounds-and-Resins/BICON-Prysmian-R391-Electrical-Weatherproof-Compound/
- CommentAuthorcjard
- CommentTimeOct 12th 2022
"condensation within the cavity"

You haven't said what the concern is as far as I can see. The cavity sides are made of rock-like material, and if you look in any river bed you can see plenty of rock like material that has been immersed in water for millions of years with little detrimental effect

Worry less about it; the stress isn't worth it! :)
- CommentAuthorwookey
- CommentTimeOct 13th 2022 edited
Posted By: Jeff Bdo you mean a cavity wall with CWI and not rendered externally in your case?

Yes. My walls are wet plaster, brick, ~65mm cavity containing blown-fibreglass fill (except for all the gaps), brick. 1960s vintage.

Sounds like you are being thorough.

I glued both the PUR and the plasterboard so there were no holes in the foil layer from fixings. Sadly I think that method is incompatible with battens (not enough glue area for the plasterboard). But then if you are filling the batten gap with insulation (not leaving a service gap), then what are they for? Holding on the VCL I guess, but you have continuous foil - you don't need an extra VCL.

As you say, there is some potential for condensation in the cavity, but brickwork is not very airtight and slightly absorbent so I reckon it'll get out just as fast as it got in if/when it does happen. I've not got sensors in mine to check if this ever occurs (I will get round to this one day) so who knows, but the building performs as expected and there is no sign of sogginess.
- CommentAuthorJeff B
- CommentTimeOct 13th 2022
Posted By: cjard"condensation within the cavity"

You haven't said what the concern is as far as I can see. The cavity sides are made of rock-like material, and if you look in any river bed you can see plenty of rock like material that has been immersed in water for millions of years with little detrimental effect

Worry less about it; the stress isn't worth it! :)

My concern is that condensation within the cavity (due to this being colder now because of the IWI) would over time turn the loose fibre CWI material into a soggy mass. Not worried at all about the cavity structure (i.e. concrete blocks) per se. The outside wall surface is a cement render painted with Pliolite paint so not exactly breathable! However I do take comfort from the Celotex calculations which indicate no condensation risk.
- CommentAuthorJeff B
- CommentTimeOct 13th 2022 edited
Posted By: wookey
Posted By: Jeff Bdo you mean a cavity wall with CWI and not rendered externally in your case?

Yes. My walls are wet plaster, brick, ~65mm cavity containing blown-fibreglass fill (except for all the gaps), brick. 1960s vintage.

Sounds like you are being thorough.

I glued both the PUR and the plasterboard so there were no holes in the foil layer from fixings. Sadly I think that method is incompatible with battens (not enough glue area for the plasterboard). But then if you are filling the batten gap with insulation (not leaving a service gap), then what are they for? Holding on the VCL I guess, but you have continuous foil - you don't need an extra VCL.

As you say, there is some potential for condensation in the cavity, but brickwork is not very airtight and slightly absorbent so I reckon it'll get out just as fast as it got in if/when it does happen. I've not got sensors in mine to check if this ever occurs (I will get round to this one day) so who knows, but the building performs as expected and there is no sign of sogginess.

Our exterior wall has a cement render painted with Pliolite paint so not exactly breathable - unlike your's which is brick.

The battens are there to screw the plasterboard onto and also to provide something to staple the VCL onto. I agree the whole construction is OTT but I'm only going to do it once! The VCL cost is minor compared to the overall cost.

We have almost completed the PIR installation (no VCL or plasterboard yet) and already the room feels much warmer. In fact the temperature increased from 19C overnight to 23C by mid afternoon with no heating on and the windows open - just the heat from our two bodies as we were working!
- CommentAuthorJeff B
- CommentTimeOct 15th 2022 edited
Posted By: Jeff BIn case anyone is interested, during my IWI project I have come across two useful products which you may or may not already be familiar with:

Soudal VapourSeal airtight acrylic mastic. Can be used in conjunction with vapour barriers to provide an airtight seal.
https://www.soudal.co.uk/pro/products/adhesives/technical-adhesives/vapourseal

R391 - a putty-like compound which can be used to seal cable entry points and is safe to be used in contact with electrical cables.
https://www.bicon-uk.com/BICON-Compounds-and-Resins/BICON-Prysmian-R391-Electrical-Weatherproof-Compound/

The VapourSeal is an interesting product. It still remains very sticky at least 24 hours after being applied and therefore ideal for holding the VPC sheeting in place before attaching the plasterboard. Also I can see that with a decent bead across the centre of the battens the penetration holes caused by the screws used to secure the plasterboard will effectively be sealed in-situ. Neat - no need for staples!

The R391 putty basically does "what it says on the (proverbial) tin"!

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