Posted by: @jarlatheoin

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Hi,

I am currently considering the next steps in the internal finish one of which is a 34mm PIR internal insulation wrap. The house has a 150mm cavity with 100mm PIR already integrated. The plan is for internal 34mm and then plasterboard to be mechanically fixed to all external walls.

I have read that you can airtight foil backed PIR by taping out the gaps and seams.

Has anyone got input on this theory or any other relevant comments?

Hoping to avoid the airtight membrane if not strictly needed.

Thanks,

Jarlath

The principal purpose of this, I believe, is to stop vapour from the house (which is generally more humid and at a slightly higher pressure than the outside) finding its way into cold voids where it will likely cause condensation and interstitial mould.  As we know from recent Evo4 problems, interstitial mould can be a nasty thing indeed.

Personally I would fit it strictly according to manufacturers instructions, to minimise risk.  For my money the risk of interstitial mould is also a very good argument for pir-bonded (foil backed) plasterboard, not loose insulation fixed to a battened out wall.  PIR is closed cell so the air gaps are sealed, and the bonding avoids an unnecessary air gap between plasterboard and insulation or in the insulation layer.  Admittedly you will still probably apply it with dot and dab so there is a gap behind, but the recommendation here (at least on the make I used) is to have a continuous run of adhesive around the edges.

@jamespa thank you for the reply, do appreciate it

In summary the recommendation would be insulated plasterboard with foil backing adhered using a dot and dab approach with a rim seal of adhesive on each panel fixed to the internal block skin of the cavity?

Would that then negate the need for a vapour barrier / airtight membrane?

1. It's much more complicated than you realise and to get the true answer you need to pay an expert.

Interstitial condensation is the unwanted side effect of badly specified and fitted IWI on solid walls, not already insulated cavity ones.

Your walls atm will allow a certain amount of moisture out through them, if you do as you plan then you'll reduce that and it has to go somewhere. If you have really effective 24/7 ventilation then that will do the job, if you don't then it'll go upwards into the attic and could cause condensation in there instead.

 As per point 1 you/someone needs to do the calcs on whether or not it's worth the cost, risk and disruption for whatever gains you'll get.

To do it properly it needs a professional risk and cost assessment.

Agree with James, fitting according to manufacturers and system specifiers instructions is key to peaceful enjoyment. 

In our loft, we had phenolic boards throughout and for that, tapping with foil was expected. So tapping over all the gaps left after filling the timber frame structure with cuts from phenolic board.... 

Posted by: @jarlatheoin

↑

Hi,

I am currently considering the next steps in the internal finish one of which is a 34mm PIR internal insulation wrap. The house has a 150mm cavity with 100mm PIR already integrated. The plan is for internal 34mm and then plasterboard to be mechanically fixed to all external walls.

I have read that you can airtight foil backed PIR by taping out the gaps and seams.

Has anyone got input on this theory or any other relevant comments?

Hoping to avoid the airtight membrane if not strictly needed.

Thanks,

Jarlath

Posted by: @dgclimatecontrol

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.. I'd look at HR fans for the wet rooms or a whole house HRV system if not already there. They balance the house whereas extract fans cause a negative pressure which sucks in cold.

That makes sense, I plan to replace our extractor fans with Heat Recovery fans as they start to pack up.. 

@dgclimatecontrol thank you for the response.

Planning on MVHR (Mitsubishi or Ventaxia) designed and supplied by BPC with self install. With that in the mix would it be correct to say moisture will be greatly reduced and with it the risk of mould?

Posted by: @jarlatheoin

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@dgclimatecontrol thank you for the response.

Planning on MVHR (Mitsubishi or Ventaxia) designed and supplied by BPC with self install. With that in the mix would it be correct to say moisture will be greatly reduced and with it the risk of mould?

One aspect of making insulation air-tight that hasn’t been mentioned is wind washing, a strange term which is if the insulation isn’t sealed (like around the edges) then the wind gets behind it it removes all of the slightly warmer air and makes the insulation ineffective. Think of it as external insulation of the garden wall(!)

re insulation thickness I did a whole house thermal model for our home and worked out the cost-benefit of extra insulation thickness and on insulation cost alone it was worth the extra thickness up to 120mm. This was to add EWI to pre-existing cavity wall (urea foam from 80s). BUT the total cost, labour and other costs extending the roof to prevent water ingress behind etc made the concept untenable on the quotes we received. My view on any other systems such as aluminium flashing and/or sealing at the top was they would be certain to let water in, and so could be ignored. I suspect that many of the poor insulation jobs on eco4 were on basically keeping the rain out of the interstitial joint as much as lack of breathability. I saw photos of over-flowing guttering onto the EWI edges for example.

@judith The concept of a thermal model for your home is impressive, but I appreciate much of it is maths!  

In our EWI we used phenolic boards. They need a small airgap between the wall and the boards, to deal with any damp. At the top, there are a few gaps around the bottom of the roof level. It made me think whether it could impact the thermal efficiency, but I still estimate it reduced the heat loss by 30-40%.   

@batpred excel mainly!