EWI, IWI and ventilation

[richer]

Can anyone help me with this specialist question?

 

I live in an 1880s brick semi (no cavity) and have renovated the house top to bottom. We were concerned about damp and following the surveys we initially gutted every room, and eventually brought down the ceilings, and have insulated (50mm Celotex equivalent) floor and walls with a contractor. The used a special Permaseal membrane after taking the plaster back to the brick work and then dot and dab over that. At the time we opted for IWI because it was too expensive to do the whole house in one go, we had to renovate the rooms anyway (including bring down cracked lathe and plaster ceilings and cornice). In the end, what decided the EWI/IWI debate for me for me was the brick front of the house which could not easily be insulated with EWI. So we went with IWI, room by room. 

 

2 years later, the render on the outside of the house needed significant work to paint and the cost of EWI wasn't much more, so we've now opted for EWI on top of the IWI inside. Because we went room by room with the IWI, above the ceilings was unlikely to be done by the builders so there will be cold bridges; this was part of the rationale for EWI, combined with the render needing work and also adding another layer of insulation at minimal extra cost. This is 50mm of what looks like polystyrene, with mesh, adhesive and a top coat of render (I think!). I thought it would be wise to do the DPC before doing EWI to keep rising damp out. Unfortunately the weather has been awful and the walls are undoubtedly wet with the EWI now applied. 

 

I'm now concerned about moisture trapped in the masonry and not able to get out and any damage this may do in the long term. One of the issues with the render was it was extremely hard and possibly cement based so not breathable and the bricks were very soft so the render could not be easily removed. Does anyone have any advice here on what the risks are and what (if anything) can be done? I feel like the best thing would be to try to allow the moisture out somehow but I'm unsure of how this can now be done and anything that will need doing will presumably effect the U value of the EWI?

 

Thoughts gratefully received!

Edited November 27, 2023 by richer

[Redbeard]

Hmmm,

 

The membrane I think you are describing has a good role to play in basement waterproofing. I would rarely suggest its use elsewhere. It appears to be used in a number of situations where good drying-out would/could have solved the issue.

 

You seem to describe thicknesses of insulation which would not, on their own, meet the target U values set put in the Bldg Regs. Added together they might, but like you I do not like the idea of possibly having made each side of the 'sandwich' impermeable.

[Conor]

The system you are describing sounds like EPS, reinforcing mesh with base coat, and silicon / acrylic render skim on top. These are somewhat breathable (EPS is like a sponge and is vapour permeable) and moisture will make its way out eventually. I'm sure our in-house render expert will be along soon. In the mean time, ignore the rest of us.