Benefits of cavity wall insulation and external wall cladding

[martin smith]

I am refurbishing a late 1960's detached house with brick and block cavity walls which is as leaky as hell in winter and summer. I am replacing all windows with triple glazed Ecoplus windows and new doors from Green Building store to reduce heat losses, sealing around all windows with compriband. The cavity was filled with blown insulation last year, Warmcell, installed by EAGA, but this has not made any noticable difference, probably due to the draughts. I am thinking of cladding the outside of the walls with 100mm of diffutherm boards supplied by NBT and rendering over with lime render. The concern I have is the effectiveness of the external insulation if the cavity is not properly sealed. It is very difficult to know how to address this problem. Any suggestions as to the best way to seal the cavity and also how effective the external insulation is likely to be would be welcome.

[Geoff Stow]

Martin Are you sure the cavity was filled with Warmcel? My understanding is that it is not suitable for cavity walls.
You could use a thermal imaging camera to check the quality of the installation.

Geoff

[martin smith]

Geoff, thanks for your reply. I was really more concerned about the effectiveness of the external wall insulation. Most properties where it has been applied are solid walls where the benefits are more obvious. Thanks Martin

[Mark Siddall]

Martin,
I've worked on a project where we super insulated a cavity walled property. Ensuring that you achieve an airtight building is important. We used the parge (which is required as the substrate for external insulation) to privide the air barrier. Ensuring airtightness at the junctions, to avoid thermal bypass, is the tricky bit. Key places to concentrate attention:

1) between the roof/ceiling air barrier and the wall - we took an air barrier membrane over the top of the old roof trusses and lapped it down and sealed it to the parge. A less costly, but more risky approach would be to take the air barrier membrane over the attic ceiling and take that through to the parge (there will be penetrations caused by the trusses).
2) between founds and wall - take the parge down 450mm below DPC level

HTH,
Mark

[David OLIVIER]

Is this really true, that you used cellulose fibre to fill a cavity wall, in which the outer leaf is naturally damp and rain occasionally runs down the inside of the outer leaf?

It will be papier mache by now.

[martin smith]

In response to Mark's comments. It is sealing the roof, wall and ceiling that I have the greatest concern with. I will have a look at the suggestion you have made with the builder. Thanks for the idea. With regards to the cavity wall insulation, I am sure that the installer said it was warmcell. They even advertise the fact on their website. Where I have made openings in the wall, ie. for a new door opening it has been visible and still in place. Not sure how much benefit it gives though.