Welcome to the AECB Forums Building Refurbishment and Retrofit Advice: wet insulation at perimeter of insulated ground floor

Viewing 5 posts - 1 through 5 (of 5 total)
  • Author
  • #31948

    Hello AECB Retrofit Hive Mind

    I had a question about insulating under a suspended timber floor (accessible by a cellar) – see attached sketch.

    Retrofitting a solid wall house (1899). I’ve full-filled the space between the joists with sheeps wool and in some cases recycled denim and cotton and stapled a breather membrane underneath to hold it all in place.

    Potential Problem:
    At the perimeter when I’ve pulled out some of the insulation that has been touching the external wall it is wet. Does this pose a problem? I worry that damp wool by the joist ends might eventually lead to rot. This wet is just the dampness of the surrounding earth due to rain.

    – Could another insulant be used like EPS that won’t retain the moisture? But this may just trap the moisture against the joists.
    – Could I leave it uninsulated at the perimeter and just make it more airtight with the breather membrane? But I might end up with cold strips around the room.
    – Could I line the area at the perimeter with a moisture barrier, like the breather membrane? But would this leave enough air movement to stop rot.

    Has anyone else dealt with this effectively before?

    Many thanks

    Mark Siddall

    Hi Flo,

    This interface is a challenge for many retrofits.

    For one example of how this has been resolved see Andy Simmonds retrofit:

    Foam could make the issue worse (traps moisture into timber as evaporation is prevented.)
    Consider treating the timber in order to help improve resistance to moisture damage.



    Thanks very much for the response Mark. I'll check those links out and report back.


    Tom Foster

    Carry the EWI down to base of foundation in a trench backfilled as a french drain! and minimise or omit the suspended floor insulation. Esp as it looks like you'd get a good deep downstand of insulation hence a good long path-length through soil, of heat passing down through the basement floor, curving out and up to surface. That 'thickness' of soil makes a good insulator. You'd also get a bone-dry basement and it would be part of the heated/habitable volume.


    Well it's a rather delayed from me, but better late than never.

    Mark: I had a look at Andy Simmonds retrofit, which was impressive to say the least. His solution was great but I wouldn't be able to get it to work in my particular cellar and ground floor without major disruption (especially while my family was living in it).

    Tom: Thanks, I like your idea and its something I did do on the east and north side of the house where I could manage to get below the joists, however this soggy insulation I was getting was coming through the south-east and south-west corner of the cellar. While the north, east and west-gennel is externally insulated, the south side of the house is directly onto the street so I can't french drain it or externally insulate it there (wish I could). I'm planning on doing internal in the future.

    I have tried to adapt a bit of Andy Simmonds ground floor retrofit idea and have wrapped breather membrane around the insulation where it penetrates the cellar walls, like a sort of breathable tanking,hopefully this will allow the timbers to breath still and not get too wet. It was a bit fiddly making these individualised little pockets for the insulation to sit in but I have attached a couple of photos of it.

    2 months later and the insulation is still bone dry at least!

Viewing 5 posts - 1 through 5 (of 5 total)
  • You must be logged in to reply to this topic.