Advice: wet insulation at perimeter of insulated ground floor

[Anonymous]

Hello AECB Retrofit Hive Mind

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

Background:
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.

Question:
– 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
Flo

[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:
http://www.passivhaustrust.org.uk/UserFiles/File/Projects/Awards2012/Grove%20Cottage_Simmond%20Mills.pdf
https://aecb.net/wp-content/plugins/aecb-publication-library/librarian.php?id=8132&file=8098
https://aecb.net/forum/index.php?action=dlattach;topic=1463.0;attach=257
https://aecb.net/forum/index.php?action=dlattach;topic=1463.0;attach=258

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.

HTH
Mark

[Anonymous]

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

Cheers
Flo

[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.

[Anonymous]

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!

[Author]

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