Green Building Forum - Cavity walls in1920s house

Cavity walls in1920s house

Osprey — Thu, 15 Dec 2022 09:57:34 +0000

I have a house built in 1926, about 10mins walk from the sea (south coast) but sheltered among others. The walls are approx 50mm cavity, pebbledash rendered. I am looking for ways to improve heat retention.

In the loft, I can see that the cavity walls are uncapped - indeed I can see no other means of loft ventilation - and the loft itself is dry. There is a good amount of loft insulation between/over the joists. There are quite a few airbricks around the property, below dpc. These connect to the space below the suspended wooden floors. I believe they also ventilate the cavity walls (no tunnel) so cold air can freely circulate through the walls and loft, with high heat loss. Obviously I want to reduce the heat loss.

1) would capping the cavity in the loft make much difference? I would need to provide other loft ventilation. I would much prefer not to use soffit vents - are other methods possible (old, tile hung with bitumen paper lining)
2)if I go for bead cwi, what happens below the floor? A fibre insulation installer said they would need to drill and install some sort of shields, at an angle, over the airbricks.
3) what happens at the bottom of the CWI? Does it go below the DPC and just lay on the ground between the walls? Doesn't that bridge the DPC?

Thanks for any help/advise!

Cavity walls in1920s house

tony — Thu, 15 Dec 2022 12:37:44 +0000

1) Yes but insignificant, and approaching zero when it is windy
2) I would fill the floor void too! If not then you need to fit cavity liners not the shields
3) I would go below dpc with the beads - canâ€™t bridge a dpc with eps beads

Use platinum or graphite beads, render will protect cavity. Need tuck loft insulation into the top of the cavity

Cavity walls in1920s house

Osprey — Thu, 15 Dec 2022 13:21:54 +0000

Posted By: tony1) Yes but insignificant, and approaching zero when it is windy
2) I would fill the floor void too! If not then you need to fit cavity liners not the shields
3) I would go below dpc with the beads - canâ€™t bridge a dpc with eps beads

Use platinum or graphite beads, render will protect cavity. Need tuck loft insulation into the top of the cavity

Thanks for your reply.
By 'render will protect cavity' do you mean from damp penetration? If so, that would be my assumption, although I would need to paint the render to hide the injection holes, and some say this would hinder its ability to 'breath'.

Re: 2, I have now read that you have suggested that in this forum before. I have read up a fair bit about insulating under suspended wooded floors, but have seldom come across that approach , other than when preceeded by the word 'don't'. Is it an accepted method (-I.e. one that would not give a future surveyor a heart attack)?
I would also have concerns about fire risk (beads incased within brick being a bit different) but does the use of graphite/platinum beads eliminate that risk.

Cavity walls in1920s house

tony — Thu, 15 Dec 2022 16:48:10 +0000

Polystyrene is used as beam and block infill, under concrete floors, no reports of fire with those

Wooden floor is a fire risk on its own.

I have examples of suspended wooden floor voids filled years ago overseas.

A surveyor would have a problem but like you donâ€™t have them with in wall cavities.

The cavity will be dry, with beads drier best without paint or coatings

Cavity walls in1920s house

Osprey — Sat, 17 Dec 2022 18:50:53 +0000

Thanks Tony.

Cavity walls in1920s house

fostertom — Mon, 19 Dec 2022 17:47:44 +0000

Yeah, sounds like you're lucky, if prepared to do it thoroughly. Filling the u/floor and the cavities with beads will make a wonderful almost unbroken upside-down 'tea cosy', completed with loft floor insulation as 'lid', all uniting together and closing all those draught-bricks, ventilated first floor space and open cavity tops.

Paint on the render can be breatheable.

Cavity walls in1920s house

Peter_in_Hungary — Mon, 19 Dec 2022 18:51:15 +0000

Of course filling the void could reach a whole new dimension if the void happens to be 4 - 5 ft. deep. Hopefully it will only be about 1 - 2 ft.

Cavity walls in1920s house

fostertom — Mon, 19 Dec 2022 20:07:12 +0000

Aha - a mid European semi-basement!

Cavity walls in1920s house

Osprey — Mon, 19 Dec 2022 20:37:34 +0000

Yes I would use properly breathable paint (silicate).
Void is maybe 450mm deep.

I guess filling with beads would eliminate my problem of sleeveless air bricks as well!
I am, however - nervous, about following this approach....!

Cavity walls in1920s house

philedge — Tue, 20 Dec 2022 08:17:25 +0000

If youve got reservations about flooding the under floor with ESP beads, maybe speak to your building control office. Theyll know your area and likely the build standards of your house. Could avoid any worries about loss of beads from the CWI and how to deal with your air bricks.

Cavity walls in1920s house

Osprey — Tue, 20 Dec 2022 17:59:47 +0000

Posted By: philedgeIf youve got reservations about flooding the under floor with ESP beads, maybe speak to your building control office. Theyll know your area and likely the build standards of your house. Could avoid any worries about loss of beads from the CWI and how to deal with your air bricks.

Yes, I may well do that. I believe I would have to sleeve or otherwise protect the electricity cables that are under there, which would be a bind, and also de-rate them, as they would no longer be in free air.

Cavity walls in1920s house

philedge — Tue, 20 Dec 2022 18:50:32 +0000

Unless youre particularly energy hungry, de rating the cables might just be an MCB swap. We run with 20amp socket circuits and have never had an MCB trip.