How to cover top of cavity at window cill?

[John Stumbles]

Chasing out the wall below the window cill in my kitchen (c. 1970s extension) I dislodged the top layer which turned out to be about 3/4" of plaster sitting on slates sitting on the bricks of the inner course, so that the slates and plaster covered over the cavity. Here's a picture, showing the cavity now revealed and a little bit of slate and plaster still crossing it: http://82.21.72.246/~john/cavity.jpgI'm a bit surprised that there was nothing supporting the outside of the slate + plaster cap so that it was only held by the adhesion of the mortar to the underside of the slate. I think a bit of levering down on the outside of it could have broken it away. I don't know if this was ever legit building practice.

I'm thinking along the lines of stuffing a few inches of rockwool into the cavity, fixing a tanalised batten to the outside wall below the windowcill and putting some artificial slate back across the gap and mortaring it all up.

Does this seem reasonable?

[Nigel Molesworth]

That's what the window sill is for.

[Phil L]

For a while yes, at least until the batten rots, which won't be long considering it's on an outside wall and the condensation involved in a kitchen.

Put a proper window board on it, either timber or pvc, this is what the groove in the bottom of the frame is for.

HTH

[dg]

Why don't you just clean the slates off and refix them? They have done alright for 30 years

dg

[jim_in_sussex]

I'm not up to date with current spec (chances are it's been changed by

2 jags), but it used to be that cavities had to be sealed with a smoke (or possibly fireproof) barrier.

the reason is to stop smoke penetration through the cavity from another part of the building.

The method I've seen most often is to lay cut building blocks on their sides on the inner leaf to bridge the cavity. A dpc (dipping at least

50mm below the block under-surface) is wedged between the block and the outer leaf. The cill sits on top of the block.

I've also seen the slate method, which I had to use once due to space problems over a window lintel (it was approved by the LA Building Inspector).

Slate is fireproof, but perhaps 2 overlapping leaves would be an improvement to lessen the chance of smoke penetration. So far as butting to the outside leaf, I can't recall if I used a dpc, but as slate is a dpc in itself, so long as no mortar bridge the gap between outer and inner leaf there should be no dpc problem.

If you mean rockwool batts sold for building into cavities, go ahead. But f you mean rockwool used between joists, I'm unsure about that as rockwool batts used for cavities have a hardened surface texture which could be a design feature to limit moisture penetration across the cavity. BTW do not use polystyrene as that requires an air gap to prevent condensation in the cavity & stop sweating on the insie wall..

IME you will find it difficult to fill much of an existing cavity from above, but every calorie saved will help your household bill.

fixing a tanalised batten to the outside wall below the windowcill

Would advise against that as although tanalised timber is supposedly rotproof it is not water resistant. You'll create more problems than you solve.

Stick to the masonry or slate solution, avoiding bridging the cavity with mortar unless there is a dpc stop in place.

Be careful - what are the water resistant & (fire) temperature properties of artificial slate?

Real slate can withstand the high temps of a house fire and is nigh on

100% water resistant.

HTH

NB 2 jags' present regs may be different - check with building control if you have any doubts, better still, someone up to date will post!

[Chris Bacon]

I should cut a slot in the mortar under the window, and replace the slate that you've removed. Not a whole lot of point in pushing rockwool down the cavity, and I'd use real slate for the job, as artificial isn't absolutely rigid. Muck up after to replace what you've dislodged.