Our bedroom is always cold at one end where there is a gable wall. This wall has fitted wardrobes mounted in front of it. The house is late 20's with cavity walls and the cavity is very well ventilated so the inner wall I guess is pretty much at the ambient temperature. I'm reluctant to fill the cavity with insulation as I don't want to end up blocking the vents below the DPC and causing vapour condensation.
So, I'm looking at moving the front of the fitted wardrobes forward and fixing insulation to the wall. I can readily spare the 2 to 4 inches of thickness that might be involved. . Someone suggested foil- backed insulated plasterboard and this looks like an attractive possibility as it wouldnt need to be skimmed because the wardrobes span the full width/height of the wall.
I'd much appreciate guidance on how to proceed - whilst giving careful consideration to vapour barriers, condensation etc -e.g. would I need to leave an air gap? Should I fix it on to battens or screw direct on to the wall? Would I need to seal all the joins and edges?
The vents below your DPC are usually underfloor vents, but it's all academic anyway, whoever installs the CWI either replaces the vents (which is included in the price) or otherwise makes sure they are clear after installation, likewise any other vents, be they gas vents, room vents or anything else...obviously cavity vents are sealed with silicone because they will become defunct.
The amount you will spend on celotex, plasterboard, plaster and vapour barriers, won't be far off the cost of CWI the full house.
partial grants are available through your energy supplier, or if you are diabled or on benefits, 100% grants are available through the warmfront scheme
I'm still apprehensive about CWI based on previous posts to this NG - particularly blocked air bricks and the number of cowboys in this particular trade. I've queried a couple of authorised WarmFront installers and got less than satisfactory answers. I'm in London and am willing to reconsider CWI if someone recommends a thoroughly professional installer.
Half my air bricks ( which vent the underfloor void ) are in a channel around the house as the level of the rear patio and side alley is just above the level of the air bricks. The previous owner of the house had raised the levels and blocked the air bricks! Strange but true. As a result, I had to replace the joists and floor boards in the kitchen because of dry rot. Hence I created the channel to expose the air bricks and hence my sensitivity about blocked air bricks.
Given that the celotex/plaster board would be at back of the fitted wardrobe, would it reallly be necessary to skim it?
I have done this extensively and can advise with some authority and experience. Firstly, I would investigate the option to put blown mineral fibre into the void because this is reversible if you have a problem. You need to ensure that this is the case by written contract with the firm and as you so rightly observe take precautions to ensure that the channels to the subfloor air vents are protected by perhaps putting in fittings of exendable plastic so it is ducted to the subfloor void and separated from the cavity. I am no expert in cavity walls so get better advice from others but as far as I understand it the idea is not to vent the cavity other than weep holes at the bottom so it is NOT at ambient outside temperature!
It is most unlikely that you got DRY rot from an absence of airbricks. I have surveyed several thousand buildings many hundreds with dry rot and NEVER ONCE found this to be the CAUSE. It is always a positive source of water and not condensation or the like. It is a contributory factor only when a postive source of water is present.
It was probably wet rot!
If it was dry rot then the cause was water not lack of ventialtion
On to the insualtion If you are determined to insulate the gable from the inside (consider outside) then this is the way I did it
I discounted the advice from the manufacturer to put on battens (too much space taken up and not necessary for water penetration risk through outside wall) I used the pink plastic / stainless steel nails made for external insualtion together with Plasterboard adhesive to fix 2" TP 10 (or any PU board with foil on) to the wall surface ensuring it was sound I then did the same with 1/2" plaster board
10 or so fixings - five in each layer Glue put on a la a tile with tile trowel
Skip with plaserboard finish and Bob's your uncle
Problem is that to do the gable properly you need to a) take off the skirtings, picture rail, cornice if present, lift floor boards (hope not the wrong way otherwise its alternate staggered cuts) and cut short on side walls and put them all back on the surface afterwards! This needs long fixings to go thru insuationi into the masonry
Read the Celotex website. They've a bunch of glossy guides on how to do various sorts of refurb. Naturally they're fond of their own products, but the information is good.
If you care about thickness, you're likely to spend extra to use the yellow foam PIR boards rather than doing it cheaply with rockwool. It's usually a couple of inches. Hacking the plaster off first gains an inch.
Cavity wall is great, but only if you're 1950s+ or so, and you have a usable cavity to work with.
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