Wall and ceiling insulation.

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Have the plasterboard off the kitchen ceiling to replace with fresh stuff and be plastered.

Have some 50mm foil backed celotex still, its a 100mm deep ceiling joist, so if I fitted the celotex and taped it up for vapour barrier, that would leave 50mm air-gap behind it, is there any issue with doing that?

I should say, its 100mm ceiling joist straight to outside, no loft etc above, just felt and slate on other side of the ceiling.

Out of curiosity if you used rock-wool style loft insulation in that gap, and it took up the whole space, would it be ok aswell? Im thinking as air could still move through rock-wool just slower?

Also, the cavity wall there has no insulation in it, while there open, would it be worth pouring the cavity wall polystyrene beads down there? The kitchens an extension, but the way its built up thats the only outside wall.

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50mm celotex is better than wool .and no don’t ‘pour’ beads in the wall. you’ll be left with voids that cause condensation areas. plus they’re meant to be bonded so they dont just pour out if you open the wall up lower down .
better off insulating the room internally with insulated pb that you can batten or bond directly to the wall.
 
Thanks, Will use the celotex on the ceiling then, and see if I can get a thin insulation backed plasterboard for the outside wall just to get that little bit lower heat loss, as you can see, the kitchens tiny so no room for decent size radiator.
 
Cheaper and easier to use insulation boards and plasterboard. The insulation backed plasterboard does not allow you to maintain a good vapour barrier. Stick insulation to the wall (everbuild pink dry fix foam), tape up all joins, then stick plasterboard to that.
 
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Cheaper and easier to use insulation boards and plasterboard. The insulation backed plasterboard does not allow you to maintain a good vapour barrier. Stick insulation to the wall (everbuild pink dry fix foam), tape up all joins, then stick plasterboard to that.
Thanks, I had wandered about the insulation backed plasterboard being that were supposed to tape up and seal over the insulation, so that confirms that this isnt the best way. Thats what I'll do. Cheers.
 
(everbuild pink dry fix foam), tape up all joins, then stick plasterboard to that.

Does the foam stick well for plasterboard onto the foil side of the insulation as well as sticking insulation to wall?
 
I've been doing this for almost 3 years now ... old solid wall bungalow with damp problems. Almost finished ...

Here's an in progress pic

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Does the foam stick well for plasterboard onto the foil side of the insulation as well as sticking insulation to wall?

Yes. Very sticky. I did some tests, sticking wood to plasterboard and foam etc. and couldn't pull it off.

The silver backing on the insulation is only stuck on (no idea what with, maybe during manufacturing?)
 
Nice work, I did 50mm in-between stud in bedroom and the room heats in 1/3rd the time that any other does now. Wish I'd know more about all this when I bout the place and started ripping it apart, could have probably done a better job of it. But its still much improved, 2 bed semi bungalow built in 1910 down to about 9 to 9.5kw total heat loss at -4 outside.
 
Wish I knew how much I was saving! I have been putting 50mm kingspan + 50mm rockwool between joists too, and chucked some more insulation in the loft. All helps. Would not have done any of it if the floors were not rotting, so in a way, good that they were!
 
Have the plasterboard off the kitchen ceiling to replace with fresh stuff and be plastered.

Have some 50mm foil backed celotex still, its a 100mm deep ceiling joist, so if I fitted the celotex and taped it up for vapour barrier, that would leave 50mm air-gap behind it, is there any issue with doing that?

I should say, its 100mm ceiling joist straight to outside, no loft etc above, just felt and slate on other side of the ceiling.

Out of curiosity if you used rock-wool style loft insulation in that gap, and it took up the whole space, would it be ok aswell? Im thinking as air could still move through rock-wool just slower?

Also, the cavity wall there has no insulation in it, while there open, would it be worth pouring the cavity wall polystyrene beads down there? The kitchens an extension, but the way its built up thats the only outside wall.

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I believe you should ideally allow some cross ventilation of the air gap, and most people would recommend that (because it's in a kitchen) as well as taping all the joints you cover the whole thing with a vapour membrane before replacing the plasterboard.
 
not necessary . use vapour boards.
if you're converting a garage for instance or ,as we are, bringing a previously unheated area into the thermal envelope and and going building reg's route, they'll insist that you ventilate and cover with vapour membrane, even when using foil backed boards.
I believe that interstitial condensation is one of the biggest risks of internally insulating.
 

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