insulating sloping ceiling-roof space

[tonyjeffs2]

Part of our kitchen has a sloping ceiling; plasterboard nailed to the sloping roof rafters. This section is not insulated. What is the best, and what is the easiest way to do it? Should I take off the plaster, lower the ceiling by 3 inches, and pack between the joists and the extra space with fibre - a relatively big job.... or could I successfully take an easier route; drill holes and inject something - expanding foam maybe?

Thanks

Tony

[dom]

Can you get access to the void from either end? If you can, you may be able to slide in sheet PU insulation (e.g. Kingspan). This needs to be of a depth to not adversely affect air circulation between the joists. If not, then battening and plasterboard is probably the answer, although there's better insulators to use than glasswool where the depth is limited.

Injection into the void - bad. There needs to be air circulation between ceiling and roofing to keep the joists dry and remove any rainwater blown under the tiles/slates as well as moisture penetrating from inside the kitchen.

[Lobster]

I would pull the plasterboard off the slopey ceiling to expose the rafters - how deep are they, 3"? If so, the next step is to fit slabs of 2" thick Kingspan or Celotex between them, flush with the inside face of the rafters. This stuff is *way* more effective than any mineral wool, inch for inch, so in a confined space like this, it's a no-brainer. Important to leave the air gap above, as others have said; so injecting anything as an alternative is really not on. (I should stick with Rockwool or whetever for any flat section, as it's cheaper and depth isn't an issue.)

You then need a vapour barrier (specified polythene sheeting) to prevent condensation; the Celotex will be foil-covered, but the joist faces will be exposed and there will be cracks etc, so I think you're best off then covering the whole lot.

If you've got the ceiling height, you could then overboard with large sheets of Celotex, followed by plasterboard, or you could just fit polystyrene-backed plasterboard; either of which will give more insulation.

I did a similar project myself not long ago in an attic bedroom, and asked various questions here - might be useful if you can find the thread.

David

[Hugo Nebula]

On Sun, 18 Nov 2007 05:46:29 -0800 (PST), a particular chimpanzee, [e-mail] randomly hit the keyboard and produced:

Neither.

The best; remove the plasterboard, install 25mm PIR foam boards (Kingspan, Celotex, etc) between the rafters, leaving a 50mm ventilated space over, then 120mm PIR under, then a polythene vapour barrier, then plasterboard.

The easiest; an insulated plasterboard under.

[The Natural Philosopher]

No, you buy the celotex foil sticky tape and tape over the joists and board and make it comptletely AIRTIGHT, then board up. No need for foiled board, but belt and braces...

Rockwool is useless in any place that is ventilated. In a wind it loses most of its insulating properties.

[Lobster]

Yeah, I was going to do that with mine but ended up covering it with the vapour membrane - achieved the same airtight result. Can't remember whether it was due to cost/availability of the tape at the time, or because the membrane would be quicker.

David

[tonyjeffs2]

Have I got rhis about right?

-------------------------------------------------------------------------- roof tiles |\\\| |\\\| |\\ \| 25mmventilation space |\\\| ::::::::::::::::::: |\\\|::::::::::::::::::: |\\ \| ::::::::::::::::::: |\\\|::::::::::::::::::: :|\\\|::::::::::::::::::::|\\ \|::::::::::::::::::: 50mm Kingspan ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 25 mm PIR foam & dpm ===================================== Plasterboard So... if the joists are 3", I'd leave a 1" ventilation gap at the top, pack a 2" layer of kingspan between the lower 2 " of the joists. 25 mm PIR accross the underside of the joists, DPM Ceiling plaster.

Thanks Tony

[mail]

I agree with David andHugh Chris

[nafuk]

Did something similar recently in our kitchen. Put 25 mm celotex between the 3" joists - held in place by friction (leaves a 50 mm airspace between insulation and roof tiles). Put 70 mm celotex over the joists - held in place by a few long screws with large washers. Cover joints in celotex with aluminium tape. Covered over with foil backed tapered edge 12.5 mm plasterboard - held in place by 100 mm plasterboard screws from toolstation.co.uk (Screwfix don't appear to sell them).

If you want more insulation (I think about 160 mm is recommended?) then attach batten e.g. 50mm to the rafters and fill the space between rafters with 100 mm celotex then overboard with 60+ mm of celotex. I would have done this but the sloping parts of the ceiling were quite small and I stuffed the large void/attice above the sloping parts with glass fibre insulation.

[tonyjeffs2]

Thanks everyone. I've got the idea and I'll do all of that. I can lower most of the ceiling very adequately by reducing the gradient, and lower the lowest point an inch or so, so I think that'll be a good job.

Tony