I have a cottage with quite a big loft, but very little vertical space in it - enough to crawl but not to stand. There is some fibre-glass insulation between the joists. I'm thinking of getting some insulating boards to place over the joists. Is this a sensible solution? I recall some years ago seeing "Knauf boards", but I haven't seen any for some time. Is Kingspan or Celotex suitable for this? Any advice or suggestions gratefully received.
On Tuesday 26 March 2013 12:14 Timothy Murphy wrote in uk.d-i-y:
Yes it is - and is approx equivalent to twice the thickness of glass wool (etc).
Ensure that the cold side (the loft in this case) has adequate ventilation from outside and that the trap door has a seal to minimise hot damp air entering. Otherwise you will get condensation and maybe rot.
What I'd do is put 75mm or even 100mm celotex from ebay over the floor joists. I'd screw them in with a few long screws and insulation washers to keep them from sliding. Then tape the joints with proper foil tape.
Then drop 18mm ply straight on top of the areas that you want to crawl or store stuff on - perhaps a run down the middle. You could come back to thinner ply in areas that you will only store light boxes on.
Perhaps 12mm ply would be good enough as this is an expensive and heavy element - if anyone else wants to answer that? I'd default to 18 as being " a safe bet".
Anyway, you'll end up with the equivalent of 6-8" wool in addition. This may not make sense if the rest of your cottage leaks like a sieve (eg all single glazed windows) - in which case 50mm celotex would probably be good enough at less expense.
I would concur with this, and have done something similar in our 70's bungalow for loft areas that I use for storage. You will probably find though that glass fibre in plastic casings is more cost effective for areas that are too low for storage.
One thing to be aware of is leaks of warm damp air from the house under the celotex sheets - leading to small damp areas of condensation. You have to seal all leaks with considerable care. I even found that air rising up oval conduit, with cable in it, above 13A sockets, was enough to lead to small areas of condensation. In the end I've sealed these with small blobs of plumbers mait, and this seems to have solved the problem, while leaving the cable removable if needed. All other holes can be sealed with patches of plasterboard etc.
Despite careful leak sealing, once you are sufficiently well insulated that the loft is much colder than the house, loft ventilation is very important, as Tim has said.
These boards are very expensive but you can get "seconds" half price or so. Just Google "insulation board seconds." to find a localish supplier. Order as many as possible to get the best deal. They have a few damages and voids. You can fix the voids with canned foam. You must leave no gaps,the slightest gap defeats the whole scheme. Cut undersize and fill gaps with canned foam is easiest. I would take up the mineral wool and replace with insulation board and more on top. I have 200mm of board in my roof space.
Just check draughts can't blow through the fibreglass. If any doubt you can block the outer edges of the fibreglass insulation with celotext stuck in with canned foam.
I'd lay chip on the insulation boards. Scrap 12mm's enough if cut to fit the joists, otherwise 18mm T&G
Such a scheme does lack any VB, you might want to add one.
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