I would be inclined to use your 100mm Kingspan solid up against the deck held in place with battens or nails and then fill the void with a couple of layers of Rockwool or similar. Use a foil backed board.
Retro fit flat roof insulation?
- Thread starter markdon
- Start date
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Ag great, that sounds good. Is there any advantage of using the kingspan on top rather than the rockwool? Ive not bought any yet, thats just what i was thinking about getting.
Thanks for the advice.
Thanks for the advice.
The Rockwool may sag if used on top and create an air gap, the Kingspan won't.
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That's true, though I'm not sure why as there are no soffits or air bricks anywhere? Would the cavity in the walls provide enough draught to flow up through the roof space?
If the rock wool is properly jammed up, like if there is too much for the gap, then the kingspan shoved in and secured, then the foil backed pb on, would that work?
If the rock wool is properly jammed up, like if there is too much for the gap, then the kingspan shoved in and secured, then the foil backed pb on, would that work?
So just spray a bit of expanding foam in the gaps above the joists. Its important to eradicate any air gaps. As mentioned its not ideal but it'll be fine.
Grand, cheers, off to find a competent workman now who can do it ASAP.
Thanks for the help.
Thanks for the help.
Last question, i promise.
I may be able to get my hands on alot of earthwool for not much money, would stuffing this in as tight as possible, totally filling the void, then something like wire mesh to hold it in place then the foil backed boards, would that be ok? Or is it best to use the kingspan stuff? The gap is between 20-30cm across the roof space, so should get a fair bit stuffed in.
I may be able to get my hands on alot of earthwool for not much money, would stuffing this in as tight as possible, totally filling the void, then something like wire mesh to hold it in place then the foil backed boards, would that be ok? Or is it best to use the kingspan stuff? The gap is between 20-30cm across the roof space, so should get a fair bit stuffed in.
I don't understand the reluctance to fit some rigid insulation up tight against the deck, you just hold it up with battens or nails as previously mentioned. What you stuff the remaining gap with underneath is up to you.
I'm still not comfortable with trying to make it unventilated. Poking insulation up from below is never going to make an airtight seal that prevents warm humid air reaching the underside of the decking boards. But stuffing it is going to prevent ventilation with cool dry air from outside.
The joist and batten arrangement of this roof will enable air to circulate under the decking, provided there are airbricks or other holes below the deck and above any insulation. It seems to me that putting insulation between the ceiling joists, but not quite reaching the battens, would enable the underside of the deck to stay dry.
When the ceiling is replaced, yes, it should be done with foil-backed plasterboard to minimise the passage of water vapour into the void, but you will never stop it completely without a continuous unperforated membrane with no joints or light fittings.
This post-construction job is not the same as laying a new roof with an entire taped deck of kinsgspan that will prevent water vapour rising to the underside of the deck.
The joist and batten arrangement of this roof will enable air to circulate under the decking, provided there are airbricks or other holes below the deck and above any insulation. It seems to me that putting insulation between the ceiling joists, but not quite reaching the battens, would enable the underside of the deck to stay dry.
When the ceiling is replaced, yes, it should be done with foil-backed plasterboard to minimise the passage of water vapour into the void, but you will never stop it completely without a continuous unperforated membrane with no joints or light fittings.
This post-construction job is not the same as laying a new roof with an entire taped deck of kinsgspan that will prevent water vapour rising to the underside of the deck.
Yes John but this how many a garage conversion roof is done and the timbers do not seemingly rot, as mentioned not ideal but OK.
Cheers for the input. The only thing putting me off is the cost. I can get enough earthwool for the job and some spare for £80, the kingspan will be £315 then need earthwool on top of that cost.
There are no air bricks in the room, once the ceiling goes up the only airflow will be from the cavity c
There are no air bricks in the room, once the ceiling goes up the only airflow will be from the cavity c
Just squeeze a load of your earthwool in then the point is it won't be ventilated at all, I can't be bothered anymore some other muppet can take the baton.
Kingspan is expensive, but has the advantage that it is about twice as good an insulator as mineral wool, so you only need half the thickness.
Modern standard for house lofts is 250mm of mineral wool, which IMO is a bit excessive, it is certainly better than 100mm, but the incremental benefit of additional thickness gets smaller and smaller with every layer.
In a loft the additional thickness is not much of a problem and the extra cost is slight, but you are going to be restricted in available depth.
I don't know what will happen with condensation, as FMT says it is often done. Water would tend to condense on the board at night, then evaporate off when sun gets on the roof. You can also get interstital condensation in wool but not in Kingspan. I can't help thinking that ventilating the underside of the deck would help this water vapour to escape rather than accumulate. I am not experienced with flat roofs.
Modern standard for house lofts is 250mm of mineral wool, which IMO is a bit excessive, it is certainly better than 100mm, but the incremental benefit of additional thickness gets smaller and smaller with every layer.
In a loft the additional thickness is not much of a problem and the extra cost is slight, but you are going to be restricted in available depth.
I don't know what will happen with condensation, as FMT says it is often done. Water would tend to condense on the board at night, then evaporate off when sun gets on the roof. You can also get interstital condensation in wool but not in Kingspan. I can't help thinking that ventilating the underside of the deck would help this water vapour to escape rather than accumulate. I am not experienced with flat roofs.
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