Air gap in timber frame floor for PIR board?

18 Apr 2022
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United Kingdom
I'm building a garden room and am about to build the timber floor frame. I will be insulating between the joists with foil-backed PIR board. I have seen many tutorials where the PIR board is installed flush with the top of the timbers, so that the plywood floor base lies on top of the timbers and board. But this would not leave any air gap between the plywood and the foil face of the PIR board, so you wouldn't get any radiant heat reflection back into the room. I am planning to install underfloor heating on top of the plywood, so radiant heat reflection back into the room would be very useful.

Is there a problem with installing the PIR board with a 20mm gap between the foil face and the top of the timbers? It will effectively be sealed when I lay the plywood on top.

Thanks for any help or advice.
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Be sure that you understand how the insulation works. Will it perform better with an air gap like you think?

Will you be introducing a condensation risk?

You should install the insulation in accordance with the guidance from your heating system manufacturer.
As Woody says, probably best consulting the floor heating manufacturer.

Generally there is no issue leaving a void between the floorboards and insulation and it’s often done to allow services under the floor. See the BBA certificates from Kingspan and Celetex for details.

You could also contact your insulation boards tech support for guidance, they’re usually pretty on the ball.

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