Floor insulation & Aluminium tape advice

[Mattso]

Hello, 

 

I just came across this forum and I'm sure I'll be spending many hours on here in the weeks to come. 

 

I'll be insulating a suspended ground floor using PIR. As I have decent headroom underneath most of the floor, I'd like to insulate from below rather than lift all the floorboards: cut PIR to size, fit between joists, attach some sort of batten, spray foam any gaps. As floor for the rooms, I'll be keeping the original floorboards, and will be filling the gaps between the boards with pine slivers and glue before sanding everything down. I'll make sure any potential air gaps will be addressed.

 

The question (1): Is there any point of or recommendation for using aluminium tape on the underside of the joists, to create an air and vapour-tight barrier? Ideally, I'd put the aluminium tape on the joists on the warm (room) side of the insulation, in order to allow the joists to get the benefit of underfloor ventilation. 

 

Bonus question: This probably depends on the answers to the previous question; I'll be removing some 50mm Celotex from between some joists where my predecessor did a so-so job. Any point in cutting those boards down to liberally cover the underside of the exposed joist, in order to provide additional insulation? This, as the aluminium tape, would interfere with the ventilation of the timber joist, therefore maybe not recommend. 

 

Many thanks, any thoughts are welcome. 

[jayc89]

Assuming the void below is well ventilated, ideally you'd have a wind tight membrane below the insulation (cold side) and an airtight membrane above it (warm side). A few images of the idea here - https://www.ecologicalbuildingsystems.com/post/ask-expert-thermally-upgrading-suspended-floors. 

[Mattso]

Thanks @jayc89. From what I gather, the PIR + aluminium tape should make any further membranes redundant as it's considered a fairly airtight combo anyways. In my specific case, I'd like to avoid lifting the floorboards to add 'stuff' on the warm side of the insulation. 

[jayc89]

4 minutes ago, Mattso said:

Thanks @jayc89. From what I gather, the PIR + aluminium tape should make any further membranes redundant as it's considered a fairly airtight combo anyways. In my specific case, I'd like to avoid lifting the floorboards to add 'stuff' on the warm side of the insulation. 

 

A membrane on the cold side is to prevent cold air impacting the performance of the insulation (thermal bypass), you could argue PIR taped well enough on the cold side offers the same thing. You still need to tape the warm side though to avoid condensation. Without tape, the risk is warm moist air from the room could get through the gaps between the PIR and joists and condense against the cold side. 

[Mattso]

41 minutes ago, jayc89 said:

 

A membrane on the cold side is to prevent cold air impacting the performance of the insulation (thermal bypass), you could argue PIR taped well enough on the cold side offers the same thing. You still need to tape the warm side though to avoid condensation. Without tape, the risk is warm moist air from the room could get through the gaps between the PIR and joists and condense against the cold side. 

I'm still trying to work out if I can get away with not taping the warm side of the PIR. If taping the cold side (underside) I guess the concern is that moist air makes its way through the gaps in the PIR (it won't go through the PIR) and get trapped by the aluminium tape, condensing on the taped timber. 

Any condensation on the underside of the PIR/ timber isn't a concern as the undercroft/ crawl space is well ventilated