Joining PIR insulation under floating plywood floor

[Hastings]

Anyone know the best way to join PIR to itself to prevent thermal bypassing caused by even very tiny air gaps?

 

Is it practically possible to tape the underside edges together?

 

I am thinking of using the same expanding foam tape that I have used in between all my insulated timber-frame wall panels.

 

Kingspan's Thermafloor TF70 product installation guide says to lightly butt together, which would surely reduce its effectiveness considerably.

 

I would rather not use PIR at all (I'm not using it anywhere else in the build) but it seems the only option due to difficulty of excavating the ground floor deep enough without disturbing the ground under the existing old stone walls.

[PeterW]

Thin bead of expanding foam on all the joints and then foil tape over the joints and you’ve pretty much got one monolithic foil covered foam slab. 

[Hastings]

25 minutes ago, PeterW said:

Thin bead of expanding foam on all the joints and then foil tape over the joints and you’ve pretty much got one monolithic foil covered foam slab. 

I'm in the camp that doesn't trust expanding foam from a can.

I did wonder about a non-setting building mastic but I don't trust them not to 'dry' and shrink later either.

Edited August 30, 2020 by Hastings

[tonyshouse]

Just but together, I use poly air barrier/vpm on top then flooring sheets glued, I prefer ply 

[Hastings]

37 minutes ago, tonyshouse said:

Just but together, I use poly air barrier/vpm on top then flooring sheets glued, I prefer ply 

Yes, that's the official method but it allows heat to easily move between the warm and cold sides of the insulation - effectively you provide a nearly continuous airway around all surfaces of each insulation sheet.

 

It's known as thermal bypass.

[PeterW]

4 hours ago, Hastings said:

I'm in the camp that doesn't trust expanding foam from a can.

 

Any evidence as to why not ..?? 

 

17 minutes ago, Hastings said:

Yes, that's the official method but it allows heat to easily move between the warm and cold sides of the insulation - effectively you provide a nearly continuous airway around all surfaces of each insulation sheet.

 

It's known as thermal bypass.

 

so a 2mm gap all round a board is the equivalent of 0.5% of the board. That is less than the tolerance in thickness of a board, so is lost in the rounding errors of a thermal calculation. 

[tonyshouse]

As I see it there is no significant air movement under a floating floor so thermal bypass is minimal. 

[MJNewton]

5 hours ago, Hastings said:

I'm in the camp that doesn't trust expanding foam from a can.

 

I wouldn't be too quick to dismiss it. It strikes me as a perfect solution - easy to use, tried and tested, gap filling, insulating, adhesive, strong and durable. What's not to like? 

[Hastings]

11 hours ago, MJNewton said:

 

I wouldn't be too quick to dismiss it. It strikes me as a perfect solution - easy to use, tried and tested, gap filling, insulating, adhesive, strong and durable. What's not to like? 

 

I have been advised by specialist suppliers of airtight building products not to use it.

There is quite a bit of information on the subject

eg. https://foursevenfive.com/blog/foam-fails-reason-5-excessive-shrinkage/

 

Hence the use of pre-compressed foam tape in airtight building construction.

[PeterW]

12 minutes ago, Hastings said:

 

I have been advised by specialist suppliers of airtight building products not to use it.

There is quite a bit of information on the subject

eg. https://foursevenfive.com/blog/foam-fails-reason-5-excessive-shrinkage/

 

Hence the use of pre-compressed foam tape in airtight building construction.

that is a US article about the boards shrinking not the canned foam itself. 
 

It’s not possible using standard building materials to get fully airtight - even window seals leak.
 

Why the target of a zero infiltration building ..? Unless you’re building a clean room ..??

[Hastings]

I don't have a target of 100% airtightness for the whole building, just in all the places I can reasonably achieve it.

[the_r_sole]

xtratherm do a tongue and groove board (other suppliers might do too, I just know the xtratherm one!)

[Hastings]

26 minutes ago, the_r_sole said:

xtratherm do a tongue and groove board (other suppliers might do too, I just know the xtratherm one!)

The floor grade product doesn't seem to be offered in T&G, by any manufacturer.

[Russell griffiths]

I used two layers of insulation so I couldstagger the joints. 

[the_r_sole]

9 minutes ago, Russell griffiths said:

I used two layers of insulation so I couldstagger the joints. 

 

genius ? can't believe I missed the obvious solution - give yourself a pat on the head!

[Hastings]

1 hour ago, Russell griffiths said:

I used two layers of insulation so I couldstagger the joints. 

 

That's not a bad idea, thanks.

Did you try and join them together with anything other than tape?

[Russell griffiths]

All sheets foamed together, but I buy better foam than is available in the shops. 

ILLBRUCK air sealing foam. 

I wont ever buy a tin of the normal cheap shite again, like chalk and cheese. 

[Hastings]

Thanks Russell - didn't know Illbruck did the foam stuff though I had been looking at their tapes as an option a while back. I've been using Pro Clima tapes and VCLs and it'd be interesting to know why they don't offer a chemical type expanding foam in their range.

Edited September 1, 2020 by Hastings

[Gav_P]

On 30/08/2020 at 22:39, MJNewton said:

 

I wouldn't be too quick to dismiss it. It strikes me as a perfect solution - easy to use, tried and tested, gap filling, insulating, adhesive, strong and durable. What's not to like? 

I think it’s brilliant stuff, but I hate it! I end up with it stuck to everything, walls, floors, ceilings, the sink, the dog, the car, trees.... and I’d only be trying to create a seal in a small hole drilled through a wall. 
 

Oh, and I end up with an elephants head size bit of foam growing out of a 10mm hole in the wall!!... which also sticks to everything within a 5m radius.

Edited September 1, 2020 by Gav_P

[MJNewton]

I think we can all relate to that experience @Gav_P! That said, most (all?) find the issues completely go away when they move to using a gun (rather than nozzles cans) - have you done this? It really is chalk and cheese. Ignore the cost of the gun as it soon pays for itself thanks to far less wastage (and being able to spread the use of a single can over several weeks if not months). 

[Gav_P]

6 minutes ago, MJNewton said:

I think we can all relate to that experience @Gav_P! That said, most (all?) find the issues completely go away when they move to using a gun (rather than nozzles cans) - have you done this? It really is chalk and cheese. Ignore the cost of the gun as it soon pays for itself thanks to far less wastage (and being able to spread the use of a single can over several weeks if not months). 

Nope not done that... I guess I love to torture myself! 
In the same way that I will struggle for 20 minutes with the wrong tool rather than getting up and going to the tool box to get the correct tool and do the job in seconds ?

[MJNewton]

Ha ha, yeah. I do that too. 

 

Definitely buy a foam gun (and cleaner) - I promise you you'll love it. One thing that might not be obvious is how controllable the gun is as the flow is proportional to the trigger throw. 

 

 

Edited September 1, 2020 by MJNewton

[PeterW]

+1 to this.  Bought a Fischer gun (old model so 1/2 price) and used either Illbruck or Soudal Low Expansion foam and Toolstations one cleaner. Only cleaned the gun every 2 or 3 cans or switching between the foams and it must have had about 3 years abuse until it finally died ..!

[Cpd]

I have exposed areas that I foamed 7 years ago and it’s still perfect, I use foam all the time and love it. Staggered joints foamed and taped is belt and braces.