Does PIR need an expansion gap?

[andyscotland]

For example, where the insulation on my warm lean-to roof butts up to the house brickwork? Or where the internal wall insulation (on battens on converted garage wall) meets the insulation on the top of the slab?

 

I have seen Ecotherm's instructions for lining a room with insulated PB on battens say to cut it 10mm less than the height but I'm wondering if that's just to do with the plasterboard / ease of fitting. I know if e.g. between studs you'd just cut the PIR to fit tight.

 

I had been thinking of cutting the boards very slightly short and then running a bead of expanding insulation foam to fill any gaps, but now am wondering if that's correct.

 

I know thermally it wants to be continuous, so if I do need to leave a gap presumably I want to fill that with ...something... as an air gap would defeat the purpose?

Edited July 5, 2019 by andyscotland
Clarification

[scottishjohn]

think its just same as PB --you never have it touching the floor ,cos it can suck up moisture and skirting will cover it anyway ,

 

[andyscotland]

Just now, scottishjohn said:

think its just same as PB --you never have it touching the floor ,cos it can suck up moisture and skirting will cover it anyway ,

 

Thanks, in my case the PIR on top of the old garage slab goes wall to wall, so the bottom end of the wall insulation would meet the foil face of that. The floor insulation is on a DPM so there should be quite a low risk of sucking up moisture?

 

I know the skirting will cover the visible plasterboard gap, but it won't resolve the thermal bridge/bypass if there's an air gap? (although the inside end of that will be covered by the VCL).

[Onoff]

18 minutes ago, andyscotland said:

Thanks, in my case the PIR on top of the old garage slab goes wall to wall, so the bottom end of the wall insulation would meet the foil face of that. The floor insulation is on a DPM so there should be quite a low risk of sucking up moisture?

 

I know the skirting will cover the visible plasterboard gap, but it won't resolve the thermal bridge/bypass if there's an air gap? (although the inside end of that will be covered by the VCL).

 

Run the dpm across the floor then up the wall a bit.

 

In my bathroom refurb I did this and also put a 1m high strip of dpm on the two external walls that sits down behind the tucked up floor dpm.

 

(I'd had some previous damp issues where the outside path was higher than the dpc and the internal concrete floor dead level with the dpc).

 

Wall pir should butt up tight to the floor pir to create a thermal envelope or you get cold bridging. Foam fill gaps.

[nod]

These are a few of the pics that I took for BC

[Onoff]

39 minutes ago, andyscotland said:

I know if e.g. between studs you'd just cut the PIR to fit tight.

 

Some cut it say 5mm less than the gap then foam fill to ensure at least one side is super tight to the studs / joists. 

 

I'm doing that above my bathroom between the 145mm deep joists at 400mm centres. Already got 100mm in there. Adding some 40mm I scored free as a top layer that'll overlap the 100mm joints. Nominal gap is 355mm between joists so I've cut the 40mm pir at 350mm. Tbh I'll probably gun foam all the edges.

 

You can only really g'tee a super straight, square edge if "factory" (though not always) or if cut on a table saw. Then you have to contend with warped sheets etc. Expanding foam takes up these defects.

 

It's why Icynene is so good.

[nod]

Just a point the tape pictured is one we use for Rendering protection It’s called Jaffa tape 

Will stick sheeting to bare block and stone No residue

At £4 a roll It’s the best I found  

[Ferdinand]

@andyscotland

 

AIUI, PIR does not expand .. it shrinks over time, so you are possibly going to get a gap there that you do not want. This has been discussed a couple of time iirc over at the Green Building Forum years ago.

 

By now the spec may have been tightened but it used to be expected to shrink up to 1-2% ie 10-20mm on a 2.4m sheet. The spec now is more like 1% max, I think.

 

One fix was to let it shrink before install. Another was to use a flexible foam.

 

I would welcome some input from someone with better and more recent knowledge than me on this.

 

This is one of the reasons I incline towards rock wool or similar for use underfloor or overceiling in renovations.

 

F

[andyscotland]

@Onoff @nod thanks, that's basically exactly what I had in mind before I started second guessing myself. Really helpful reassurance. Only slight difference is the wall battens are on DPC strips and the floor DPM taken up the front of them. That way if (despite the healthy condensation analysis) there did end up being any moisture on the inside of the blockwork it will run/drip down behind the DPM onto the garage slab.

[andyscotland]

@Ferdinand thanks - good point re shrinkage. By flexible foam do you mean pre-formed strip/roll rather than expanding foam? E.g. brickfill or similar, compressed into the joint so it will expand to fill any gap that appears over time?

 

Re letting it shrink before install - what sort of timeframe is that? I've had the insulation on site a few months (made sense to order the same time as some other materials) so will potentially have already shrunk a bit but guessing may have more to go.

[Onoff]

19 minutes ago, andyscotland said:

@Ferdinand thanks - good point re shrinkage. By flexible foam do you mean pre-formed strip/roll rather than expanding foam? E.g. brickfill or similar, compressed into the joint so it will expand to fill any gap that appears over time?

 

Re letting it shrink before install - what sort of timeframe is that? I've had the insulation on site a few months (made sense to order the same time as some other materials) so will potentially have already shrunk a bit but guessing may have more to go.

 

Compriband would probably be the ultimate albeit expensive gap filler!

[andyscotland]

@Onoff yeah I thought that this afternoon but I think it probably is prohibitively expensive for this application. And overkill really, doesn't need much by way of weatherproofing.

Edited July 5, 2019 by andyscotland
Didn't tag properly

[Onoff]

2 hours ago, andyscotland said:

@Onoff yeah I thought that this afternoon but I think it probably is prohibitively expensive for this application. And overkill really, doesn't need much by way of weatherproofing.

...I was kidding!

[andyscotland]

@Onoff there should be an embarrassed laugh / face palm emoji

[Onoff]

5 hours ago, andyscotland said:

@Onoff @nod thanks, that's basically exactly what I had in mind before I started second guessing myself. Really helpful reassurance. Only slight difference is the wall battens are on DPC strips and the floor DPM taken up the front of them. That way if (despite the healthy condensation analysis) there did end up being any moisture on the inside of the blockwork it will run/drip down behind the DPM onto the garage slab.

 

My build up is similar as in wall battens on DPM. I dug up the existing concrete floor, laid compacted type 2, then sand blinded. I then made in effect an EPS "well":

 

 

Over that I laid a DPM:

 

 

150mm of pir on top of the DPM:

 

 

Where it goes up the wall a bit isn't stuck:

 

 

I then put 1m high DPM on the two external walls:

 

 

The 1m high DPM comes down and tucks under the battened wall bottom member:

 

 

The floor DPM comes up over:

 

 

There's a min gap behind all the battens of 5mm foam filled. Once I'd pir'd in between all the battens I put a vcl over the whole lot:

 

 

Looking back it seems awfully complicated! 

 

Who cares! 

 

 

 

[andyscotland]

@onoff - looks great! I like the routed out detail on the top of the bottom batten. We fixed vertical battens and then top/middle/bottom noggins but I suspect it would have been much quicker to get the centres / plumb (and possibly more sturdy) doing it your way. There's still a couple of walls to do, and the battens inside the insulation for the service void - might try your technique for the next ones.

 

Is that proplex / correx type board on top of the floor PIR in your batten pics? I've been trying to get my head round a bit of an upcoming sequencing / materials-double-handling issue, would be easier if I could get the floor PIR down now but leave the chipboard till later but I've been concerned about walking / working over it too much and damaging it.

[Onoff]

8 minutes ago, andyscotland said:

@onoff - looks great! I like the routed out detail on the top of the bottom batten. We fixed vertical battens and then top/middle/bottom noggins but I suspect it would have been much quicker to get the centres / plumb (and possibly more sturdy) doing it your way. There's still a couple of walls to do, and the battens inside the insulation for the service void - might try your technique for the next ones.

 

Is that proplex / correx type board on top of the floor PIR in your batten pics? I've been trying to get my head round a bit of an upcoming sequencing / materials-double-handling issue, would be easier if I could get the floor PIR down now but leave the chipboard till later but I've been concerned about walking / working over it too much and damaging it.

 

I made the battened walls on the floor then raised into place.

 

The battens were nominally fixed with x5 concrete screws. Here and there where they didn't take I used Fisher resin and studs. I really damaged my pir by walking on it for so long! Ended up repairing with foil tape. Discovered Correx too late!

 

When the opposing battened walls were up I screwed screeding rails to them (Unistrut) and used a drag board across to get the wet concrete dead level.

 

 

 

[andyscotland]

@Onoff aha, that makes sense. We're using 25x50 for battens - I'm very tight on space - which I think might be a bit wobbly to build on the ground. But I might still do the top/bottom runners continuous and routed like yours.

 

I will stick to the plan of getting the floor insulation and chipboard down at roughly the same stage!

[Onoff]

16 minutes ago, andyscotland said:

@Onoff aha, that makes sense. We're using 25x50 for battens - I'm very tight on space - which I think might be a bit wobbly to build on the ground. But I might still do the top/bottom runners continuous and routed like yours.

 

I will stick to the plan of getting the floor insulation and chipboard down at roughly the same stage!

 

My room was so out of square and I wanted inset, illuminated pockets that I battened everything square. Tbh it was more like hanging stud walls on the walls.

[Pocster]

Sunken bath surely !

 

 

[Ferdinand]

Quote

Looking back it seems awfully complicated! 

 

Whodathunkit ?

[Ferdinand]

2 hours ago, pocster said:

Sunken bath surely !

 

 

 

is that not a NZ version of a French Loo, designed to stop drunken Ozzie’s missing (without the need for a Golden Shot Up a bit! Left a Bit! FIRE ! Facility) ?

 

And it looks darned uncomfortable for a bath.

Edited July 6, 2019 by Ferdinand

[scottishjohn]

4 minutes ago, Ferdinand said:

 

is that not a NZ version of a French Loo, designed to stop drunken Ozzie’s missing (without the need for a Golden Shot Up a bit! Left a Bit! FIRE ! Facility) ?

better not show SWMBO  this she,ll want it