Filling gaps in PIR boards over a slab.

[Tony K]

Afternoon all.

Once I finish the DPM detail, I will be laying 100mm PIR boards down over my ground slab before UFH and screed go over the top. 

I will of course be taping the joins between the PIR boards, and am minded to fill any gaps around the edges or between boards with spray foam. 

I have seen a few useful debates about filling gaps in ceiling or roof PIR insulation, but not for ground floors. Regarding ceilings/roofs, there are those that argue in favour of cutting your PIR undersized relative to the gap between joists, leaving about 5-10mm gaps all around deliberately so that you can foam-fill the edges, thereby achieving a better tightness than you might if relying on cutting boards to size.

 

Is this practise advisable where the floor is concerned?

Does anyone have any recommendations on techniques, tools and materials for this purpose please?

 

Thanks

[jayc89]

I used foam to fill any large gaps. Let it try, trim and tape over. 

[JohnMo]

You really should be aiming at a really tight fit.  Any gaps that remain spray foam then tape.

 

Remember to add the perimeter boards before the fill insulation, the use a thin DPM membrane on top of the insulation.  This serves two jobs, it's good to prevent screed getting below the insulation, if it does the insulation will just float.  It also stops a chemical reaction between the aluminium coating on the board and the concrete within the screed.

[Ferdinand]

If you are using expanded foam, make sure there is room for it to expand 🙂.

 

I think it is normal recommendation to mist the cavity with water first.

[Tony K]

1 hour ago, Ferdinand said:

If you are using expanded foam, make sure there is room for it to expand 🙂.

 

That's my concern!

 

I can see the logic in leaving extra gaps to the sides of PIR and filling with an appropriate foam when fitting between timber joists. I suppose those joists may swell or shrink, or just move a little over time and what was a snug fit the day you fitted the PIR might be less so (or too much so) later on. 

 

In my case, I'm putting PIR onto a concrete slab, inside blockwork walls so I don't expect movement. I reckon I'll fit the PIR as snug as possible, then add a small bit of spray foam where there are gaps. Then wait a bit, then add some more, then repeat, then panic when the PIR starts rising up, then rip it out and swear a lot. 

[Ferdinand]

4 hours ago, Tony K said:

 

That's my concern!

 

I can see the logic in leaving extra gaps to the sides of PIR and filling with an appropriate foam when fitting between timber joists. I suppose those joists may swell or shrink, or just move a little over time and what was a snug fit the day you fitted the PIR might be less so (or too much so) later on. 

 

In my case, I'm putting PIR onto a concrete slab, inside blockwork walls so I don't expect movement. I reckon I'll fit the PIR as snug as possible, then add a small bit of spray foam where there are gaps. Then wait a bit, then add some more, then repeat, then panic when the PIR starts rising up, then rip it out and swear a lot. 

It will be nothing like that difficult. 🙂