Insulating subfloor

[kennydies]

I know this subject has been done to death but I am still no cleared on the pro's and cons of fitting celotex between floor joists.

There seems to be 2 method:

1. Friction fit in between the joists but it is not clear if this is airtight or not. If it isn't it negates the insulation pretty quick.
2. Undercut the board and seal all edges with expanding foam. This makes it pretty air tight and seems the easier option. This method would require batons where option 1 wouldn't

What is the common method for doing either and are there any pros and cons to either?

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[freddiemercurystwin]

Using friction alone is not a good method for suspended floors, over time as the floor expands and contracts the insulation will just drop out.

Instead of battens, panel nails can be used, set at the height of the bottom of the insulation.

 

[kennydies]

So regardless of how the board is supported. What is the consensus undercut and foam or friction fit?

 

[freddiemercurystwin]

Foam

 

[kennydies]

sold. Good job just bought 6 cans off ebay

 

[roryfirth]

Hi guys

I'm about to do the same thing and was going to screw/nail the Celotex boards through the joists to keep them in place. Will this not be sufficient?

How much should you undercut the boards by to use the foam method? And is it a simple case of squirting the expanding foam between the board and the joist?

Thanks
Rory

 

[freddiemercurystwin]

You need the nails whatever, you use the foam to fill in any gaps. Personally I would not rely on foam alone.

 

[roryfirth]

OK. My plan was to cut the boards so they are a snug fit and then secure them with screws or nails.

 

[geraldthehamster]

Hi guys

I'm about to do the same thing and was going to screw/nail the Celotex boards through the joists to keep them in place. Will this not be sufficient?

How much should you undercut the boards by to use the foam method? And is it a simple case of squirting the expanding foam between the board and the joist?

Thanks
Rory

What you did is pretty much what I did. I also used a few offcuts to prop the sheets.

Once the foam has grabbed them and gone off, they'll be rigid as fsck, unless you're planning on jumping up and down on them. Gap could be between 5mm and 15mm, it's not critical.

Cheers
Richard

 

[roryfirth]

Great, thanks for the advice.

I'll be putting 18mm board, 5mm insulation and 6mm laminate on top, which should dampen some of the impact

 

[freddiemercurystwin]

5mm insulation

 

[geraldthehamster]

Great, thanks for the advice.

I'll be putting 18mm board, 5mm insulation and 6mm laminate on top, which should dampen some of the impact

I meant, unless you're planning on jumping up and down directly on the Celotex. You'd be OK jumping up and down on the finished floor.

Why not lay a proper floor?

Cheers
Richard

 

[roryfirth]

I need to raise the level of the floor so it meets the door threshold and need some kind of air gap underneath to keep the council happy.

 

[kennydies]

I was going to make a gap just wide enough for my expanding foam tube.

Saves wasting foam, I was going to baton just bigger than the gap using offcuts from the old floor..

 

[drpepe]

I've used tape along every edge, to further hold the already wedged insulation, and also to stop drafts. I used expanding foam around the walls, and screws or battens for extra support where i was concerned there might be issues.

Worked well in 2 houses.