Supporting PIR between joists on projecting wood screws.

[John Brown]

I'm ducking as I write this, but could somebody gently explain why it would be such a heretical idea to drive in a couple of wood screws on all four sides at the bottom of my shed floor joists, rather than have to cut and fix 144 battens, just to support 100mm chunks of Celotex/Kingspan?

 

[MikeG.]

A nail is easier, quicker and cheaper.

How are you keeping vermin and insects out from under this suspended timber floor?

 

[John Brown]

I don't know the answer to that.
Vermin? All the neighbourhood cats, maybe. Insects, I dont know.
Being in an AONB and a conservation area, and having to accommodate tree roots, I maybe don't have all the options. I think I was lucky to get PP for anything.

 

[Fitzroy]

The insulation needs to stop airflow to the underside of the floor, gaps around the edge or if it drops away from the floor will reduces its effectiveness. My view is you need a good close fit with all gaps filled with spray foam. I think you stand a better chance with battens than point supports. Any load on the insulation will force the nail into the board and a gap will appear under the floor. I’ve got PIR insulation in a few places in my house and it’s effectiveness is crap on early jobs where I couldn’t oversee the builder and ensure no gaps!

 

[John Brown]

Ok. Thanks.
Do you have any breathable membrane in your floor?

 

[MikeG.]

Don't put membranes in suspended floors. They're not necessary, and they are a potential trap for spilled liquids. Knock a cup of tea over and it will sit on top of your membrane for the next few decades.

 

[MikeG.]

The insulation needs to stop airflow to the underside of the floor, gaps around the edge or if it drops away from the floor will reduces its effectiveness. My view is you need a good close fit with all gaps filled with spray foam. I think you stand a better chance with battens than point supports. Any load on the insulation will force the nail into the board and a gap will appear under the floor. I’ve got PIR insulation in a few places in my house and it’s effectiveness is rubbish on early jobs where I couldn’t oversee the builder and ensure no gaps!

I usually specify that the PIR be cut 10mm undersize, and spray foam applied all round. The nails are only a temporary support for the insulation whilst the foam goes off, because after that they are glued in place. The only downside of this is that builders can cheat, and just put a bead of foam at the front edge. Doing it yourself, however, you can ensure that the foam is the full depth of the insulation. So my suggestion is to do one or the other.....friction fit or foam, but not a hybrid. If you want to use foam, which produces the best results, then allow room for the foam. You'll need to arm yourself with a box full of wedges.

 

[John Brown]

Thanks. Are the wedges instead of the nails as a temporary support, or in addition to?

Do I vaguely remember something about dampening the timber for better foam adhesion?

 

[John Brown]

Don't put membranes in suspended floors. They're not necessary, and they are a potential trap for spilled liquids. Knock a cup of tea over and it will sit on top of your membrane for the next few decades.

Thanks again.

 

[MikeG.]

...... Are the wedges instead of the nails as a temporary support, or in addition to?........

The wedges space the insulation off the timbers. They can work to hold it at the correct depth, but it's a fiddle, and just easier with a few nails.

 

[John Brown]

Ok. I see.
Sit on nails, roughly centre with wedges.
So the wedges are left in place?

 

[John Brown]

One last question(for now..).
Any recommendations/warnings re. foam guns?

 

[MikeG.]

No, they're cheap and effective. Buy some cleaner when you buy the foam, and use it now and then. Close everything down properly overnight.

Pull the wedges out when the foam has set, and spray into the gaps they leave. You learn not to get foam on the wedges!

 

[lurker]

Thread hijack!

Can someone describe alternatives for vermin proofing.
Something that I will need to do soon.

 

[gregmcateer]

Thread hijack!

Can someone describe alternatives for vermin proofing.
Something that I will need to do soon.

(hammer)

 

[MikeG.]

Thread hijack!

Can someone describe alternatives for vermin proofing.
Something that I will need to do soon.

Where? Under a floor?

 

[lurker]

Under and around a wooden shed base.

 

[MikeG.]

There are no good answers, I'm afraid. One of the reasons I always advocate a plinth.