Insulating suspended timber floor

[Greenbeast]

Will be gutting and re-decorating the lounge soon, the floor is rotting and will be replaced with new joists and pine t&g floorboards (exposed).

What's the best course of action with regards to insulating?

Kingspan directly beneath the boards (between joists with foamed gaps)?
Another alternative?
Do i need to worry about moisture or moisture barriers?

[Richard Owen]

Is the space under the joists vented?

How much space do you have under the joists?

My preferred method (if you have the space) is to put a breathable membrane under the joists, full fill between the joists with PU foam (foam filled and taped), then a thin covering (a thin ply board works well) and then the T&G.

Air movement kills insulation value, the lower membrane and covering boarding are there to prevent that.

[Greenbeast]

Yes it is well vented.

About 8-12"

Thanks for your thoughts

[A.L.]

hi,

with a water vapour permeable air barrier membrane properly installed below joists why not fill generously with loft insulation? much cheaper than kingspan and three inches equals two inches of kingspan

or if joist spacing/thickness/depth suitable try Kingspan frametherm slabs www.knaufinsulation.co.uk/products/glass_mineral_wool_slabs/frametherm_slabs.aspx

interstitial condensation will not be a problem, anything like a vapour barrier on warm side will tend to hold liquid spills against flooring

[Greenbeast]

hi,

with a water vapour permeable air barrier membrane properly installed below joists why not fill generously with loft insulation? much cheaper than kingspan and three inches equals two inches of kingspan

Tyvek (or equiv.) installed to allow moisture down but not up?

[dtl]

If you are getting a builder/joiner to do this work then beware.......they will probably not fill the air gaps to make your floor air tight ie around the edges of the floor where it meets the wall and the gaps where the kingspan is not tight against the joists.....in my experience probably best doing it yourself.

I have had this problem twice now with two different sets of joiners, even though I went through it in great details during the bidding phase.

[Greenbeast]

well the builder/joiner is my step-dad and no doubt i'll be participating.
But yes i'll make sure air gaps are sealed!

[Richard Owen]

Tyvek (or equiv.) installed to allow moisture down but not up?

Just get the cheapest permeable membrane from your builder's merchant.

[JohnS]

As others have said, the advantage of rockwool or its equivalents is that it can be pushed in to fill all air gaps where as kingspan may leave small gaps.

Why pine boards and not hardwood? 

[dtl]

If he decides to use kingspan to fill to the maximum joist depth then the insulation value of the kingspan will far exceed that of the equivalent thickness of rockwool.

[EccentricAnomaly]

If what to do is question one, then why is the existing floor rotting seems to be question zero.

[Greenbeast]

Why pine boards and not hardwood? 

Well our new(ish) dining room floor is pine already, i guess it's down to cost really.

If he decides to use kingspan to fill to the maximum joist depth then the insulation value of the kingspan will far exceed that of the equivalent thickness of rockwool.

Well this will be down to cost. i'll have to do the sums and see what we can justify

If what to do is question one, then why is the existing floor rotting seems to be question zero.

I don't know if it is getting worse currently but a few years back we realised the floors in the dining room and lounge (and adjoining hall) were bouncing  unreasonably.
My step-dad did some investigation in the dining room and found the joists to be rotting, when he took up the dining room floor he discovered the vent brick had been blocked and so there hadn't been adequate ventilation. This was rectified and a nice new floor was laid.
Oddly i have found the air space under the lounge quite breezy so i'm not sure and in fact it might not be the same problem.

[A.L.]

hi again,

Tyvek housewrap is 'windtight' rather than 'airtight' but is cheap and will do.

as an example, a 4m x 5m room floor with two exposed edges will save about 74% heat loss with 150mm mineral wool or about 81% with 150mm kingspan at considerably greater material cost.