Underfloor insulation

[Carol]

Does anyone have experience of adding insulation between joists in a suspended floor. Cottage is an 1880s semi-detached workers cottage and seriously draughty. Do we need a moisture barrier under the boards? Found a useful pamphlet by Historic England but suspect this mostly applies to ‘better quality’ buildings!

 

[Craig89]

I used 100mm celotex between my joists, supported on roofing lath. Relatively easy to do for me as I had installed new joists so the spaces inbetween were parallel. I Used expanding foam to seal all around the perimeter against the walls.

If the floor is draughty that suggests good sub floor ventilation which is a good thing. So as long as the timber is dry, probably supported on dwarf walls with slate used as damp proof. I don't see the problem.

Some people have used rock wool type insulation supported on chicken wire

 

[erikwoolf]

We haven' t started yet but we are going to use flexible woodfibre (10cm) and sandwich that with 2 membranes, a vapour permeable/airtight underneath and a vapour impermeable above. You need to make sure that you have good air ventilation underneath. We have also started monitoring with hygrometers the moisture levels around the joist already to get an idea of what the situation is and planning to leave them after the installation (using wireless ones) to keep an eye in the long-term.

 

[noj]

I'd question the impermeable membrane. If the insulation is breathable, as is the lower membrane then why add something that isn't?

 

[Carol]

Thanks for this, really useful.
C

 

[erikwoolf]

I'd question the impermeable membrane. If the insulation is breathable, as is the lower membrane then why add something that isn't?

This is what is been proposed in a couple of guides I have read, for example here.

 

[a twig]

They are claiming that the impermeable membrane stops warm damp air from above the floorboards inside entering the insulation / crawl space and causing damp…

Given that most breathable membranes usually function through a pressure differential from having warm air one side and cold the other, isolating the breathable membrane by using an impermeable one and a load of insulation seems self defeating.

Maybe ring them and have someone talk you through it, might make more sense than that article

 

[a twig]

The only “benefit” I can think of is if the joists are getting wet from damp issues in the walls, then this would still allow them to dry out to a limited extent.

At the end of the article they talk about doing it without membranes and their justification for the impermeable one is thin at best

 

[Roger440]

They are claiming that the impermeable membrane stops warm damp air from above the floorboards inside entering the insulation / crawl space and causing damp…

Given that most breathable membranes usually function through a pressure differential from having warm air one side and cold the other, isolating the breathable membrane by using an impermeable one and a load of insulation seems self defeating.

Maybe ring them and have someone talk you through it, might make more sense than that article

Thats correct. Impermeable membrane on the "warm" side. T0 prevent warm, moisture laden air entering the joists and insulation. It can breathe through the bottom.

Least permeabe inside, most permable outside.

Given that he joists should be dry, and this isnt a wall, it makes sense.

The question is how to deal with the interface where the joists enter the wall. Insulate round this at your peril.

 

[a twig]

That last point is the crux though isn’t it, unless the joists are in hangars and isolated from the wall to prevent any moisture getting into them, then any restrictions on that moisture being able to wick / evaporate away from the timber surely won’t be good?

An impermeable membrane takes away at least 25% of that drying surface?

 

[noj]

There's a survival technique where you stretch a sheet of plastic over a hole in apparently dry ground and collect water. That's not a system I'd choose to apply over my joists...