Breather Membranes - all created equal??

[Piers]

Please can someone give me a quick explanation of what's the difference between all the different brands and weights of breather membrane.

 

We're putting up a timber, stick-built outbuilding, constructed on screwpiles with a crawl space underneath. I think we need to install a breather membrane underneath the floor joists but not sure what type. Most breather membranes are "certified for roof construction", I can't find many that are specified for timber floors and none that are readily available at the usual merchants/Screwfix/Tool station etc.

 

My floor construction from outside to inside is:

 

Screwpile

Groundbeam

Breather Membrane

Joist 150mm with a) 130mm insulation between supported by battens or b) 150mm supported by breather membrane

OSB subfloor

Vapour Control Layer

Floor finish (prob engineered wood)

 

My questions:

 

What type(s) of breather membrane can I use?

 

Do I need to tape the overlap of the membrane? If so, what type of tape?

 

Is there specific VCL for under floors? Do I need to tape overlaps? What type of tape?

 

Thanks!

[Mr Punter]

It is quite unusual to have a timber ground floor where you are.  I assume you are using rigid insulation by the thickness.

 

On a roof or wall the breather membrane is there to keep the rain out and let the structure breathe.  With a floor, the rain is not an issue so it may be OK not to have a membrane. You will need plenty of ventilation under the floor to take away any moisture that may condense in the timber, especially where it meets the substructure.  150mm is quite skinny for floor joist but I guess there are sleeper walls or intermediate supports.

 

For VCL above you can use normal DPM (and tape the overlaps if you really want to).

[Ed Davies]

14 hours ago, Piers said:

I think we need to install a breather membrane underneath the floor joists

 

I think it's worth being clear exactly what jobs you want this membrane to do. Things I can think of to usefully do under insulation in timber floor above a crawl space (I'm also building this) are:

 

1) support the insulation.

2) prevent windwashing of the insulation (wind blowing through the mineral wool or whatever and taking all your lovely heat away).

3) keep out critters.

 

None of these are particular strengths of breather membranes.

 

What I'm doing with mine is supporting the insulation on sarking boards (150x19 or 22mm treated timber boards) nailed to the bottom of the joists. They're fitted slightly wet then dry out to leave a mm or so gap between the boards (i.e., every 150mm) to allow evaporation from the structure above. https://edavies.me.uk/2018/03/some-flooring/

 

I could just drop the insulation directly on these and it'd probably mostly work fine but I'm a bit worried about windwashing as I'm on a very exposed site (hilltop overlooking the Moray Firth). I could lay a membrane but I'd want to staple and tape to the joists so I'm considering laying 25mm EPS sheets first (and taping those at the edges). The incremental cost of 25 mm of EPS per m² vs the extra volume of mineral wool is a wash against the cost of cheap membrane but I think it'd be less hassle to fit and easier to get a good seal round the edge.

Edited June 4, 2019 by Ed Davies