Ventilated wood flooring between joist insulation - which windtight, breathable membrane at the ventilated, sub floor level?

[andeebee]

Throwing away all my desires and plans for going fully UFH with a solid, insulated sub floor in my 1960's dormer, due to proposed builder quotes, I have reverted back to plan B, which was to insulate between the joists and stick with radiators in the rooms.

 

Following the great guide on the subject on Ecological Building Systems website, my question is is it really necessary to use Pro Clima Solitex Plus as quoted, or could I get away with another, cheaper material in this layer - like some of the better roofing membranes that are available?

 

I would anticipate the membrane would not have to handle a great deal of moisture moving from the insulation to the ventilated space and act primarily as a windtight layer.

 

I have no doubt the Pro Clima is good stuff, but is also expensive when compared to good old Tyvek roofing membrane, which also comes in larger sheets, so less taping

 

Any thoughts or experience in this area?

 

[Redbeard]

You don't mention the 2nd membrane, above the insulation. Will you be using the DA membrane as suggested? I think I would use Intello Plus instead of the DA, but otherwise I support the system. No combination of membranes will make up for inadequate sub-floor cross-ventilation, so sort that first.

 

Before this Best Practice recommendation came out I used to use generic roofing membrane as the under-layer and as far as I can tell it behaved OK. 

[andeebee]

Yes, I will be trying to follow their example as closely as possible, using the upper DA membrane.

Im not opposed to using the materials stated, Im just conscious they are a merchant selling those products and in a world where I had a bit more cash, I would probably tick the boxes and get it all delivered.

Just wondering if I could save a few £'s by using other materials

[Sparrowhawk]

Welcome to the forum @andeebee. I asked a similar question a while back and the replies may be of use.

I've bought the membrane and insulation, but haven't installed it yet. Partly because other jobs have taken precedence, but also because the first area to insulate has a staircase and stud walls built on top of the floorboards, so it's an absolute pain to lift them!

[andeebee]

Ah, thank you SparrowHawk. I missed your post, possibly due to the use of PIR as I intend to use woodfibre.

I have had a read through and there is some good info on there to digest.

 

The top of joist topic is one I hadnt considered, but was aware this thermal bridge will remain, regardless of the U value between the joists. Did you find a solution to that?

[andeebee]

Just noticed, on page 20, in the downloadable guide that Ecological Building Systems provide for implementing this system, they advise the use of wood fibre cladding under the joists. This would provide the windtight and breathable layer whilst reducing the cold bridge into the floor joists and negating the need for the lower Pro Clima membrane.

Would imagine they would be a pig to fit though... 

[Sparrowhawk]

9 hours ago, andeebee said:

Just noticed, on page 20, in the downloadable guide that Ecological Building Systems provide for implementing this system, they advise the use of wood fibre cladding under the joists. This would provide the windtight and breathable layer whilst reducing the cold bridge into the floor joists and negating the need for the lower Pro Clima membrane.

Would imagine they would be a pig to fit though... 

As the caption says this is "from below". If you've enough space to get under your ground floors then this is a nice approach - though cutting and taping/gluing a bit of membrane at the top of each joist across the floorboards must be a pain compared to draping it.

 

In my house if I have 20cm below the bottom of the joists I count myself lucky!