Hello and welcome to this week’s ‘Back to Earth’ podcast, with me, Chris Brookman.

This is a show for building professionals and self-builders, all about the use of sustainable building materials.

This week we have three or maybe four main areas of questions.  We’ve had quite a few enquiries about roof insulation this week.  We have a load of enquiries about a floor construction without using concrete.  We’ve got enquiries about fixing fixtures and fittings to walls after they’ve been insulated and then we’ve got a load of questions about render and what they can be applied to, how you care for them, how long it takes to dry and how you clean them.

So starting with the roof structure. This is quite an interesting one actually, this one.  This is an outdoor farm building that’s basically being converted into a hot tub and sauna room, which is not one that I’ve covered before.  Perfectly possible using wood fibre.  Actually, really, really good because wood fibre’s really good with moisture, but the absolutely critical thing because you’ve got a sauna inside – which is obviously very, very warm and moist – it’s absolutely critical to get your vapour control layer installed absolutely perfectly to make sure you don’t get moisture leaking through from the interior of the building and obviously escaping out into the insulation during the winter.  That obviously would be a problem. So absolutely critical on the vapour control layer.

The rest of the installation for a roof structure using wood fibre; very simply, flexible wood fibre between the rafters. In this particular case we’ve got 150ml between the rafters, 100ml on top of the rafters in the form of a wood fibre board, the UD top board, and then it would be counter battened, battened and slated. Really, really good from a thermal mass point of view and acoustic point of view, so it’s going to be very quiet in there, but yeah, as I say, really important on the vapour control side to get that right.

The other project that we’ve been looking at particularly was a flat roof construction. Now, flat roofs generally people assume – wrongly unfortunately – that the wood fibre can’t be used in an unvented flat roof situation.  But actually it can – and it’s actually really, really useful in that scenario because it can be used in between your joists which normally that area wouldn’t be used by insulation.

So a normal build up would be – starting on the inside, you’d have your plasterboard finish. The critical component to the whole thing is a variable vapour control layer. Now there’s lots on the market. We supply one called Udi Steam 10 Plus, which is one from Unger Diffutherm but equally there’s Pro Cleamer and there’s a whole load of different ones on the market.  They all do a pretty similar job, but they are absolutely vital to the functioning of that flat roof.

So essentially in the winter months the membrane stays very vapour tight and prevents most of the moisture from leaking up into the roof, but obviously over a period of time you do get small amounts actually condensing in the roof structure and in the insulation.  That’s fine. So long as it keeps within a certain level the wood fibre is really easily able to cope with small amounts of moisture.

And then generally in the summer, the roof structure heats up and all of that moisture is actually driven towards the interior. Now, the vapour control layer – because it’s variable – what it allows you to do is the membrane opens up and allows that moisture to come back into the interior of the building and allows the roof to dry out completely before everything cools down again and you go through another winter. So it’s absolutely critical to the functioning of that roof.

Once you’ve got that membrane in, you have your insulation between your joists, as I said. If you need to, you can have a layer of sarking board on top of the joists to reduce thermal bridging. Then you have your ply deck and single ply membrane or GRP, whichever you’re using for your finish.  A really, really effective way to do a flat roof.

So, moving on to dry floor construction.  </sp