Roof & floor insulation, fixing fixtures & fittings and renders


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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.  Normally when you’re building a solid floor you have a layer of hard core, you’ve got a layer of concrete and then you put your insulation in and then all your floor finishes. With all that moisture, that moisture’s got to go somewhere and if you’re dealing with a very moisture sensitive building, like a church in this particular case, the last thing you want to do is chuck a load of moisture in the bottom of it, which has then got to come out through sensitive stonework normally. 

So with the particular system that we generally recommend, there is absolutely concrete in the floor, it’s very quick to install and, as I say, it’s dry – so as soon as you’ve got everything in place, you can turn the heating on, you can finish the floor and you’re done.

Now the floor normally starts with a layer of what’s called foam glass, and there’s lots of different products on the market.  There’s one called TECHNOpore. There’s Glypore, there’s lots of different pores! They’re all essentially foamed recycled glass and then broken up into an aggregate, so they’re normally either a 25ml or a 50ml aggregate.  That’s poured straight down onto – it can be poured onto a damp membrane or just a geotextile, whichever, but it’s basically put straight onto the ground and then you use a plate compactor to compact it in.  It’s actually quite difficult to get completely level, so on top of that we normally recommend a geotextile and then with a layer of either very, very coarse sand or something like 10ml stone chippings over the top of that geotextile so that you can screed that layer completely flat and then you’ve got a dead flat surface to come up off.

So once that’s in place you put a 22ml wood fibre layer over the top. We normally recommend the Udi top board.  It’s a 280 kpa board, so it’s a really strong, very high density board.  Once that’s in place, then our 45ml lithotherm tiles go over the top. Now they’re basically a screed replacement. Very high density tile that interlocks. So it’s got a tongue and groove profile on it and it allows you to layer a dry, very high density layer with grooves in it, to take pipes.  So once all the tiles are in, you can lay your pipes. Use a bit of dry mix screed around the ends of the pipes and then you have a finished surface that you can tile onto or you can put your floorboards onto. That’s basically what we’re doing in this project.  This project’s for a church in south-west London and that way they’re not going to add any moisture to the building. They can quickly get that floor in and as soon as it’s down they’ve got a very high compressive strength floor that they can actually use scissor jacks and all sorts on.

Moving on to how to fix fixtures and fittings to insulated surfaces. One particular question that we’ve had has been about you how you fit reasonably heavy radiators to internal wall insulation once it’s been installed. 

So there are lots of ways. You can install timber batons into the wall before you put your wood fibre boards on, but to be fair that’s a bit of a faff.  Generally, the best way to do it is to forget about where the radiator’s actually going to go, put all your insulation in and then we do a fixing from Fisher called Thermax, which is basically a massive screw and a massive Rawlplug that is drilled into the wall. The screw screws into the Rawlplug and then on the end of it is a big thermally broken plastic head, so that decouples anything that you fix into that head from the main shaft at the fixing. You finish that plastic head flush with the surface of the wall and then you can screw your radiator bracket into those points. Really, really simple and they come from anything between 80 up to about 200ml long, so irrespective of whatever depth of insulation you’re using, there’s normally a fixing for it.

And then the final area that we’ve been working on this week is renders and as it’s getting towards the end of the year, people are becoming very conscious of drying times and also what the weather’s like with regard to whether finishes are going to dry or not.

Our main topic of conversation has been what’s the setting time of renders? For most of the Baumit renders, certainly the external ones, they will typically set overnight, but you do need to leave it at least 1mm a day to cure before you put any finishes on it.  If you’re using MP69 you want to be leaving 16 to 18 days really to let it cure before you put your finish on. When you’re using things like MT55 onto either render boards or whatever, then again you want to leave it for the best part of a week to cure.

With regard to the finishes though, Baumit do an accelerator for their silicon finishes, so if you are using any of the Baumit silicon top products and it is getting on to autumn, then the speed top is a 250ml bottle of additive that you add to each tub and it will guarantee that it will set in about six to eight hours.  So should you be using anything at this time of year and it’s cold and damp, then that’s a great way to make sure that you can get it on the wall and get it set.

And the final question regarding the renders has been about what do you with renders that have got discoloured? We’ve got one particular project where a gutter had broken, got a load of water running down the wall regularly and basically bringing all the muck out of the gutter with it.

The best thing to do really is to give a gentle pressure wash. You can gently pressure wash silicon renders as long as you keep about a foot away from the wall or so with the head of the pressure washer. Then there are various render cleaners on the market. Baumit do one, but equally plenty of other manufacturer’s do. They’re largely just strong soaps, and if that doesn’t work normally the best thing if it’s just spot discolouration you can actually just get a tub of the original render, stir it up, get a paint brush and dab the paint brush into it and actually stipple it into the surface of the render. That will obviously cover up any stains. Where you don’t have any render left over, then the next best thing is to use the silicon paint that goes over the top.

So that concludes this week’s show. If you have any further questions about sustainable building materials, please feel free to email me at or alternatively give me a ring on 01392 861763.

Check out our next show this time next week.

Thanks for listening!