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chronyx

Kingspan/celotex Between Roof Trusses

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This is not a self-build question as such but this seem the best place for this post...

Are there any potential problems with installing sheet insulation between the roof truss timbers? I know condensation can occur with cavity wall insulation although there should still be an air gap between the roof and the insulation.

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This is not a self-build question as such but this seem the best place for this post...

Are there any potential problems with installing sheet insulation between the roof truss timbers? I know condensation can occur with cavity wall insulation although there should still be an air gap between the roof and the insulation.

Thats the most important part ,you still need to have airflow between the roofing felt/membrane to allow the timbers to breath

Can't it be fitted to the underside of the joist ,xps sheets are a good option for this as they have a greater R value so it requires a thinner sheet

Edited by long time lurking

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Both! So trying to avoid excessive heat/cold/temp variations. Also any energy efficiency benefits, although there's plenty of fibreglass under the boarding already.

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What for? are you specifically trying to keep the loft warm? For storage? For living space?

There are two types of insulation of roof spaces -

1. 'cold roof' in which the loft is not ininhabited and quilt is put at ceiling level (320mm? regs at moment?), whereby the object is to keep everything below that level warm.

2. 'warm roof' in which the loft is inhabited and rigid board is fixed between the rafters so that the roof area can be utilised.You will then probably finish off with an insulated plasterboard as well.

2 is more expensive.

If it is a modern building your roof trusses will not be designed for the usual floor loadings, they are only flimsy things designed only to take the weight of roof tiles and some storage - so take care what heavy furniture you put in!

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Thanks frankief. When you say rigid board do you mean sheet insulation?

If I do put anything up there besides items in storage it will just be a desk for a computer to keep the lounge and bedroom free of computer, paperwork etc. So a small office of sorts, but nothing too heavy.

I suppose I could bore out of the side wall and install an extractor fan to remove some heat too.

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Yes, what you would call Kingspan (other makes are available), it's a compact insulation with a silver face and comes in sheets.

I'm not sure you have modern trusses if you have headroom enough to put an office up there, the diagonal struts would be in the way, BTW Do not saw through these under any circumstances! :wacko:

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That's getting a bit involved for me, think I'll just leave some air gaps! :D

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Warm roof construction has to be done correctly - full layer of insulation at correct thickness above any structural timber so that it is also 'warm' - you can't just fill between the joists. Also insulation fixings and joints have to be dealt with correctly to prevent thermal bridging.

Cold roof insulation must have adequate ventilation and roof timbers must be allowed to breathe fully inside and out.

Like cavity wall insulation, badly done it can be a real problem.

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Sounds like more aggro than it's worth, thanks though I'd rather know now than afterwards!

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Just for completeness:

1. Secondsandco is a good source for insulation. Basically Kingspan rejects. I've bought truckloads of it

2. Changing a thermal element needs to go through building control. They will check you haven't done anything silly.

3. I went for warm roof (160mm Kingspan ~= 320mm wool) with breathable membrane (Daltex good). All the tiles came off to be able to do it but it has been worth it.

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Warm roof construction has to be done correctly - full layer of insulation at correct thickness above any structural timber so that it is also 'warm' - you can't just fill between the joists. Also insulation fixings and joints have to be dealt with correctly to prevent thermal bridging.

Cold roof insulation must have adequate ventilation and roof timbers must be allowed to breathe fully inside and out.

Like cavity wall insulation, badly done it can be a real problem.

I have worked on only 1 warm roof. Bankfoot Church in Perthshire. Trusses,12mm OSB, 100mm Kingspan, Roofshield membrane,50mm x 25mm counter battens,50mm x. 25mm tile battens. 1500sqm of Sandtoft Britlock tiles. The worst part was fixing down the counter battens thus also the Kingspan using 150mm Heli-fixings every 400mm up the rafter.

I would think that English tradition roofs (felt over rafters, no Sarken) would make warm roofs harder to construct.

Unhinhabited roof spaces should be as near to the outside temprature to cut out condensation build up. Rot will develop at the eaves and near the ridge so be well ventilated.

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I have worked on only 1 warm roof. Bankfoot Church in Perthshire. Trusses,12mm OSB, 100mm Kingspan, Roofshield membrane,50mm x 25mm counter battens,50mm x. 25mm tile battens. 1500sqm of Sandtoft Britlock tiles. The worst part was fixing down the counter battens thus also the Kingspan using 150mm Heli-fixings every 400mm up the rafter.

I would think that English tradition roofs (felt over rafters, no Sarken) would make warm roofs harder to construct.

Unhinhabited roof spaces should be as near to the outside temprature to cut out condensation build up. Rot will develop at the eaves and near the ridge so be well ventilated.

I'm not sure if planning permission is required as a warm roof raises the ridge height. They also keep revising building regs - increasing the thickness which makes fixing even harder.

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I went for a wamr roof option because the lower part of the roof forms the ceiling of the bedroom. I went for a multifoil product combined with a breather membrane over the top of the rafters with an air gap of 50mm then 50mm kingspan between the rafters and 100mm beneath the rafters. When it snows our house is the last one to lose the cover on the house, by quite a margin. It also made a difference to the sound quality in the bedrooms beneath the insulation. Much quieter!

I'd only suggest going to the expense of a warm roof though if you are going to have an occupied space under it, otherwise a much cheaper option is just to install blanket insulation in the ceiling joists and above.

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