Advice please, before I start building a little roof for my kitchen extension next week.
I've built a cavity wall of brick and block.
My plan is to stick kingspan seconds to it with foam, then plasterboard. Should I seal the blocks first with concrete sealer or paint in case of moisture? Should I seal the cut top of the kingspan somehow?
At the top I plan to put two 1.7m 2x6 timbers each side, then four 1.2m joists wedged between them on hangers. Above that 18mm OSB. Above that about 6 inches of kingspan. Above that 11mm OSB. Above that EDPM rubber roofing.
But where the side timber is bolted to the blocks there is a cold bridge, (more so at the bolts). So maybe extend the kingspan up to the top layer of OSB, and bolt the side timber through it. Or would that wobble?
Are there not information items on this sort of thing available when you obtained the planning permission. I often see what aware called non material amendments, which is silly as its precisely what they are like changes in cladding or insulation materials. Brian
Hmm, I'm a bit surprised you didn't put the kingspan inside the cavity. Doing it internally takes up what limited space you have.
Is this foil-backed kingspan, or just bare foam? I think the foil should provide a suitable vapour barrier. If not, you would probably need something as vapour barrier on the inside to stop condensation seeping into the wall. I'm assuming any moisture from the outside (driving rain etc) would dry out given air circulation in the cavity and the outside face. I assume you have DPC underneath?
You can seal the cut foam with foil tape, which should be fairly straightforward. Stops any moisture getting into the foam and reducing its insulation properties.
You can get thermally broken fixings - eg these for the roof deck:
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(that channel is good for understanding roof thermal performance) I'm not sure about thermally broken bolts, but Google suggests some are available.
Normally with insulation in the cavity the block side would be 'warm', and so would the wall plate, meaning the joists can be 'warm' too. The insulation would form a continuous layer up through the cavity to meet the insulation in the roof (as much as is possible).
Here's his model of a similar setup:
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in the warm roof setup he puts in insulation between the joists over the top of the wall to avoid most of that cold bridge. I'm guessing the ends of the joists are ventilated so most moisture is protected by the vapour barrier on the inside, and moisture in the joists can escape (slowly) through their ends.
One thing you could do to reduce the joist cold bridging is to add another layer of insulation (eg 25mm) on the inside of the joists. You can get plasterboard bonded to foam which makes it roughly the same to install as a plasterboard ceiling. That means the 'warm' side is continuous through the wall and ceiling.
Yes, I'm a bit surprised to be discussing it at this stage - I'd have thought building control (probably no planning permission needed) would at least ask about U-values.
OTOH if this is not 'habitable' space (ie it's a porch or similar, with an exterior grade door to the house) it might not need full insulation to building regs. But it would then need improvement to make it habitable space later.
It does pay to think about these things first, though - saves making mistakes you can't put right later. Detailing is always the tricky bit.
It is for a kitchen. Building Control will be visiting again soon.
I had so much to learn about with the planning, party wall act, foundations and brickwork I left details of insulation until now, though planned it to be internal insulation from the start.
Is the space where the ladder is going to be 'inside' ?.
Why is there no insulation inside the cavity ?. You could have gone for full-fill rockwool or 50mm PIR, plus 50mm cavity.
The latter is no longer possible but depending on how clean the block+brickwork mortar joints are you can still push fullfill rockwool slabs (NOT loft insulation) down inside the cavity if you can introduce long strips of thin bendy but smooth-surfaced bendy material inside the cavity, one against the outer leaf and the other against the inner leaf then slide/push the rockwool slabs in between the sliders which you pull out afterwards.
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