How to minimise cold bridging at my new roof next week please?

[George Miles (dicegeorge)]

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?

I've made a little video at
https://youtu.be/kc2tBjZ097k

Advice please

George

[Brian Gaff]

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

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[Theo]

"George Miles (dicegeorge)" <diceg...@gmail.com> wrote:
> 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.

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.

> 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?

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.

> 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?

You can get thermally broken fixings - eg these for the roof deck:
https://www.youtube.com/watch?v=RBINlVX0jOc
(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:
https://www.youtube.com/watch?v=wyNviteSOzY
and 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.

Theo

[Theo]

Brian Gaff <brian...@gmail.com> wrote:
> 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.

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.

Theo

[George Miles (dicegeorge)]

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.

george

[Andrew]

On 05/10/2022 21:55, George Miles (dicegeorge) wrote:
> 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.
>
> george
>

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.