Wide cavity vs ewi

Jcanty85

Hi all,

We are just planning out our self build and are between either block on flat with ewi or a wide cavity (either full fill bead or partial fill board). At the moment I'm leaning towards the ewi as it seems like a better end product from a thermal conductivity point of view however as the site is quite exposed I'm nervous about the durability. I'm just wondering what people's opinions of the pros/cons are?

Thanks all

BryanF

Have read the ten plus years of very detailed threads on this topic?

Calahonda52

Jcanty85 wrote: »

Hi all,

We are just planning out our self build and are between either block on flat with ewi or a wide cavity (either full fill bead or partial fill board). At the moment I'm leaning towards the ewi as it seems like a better end product from a thermal conductivity point of view however as the site is quite exposed I'm nervous about the durability. I'm just wondering what people's opinions of the pros/cons are?

Thanks all

You can't really look at this in isolation to the rest of the build.

Jcanty85

I've been through every board here but I still have not arrived at a clear winner. I appreciate it can't be taken in isolation however as a starting point I thought it would be a good start. Hoping to get U value under 0.12 but in order to get this with wide cavity/Eco bead or similar I'd have to nearly go to 300mm. This will naturally throw up a lot of issues from a detail PoV so I was wondering if a 6 inch pumped cavity with (budget allowing) 4-5 inches of ewi but a better option or what people's opinion on this would be?

BryanF

Go with ewi or cavity insulation

Why are you aiming for .12w/m2k ?
Passive standard is .15
Have done phpp calc ?
Sorting out thermal bridging or better glazing, may be a better spend

Jcanty85

BryanF wrote: »

Go with ewi or cavity insulation

Why are you aiming for .12w/m2k ?
Passive standard is .15
Have done phpp calc ?
Sorting out thermal bridging or better glazing, may be a better spend

Yeah you're right, .12 was a bit unrealistic, was just my attempt at future proofing. A 200mm wide pumped cavity gets us to .15 so maybe the extra money would be better spent elsewhere.

sydthebeat

Jcanty85 wrote: »

Yeah you're right, .12 was a bit unrealistic, was just my attempt at future proofing. A 200mm wide pumped cavity gets us to .15 so maybe the extra money would be better spent elsewhere.

100% correct.

Insulation really suffered the law of diminishing returns at these levels.

Look at resolving thermal bridges instead

Still waters

block on flat with Ewi all the way

BryanF

Still waters wrote: »

block on flat with Ewi all the way

Why?

Jcanty85

Think the full fill wide cavity will be the way forward. Solid wall inside/out plus a good bit saved on the cost of rendering. Following on from this what are people's opinions on the thermal foundation blocks from a thermal PoV. It might be old fashioned but as my site is suitable to strip foundations I am hesitant to opt for insulated raft due to weight issues (blockwork/hollowcore slab). Hoping to only need two blocks on flat plus a block on edge so I was thinking of rolling a sheet of plastic out over the compacted fill (will not be much), then building two thermal blocks on flat and one on edge. 300 eps under floor. Dpc/radon under insulation and over block on edge. Additional bottom poly is only intended to keep thermal blocks from getting damp (site is sloping but just in case). I know structural advice is not permitted but any opinions on thermal bridging is appreciated. Thanks in advance.