New build insulation advice

plus353

Hi - we are building a new house (self build) and are weighing up the different isulation and building options. We have got a couple of quotes... initially we were looking at timber frame construction with rainscreen. To meet A3 we would still have to install external insulation according to the quotes and BER estimate. This had us looking at other options such as poured concrete walls and standard block walls. The advice I've been reading is kicking us towards standard caviity block construction - single leaf wall - with a tonne of external insulation. From what I've read, this moves the dew point outside of the blocks and keeps it all warm and dry. Also, the stuff I've seen recommends not insulating the internal side of the wall as this brings the dew point back in (or reduces the thermal mass).

In the quote I got for a poured concrete wall system, 150mm of external insulation (EPS) was specified.

Any advice is appreciated - I was wondering about 150mm to 200mm of EPS on the outside of cavity blocks (with acrylic render on top of that), with the internal being plasterboard on 2x1 studs with no internal insulation. Thoughts?

MicktheMan

Why cavity block and not solid block on flat?
Go with the thicker insulation, relatively little diff in price.
Where and what will be your airtight barrier?

plus353

MicktheMan wrote: »

Why cavity block and not solid block on flat?
Go with the thicker insulation, relatively little diff in price.
Where and what will be your airtight barrier?

Hiya - I was thinking the cavity blocks just for strength because the walls are 3.6 metres high. Don't know if it's needed though - just taking advice from a builder on that. You think standard blocks would be stable enough?

For a barrier - I'd looked at the waterproof sealant as a vapor barrier (inside & outside the blocks?). I hadn't considered the air tight barrier.. Maybe the same type of plastic (not actually plastic) type sheeting you'd use inside a timber frame - behind the studs on the inside. Would that work?

A single leaf wall out of standard blocks with 200mm insulation would defo be cheaper!!!

MicktheMan

plus353 wrote: »

Hiya - I was thinking the cavity blocks just for strength because the walls are 3.6 metres high. Don't know if it's needed though - just taking advice from a builder on that. You think standard blocks would be stable enough?

For a barrier - I'd looked at the waterproof sealant as a vapor barrier (inside & outside the blocks?). I hadn't considered the air tight barrier.. Maybe the same type of plastic (not actually plastic) type sheeting you'd use inside a timber frame - behind the studs on the inside. Would that work?

A single leaf wall out of standard blocks with 200mm insulation would defo be cheaper!!!

What professional(s) is / will be advising you?

plus353

MicktheMan wrote: »

What professional(s) is / will be advising you?

I have a local builder and architect. I have 20ish years of limited energy, electrical and minor timber frame experience - but almost exclusively in America. A little different building regs there For our building regs here, I'm relying on my local builder and architect. The local builder has done work for my family for the past ten years.

Calahonda52

plus353 wrote: »

....with the internal being plasterboard on 2x1 studs with no internal insulation. Thoughts?

This will negate the thermal mass idea for external walls.

4" solids on flat, as suggested, with external insulation would be much quicker than cavity block and much more serviceable as an end product.

Shanaknocker

I am thinking of building with standard block on the flat with external insulation (EPS). I have researched the heck out of this and been advised from several builders. Over half of the builders recommend cavity wall, most of them have have been i the game a long time. New builders, insulation guys and Ber guy informed that if done right, with correct render it will be water tight. As for air tight, sand and cement will do this on the inside before skimmed.
This way may cost a bit more, however you will get better air airtightness. Less thermal bridgeing, heigher thermal mass.
I am using a passive slab for the foundation, which will not need an outer ring beam to support outer leaf. I using Passive cills and triple glaze. Get the fabric right and the rest should be easier which is where im stuck. What heating system.