Insulation for wall for a new extension

Subotai

I am getting an extension built. The builder has suggested the composition of the external walls to be as follows:

External rendering, 100mm block, 50mm cavity, 50mm Kingspan insulation board, 100mm block, 50mm kingspan insulation board, then internal plasterboard.

Is this a good design for the walls? Would walls built in this manner give good insulation and heat retention?

bduffy

Subotai wrote: »

I am getting an extension built. The builder has suggested the composition of the external walls to be as follows:

External rendering, 100mm block, 50mm cavity, 50mm Kingspan insulation board, 100mm block, 50mm kingspan insulation board, then internal plasterboard.

Is this a good design for the walls? Would walls built in this manner give good insulation and heat retention?

100 mm of insulation is significant....especially if it's foil backed Polyurethane as I'd suspect you're being quoted. Is probably expensive if you're using that much but it pays for itself.

sydthebeat

Subotai wrote: »

I am getting an extension built. The builder has suggested the composition of the external walls to be as follows:

External rendering, 100mm block, 50mm cavity, 50mm Kingspan insulation board, 100mm block, 50mm kingspan insulation board, then internal plasterboard.

Is this a good design for the walls? Would walls built in this manner give good insulation and heat retention?

Absolutely terrible specification.

That wall would be guaranteed to form mould internally and then all the health risks that go with that.

Keep all the insulation in the one plane, so 100in in the cavity, and keep the block work as a thermal store internally.

The builder is just trying to make life easy on himself by using plasterboard internally.

So again, it's a terrible specification and in no way should you accept that.

BryanF

bduffy wrote: »

100 mm of insulation is significant....especially if it's foil backed Polyurethane as I'd suspect you're being quoted. Is probably expensive if you're using that much but it pays for itself.

Better to put all insulation in the cavity and plaster block internally

Gumbo

I would fully fill the cavity. The builder is already filling the cavity so why not just it 100mm xtratherm full fill boards.

Subotai

Thanks for the replies. I'll ask the builder to change the spec to 100mm internal insulation with no air cavity.

Calahonda52

Subotai wrote: »

Thanks for the replies. I'll ask the builder to change the spec to 100mm internal insulation with no air cavity.

There was never any air cavity so why the proviso now.?
was the original spec wrong?

Couple of consequential effects:
how will builder deal with sockets and wiring plumbing etc with the solid block internally.
A lot of them use the internal insulation because its easier to gunter with a Stanley knife or a saw.
Maybe put a row of thermally better blocks as the first and last row in the internal leaf, my mind has gone blank for the correct name..

Subotai

The original spec was
100mm block, 50mm air cavity, 50mm insulation, 100 mm block, 50mm insulation.

So with the need for electric sockets, etc how is it normally done? I mean if having insulation on the inside causes mould to form? How are they normally fitted?

Gumbo

Subotai wrote: »

The original spec was
100mm block, 50mm air cavity, 50mm insulation, 100 mm block, 50mm insulation.

So with the need for electric sockets, etc how is it normally done? I mean if having insulation on the inside causes mould to form? How are they normally fitted?

Walls are chased out.

For the guys that are better at calculating due points, condensation areas etc
What about the inner block, sand/cement rendered. Then fix vertical battons to the inner leaf and fix plaster board to Thea battons?

Small void that services can run through but will this "cavity" create other problems?

Calahonda52

op, thanks, I assumed the 50mm cavity and 50mm insulation was the same space: my bad

kc: that design loses the thermal mass concept: it may drag the dew point in a bit further but but I don't see it as an issue.

sky6

kceire wrote: »

Walls are chased out.

For the guys that are better at calculating due points, condensation areas etc
What about the inner block, sand/cement rendered. Then fix vertical battons to the inner leaf and fix plaster board to Thea battons?

Small void that services can run through but will this "cavity" create other problems?

The Key issue is to make sure to fill and seal the bottom of the Wall well to prevent moisture getting into the cavities, otherwise you can develop mould problems.
I done our Bathroom outside Wall exactly as you outlined but then sealed the bottom using spray foam. That was 3 years ago and still no sign of any problems.

Calahonda52

sky6 wrote: »

The Key issue is to make sure to fill and seal the bottom of the Wall well to prevent moisture getting into the cavities, otherwise you can develop mould problems.
I done our Bathroom outside Wall exactly as you outlined but then sealed the bottom using spray foam. That was 3 years ago and still no sign of any problems.

Would you like to expand a bit more on this please?
Including, but not limited to:

• Bottom of what/where?
• Moisture from where?
• What thickness/depth scary foam?
• What type of spray foam?

Thanks

coco0981

Sorry for hijacking thread but I am in same boat as op as I am planning an extension on house. We were going to go with a 150m cavity filled with ecobead platinum silver. This should get near to 0.20 u value I am told. Does this sound like a good option?