Extension external wall construction

[mravant]

Hi

New here and have read through quite a few posts and it has been really informative as we are planning a two storey extension.

The first question(of many im sure) is that we are having an extension built shortly 3.5m x 7.1m (two storey), with the front of it being brick, cavity and then block construction and the side and back of the extension will be finished in render (to match the exisitng house).

A builder that we do like suggested that he would build the side and back of a single block 215 x 215 x 440 and have a 100mm Kingspan insulation and then batton and plaster board as opposed to Block, cavity and then block again.

My main question is is this ok and would there be a massive heat loss between the two?

Thanks

 

[jeds]

It's possible but not the normal construction for the UK. Solid walling is very popular in mainland Europe though. Of course you would have to prove the U-values and satisfy BC but can't see a problem with that.

My advice is get costs for both and take it from there. Solid walling is a simpler build and therefore should be cheaper. Which is fine as long as you benefit from the saving.

 

[mravant]

Hi John,

Thanks for the info.

The builder stated that im probably looking at an extra £1500 to have the double block work.
He only stated the single block option becasue the external dimension is only 3.5m and we would loose another 10inches if we had double block.

My concern was that would the room be as warm as it would have been with double block, would there be a big difference between the two.

I spoke to building regs this morning and they were happy with the single block with 100mm insulation.

 

[RonnyRaygun]

He only stated the single block option becasue the external dimension is only 3.5m and we would loose another 10inches if we had double block.

Why would you lose another 10 inches? The thickness of the wall wil be almost identical.
215 block, 12.5mm render, 100mm Kingspan, 12.5mm plaster is 340mm.
2x100mm block, 12.5mm render, 100mm cavity, 12.5mm plaster is 325mm.
You should be able to comply with regs with less than 100mm insulation depending on what blocks you use so you could gain a bit of space that way.

My concern was that would the room be as warm as it would have been with double block, would there be a big difference between the two.

The U value depends on the materials used and their thicknesses, not whether or not there is a cavity.
As the solid wall would be using PIR board or similar - rather than Rockwool - then the U Value should be better, as long as the blocks are the same type.
You can use Kingspan in a cavity but have to leave a gap so for a 100mm cavity the U value would be less.

 

[mravant]

Stupid me was presuming that the 215 blocks would also be used on the inside....

Im not a builder

 

[mravant]

[/quote]
The U value depends on the materials used and their thicknesses, not whether or not there is a cavity.
As the solid wall would be using PIR board or similar - rather than Rockwool - then the U Value should be better, as long as the blocks are the same type.
You can use Kingspan in a cavity but have to leave a gap so for a 100mm cavity the U value would be less.[/quote]

Could you please tell me what would be ideal materials which will give an excellent U value for single block?

I want to be able to tell the builder what i want, he suggested 215 block and 70mm Kingspan. I just want the most warm i can get.

Thank you

 

[freddiemercurystwin]

Any designer or builder (if not looking to exceed the regs) will (or should) spec the thickness of insulation required to meet the Building Regs, your worries about it being substandard are unwarranted.

 

[mravant]

I just dont want to be in a position afterwards regretting that for an extra £1500 that i could have had a warmer room.

 

[freddiemercurystwin]

Just wack in 140mm of Kingspan that should keep you toasty, to get more technical would be a world of pain for you! There are dozens of different types of block specifications and insulation specifications all with their own costs. If you're really interested in warmth and saving money your builder should be exploring SIP panels.

 

[RonnyRaygun]

I just dont want to be in a position afterwards regretting that for an extra £1500 that i could have had a warmer room.

Remember that you probably won't actually notice your room being any warmer, you'll just spend more on heating the room (unless the thermostat is in a different room and cuts in too soon). Anyway, it would take you a long time to save £1500 in heating costs due to slightly more insulation in one wall of your house.

Also, insulation and U-Values obey the law of diminishing returns so the thicker your insulation gets, the less you save.

70mm Kingspan will exceed current building regs. You can also get composite insulated plasterboard which would save space compared to plasterboard on battens.

 

[mravant]

Thanks.
So it does sound like my potential builder knows his stuff...

I just got very worried initially when he suggested single block, 70mm kingspan and battens as opposed to everyone else saying double block with a cavity.

 

[mravant]

Ok...
After a long meeting this morning we have cleared a lot of questions with the builder, have much more confidence in him now.

He has suggested the he will use 100mm thermalite block for inner and outer wall and he wants to put 75mm Kingspan in the cavity with a 25mm air gap.
Does this sound better than using 100mm drytherm?