Cavity insulation advice

MizMix

Hi,

Would appreciate some advice on our wall spec which is currently:
215mm block (laid flat)
150mm xthratherm/KS PIR
100mm blockwork

A builder is currently pricing the job and expressed concerns over the insulation spec- feels there's a better way. That the blocks on flat is overkill (we're planning on engaging an engineer as we think some areas such as this and foundations may be overspecced- almost every builder who has looked at this has mentioned the foundations being OTT but we know we need an engineer to verify). He has suggested (and said he checked this with a Kingspan 'engineer') and a better method would be:
42.5 mm insulated warm boards (internal)
100mm block
140mm cavity (100mm PIR)
100mm block

He explained with 215 blockwork it doesn't hold the same thermal properties and you only get a return of high heat from the first 100mm masonry. This makes sense to me (we will need to check with an engineer) but my concern is the insulation is in two places. I don't really understand the technicalities around it but isn't best practice to ALWAYS keep your insulation in one place. I have done a search but would like to know what the reasons for this are and why PIR plus warmboard is not the best way anymore.

sydthebeat

get your engineer engaged first.

after that youll know if you need a 215 inner leaf or not, and all these queries could be a waste of time

Calahonda52

Who has come up with the design so far that is deemed OTT by the builder?

Of course the kingspin sales guy would suggest internal and cavity insulation( CI and II), its more product:
You will get sfa thermal mass if you slap on the II, which is largely, if not totally discredited for new builds there days, buy kingpin have not read the memo, why would they...
Whats the first floor: hollow core.

You are correct about all insulation in one place so perhaps 215 on flat and EWI?

Gumbo

MizMix wrote: »

Hi,

Would appreciate some advice on our wall spec which is currently:
215mm block (laid flat)
150mm xthratherm/KS PIR
100mm blockwork

A builder is currently pricing the job and expressed concerns over the insulation spec- feels there's a better way. That the blocks on flat is overkill (we're planning on engaging an engineer as we think some areas such as this and foundations may be overspecced- almost every builder who has looked at this has mentioned the foundations being OTT but we know we need an engineer to verify). He has suggested (and said he checked this with a Kingspan 'engineer') and a better method would be:
42.5 mm insulated warm boards (internal)
100mm block
140mm cavity (100mm PIR)
100mm block

He explained with 215 blockwork it doesn't hold the same thermal properties and you only get a return of high heat from the first 100mm masonry. This makes sense to me (we will need to check with an engineer) but my concern is the insulation is in two places. I don't really understand the technicalities around it but isn't best practice to ALWAYS keep your insulation in one place. I have done a search but would like to know what the reasons for this are and why PIR plus warmboard is not the best way anymore.

Who says they are OTT and more importantly, why isint the structural design in the tender pack?
The architect will have most likely marked some structural items just for clarity and pricing purposes, but you have no idea what the structure will be until the engineer comes on board.

Metric Tensor

What the Kingspan engineer is saying is that in his opinion (which may be correct) there is no additional thermal mass benefit from a 215mm block above a 100mm block - but then he goes and covers the 100mm block with an internal layer of insulation thus totally negating its thermal mass anyway!

Regardless of all of this no-one puts a 215mm inner leaf in a house for thermal mass reasons - they do it for structural reasons. You engineer will tell you if it's advisable/necessary when you take them onboard.

I have no idea how you managed to have a foundation spec without an engineer being involved!

MicktheMan

MizMix wrote: »

and a better method would be:
42.5 mm insulated warm boards (internal)
100mm block
140mm cavity (100mm PIR)
100mm block

Nooooooooooooooooo

MizMix

That was my reaction too...we had engaged an energy consultant early on (although a few years ago now) and one thing I remember as being non debatable was all your insulation in one place.

WE didn't engage an engineer as we assumed our architect could do the construction drawings fully- the only time an engineer was mentioned was if our walls went wider than 200 I think. We can see it was potentially a false economy as budget was tight (when we were quoted for construction drawings we assumed this included everything) I imagine the foundation spec is what the architect specs all the time. There's been too many questions raised when this went out to tender.

EWI isn't an option due to cladding on the outside of the first floor.

kieran.

MizMix wrote: »

EWI isn't an option due to cladding on the outside of the first floor.

If your doing cladding on the outside on the first floor is there an external leaf of block work or is it a rain screen arrangement? This would explain the 215 inner leaf possibly. If so could you loose the external block work leaf on the ground floor and reduce the foundation size.

Have you done site investigation yet, knowing were your building and my experience of the area there would be a very high chance you will need piles and ground beams.