Cavity wall insulation new extension

[iwanttodiy]

Hi

We are getting an extension built and the builders have specced 100mm insulation. I originally thought it was going to be 100mm PIR Insulation, however seems it’s going to be a 100mm cavity slab.

I understand that PIR has higher performance rating than slab. However I believe if it was PIR, this reduced insulation value as we would have to do 50mm and leave 50mm air gap.

What is the general views about what is better?

 

[Notch7]

100mm full fill rockwall insulation batts with lightwieght internal blockwork will comply with part L -assuming you are building with elemental values.

PIR insulation is only used as part fill in cavities, although there is a special PIR board available for full fill now.

 

[iwanttodiy]

Yeh, I was hoping for the builder to use fullfill 100mm boards. However they are like 5x the price of the cavity slabs.

I was hoping to go for high insulation values, but is the additional cost worth it?

 

[^woody^]

It depends on how good your builders are.

Weren't you told about PIR requiring cavities in your first thread?

 

[freddiemercurystwin]

Why yes he was woods.

 

[iwanttodiy]

Yes, but this is the fullfill cavity wall insulation which doesn’t need a cavity? It has an outlayer which prevents moisture.

Considering they specc’ed it and decided to use slabs instead suggests to me they probably don’t know how to install the high spec stuff.

 

[iwanttodiy]

Here is the product spec - https://www.travisperkins.co.uk/Xtratherm-Cavitytherm-Full-Fill-1200mm-x-450mm-x-100mm/p/277970

If I was diy-ing, I’d probably try out the fullfill stuff, as I’ve already got the builders in, i will just leave them to do what they are used to.

Reduces any mistakes and would be faster. As it’s only an extension, to a house with solid brick walls, with this, it would be better insulated than my whole house

 

[stuart45]

Those boards are great for random stonework, but expensive and a pain for brickwork.

 

[iwanttodiy]

Thanks all, I do get the feeling that for a smallish extension that insulation is insulation and probably don’t need to go over the top with expensive products. So long as the insulation is relevant and appropriately installed.

 

[garyo]

My last house had a 2-storey extension with 80mm full-fill rockwool, thermalite block, and dot-and-dab (i.e. nothing fancy) and that end of the house was like a sauna compared to the original house (1950's, 2 skins of red brick with 50mm cavity full of retro-fit polystyrene balls).

 

[iwanttodiy]

That is good to know.

That gives me more confidence that my extension doesn’t need high spec insulation to keep in heat

 

[John D v2.0]

My last house had a 2-storey extension with 80mm full-fill rockwool, thermalite block, and dot-and-dab (i.e. nothing fancy) and that end of the house was like a sauna compared to the original house (1950's, 2 skins of red brick with 50mm cavity full of retro-fit polystyrene balls).

Sounds like the person who specified the heating in the new part didn't do it right in that case. A well insulated house should have more stable temperatures, unless of course it's got a lot of glass.
A green house is always boiling even with single glazing.

 

[^woody^]

My last house had a 2-storey extension with 80mm full-fill rockwool, thermalite block, and dot-and-dab (i.e. nothing fancy) and that end of the house was like a sauna compared to the original house (1950's, 2 skins of red brick with 50mm cavity full of retro-fit polystyrene balls).

You should have fitted a better fan.