Choices for cavity wall insulation: Rock wool or PIR boards

PIR e.g. Celotex or Kingspan.

Is Rockwall preferred over PIR panels like Celotex? Based on the thickness required and it seems 65mm of rockwool isn´t thick enough to insulate sufficiently, what mm of Celotex / Kingspan / Xtratherm is required?

I´ve heard 40mm is thick enough (according to noseall )

40mm is about right, may need it a bit thicker, depends on the blocks/bricks spec, new build or extension.

Rockwool is a quilt insulation designed for full fill installations (as opposed to partial fill like Celotex or Kingspan) so can be used but you will need to be thicker than 65 more like 90mm or thereabouts. Again depends on the same things as mentioned. Its a cheaper insulation mm for mm but as you need a thicker layer will prob work out similar price.

ok, I would think then, as costs overall are similar, The PIR sound a better bet as they mean the cavity can be thinner and I think its sounds easier to work with.
Didn´t realise the blocks/brick spec would be so important , I´m guessing its a width thing, the thinner the layer, the tihcker the insulation should be.

No the cavity is unlikely to vary much, if you have partial fill rigid insulation it needs an air gap between it and the outer skin of blockwork. If you have full fill you generally use a lower spec insulation that needs to be thicker hence cavity will be similar whichever you opt for. Price is difficult to work out very accurately. Regards the type of blocks used its not so much the thickness of the block but its 'thermal resistance' that matters. Generally a dense block has a lower thermal resistance than a lightweight block as it has more air inside it which helps it retain heat more efficiently hence such a block wall requires (slightly) thinner insulation to get the overall wall up to spec. There are dozens of different blocks to use. Do you have a specific question if so you'll need to answer the queries in my first post.

Hi Freddy,

I don´t know the brick/block specs yet. I may be able to find out next month (when I move in , . Once I have them I can go from there. There was an extension prior to the current owner and it needs a second skin plus insulation. So at least I would know what bricks I´d be putting up.

I am unsure how easy it is to lay bricks, in terms of the foundation I should be putting down...

So its single skin at the moment then. You'll need to do some digging around once you move in then to find out what foundations are and beef it up if neccessary. Or do your internal skin in timber instead.

Ok cool, the surveyor suggested more brick for building regs as the previous owner put on this extension with single brick which the surveyor said was of outhouse style construction, then the current owner, the one who I´ve bought from has knocked through/out the original exterior wall.

Timber would be cheaper and something I believe I could manage easier than brick. I presume plywood then as the internal skin with either rockwool then plasterboard or PIR material, e.g. Kingspan with plasterboard bonded onto the back of it.

The roof is flatop and is 10 years old. The front porch is also flattop and is 15 years old so should be replaced with a pitched one within the year, could potentially do the extension roof at the same time. Whether timber or brick I wouldn´t think it should affect putting a new roof on

I´d like to put a cloakroom in this extension (it would be next to the end of the kitchen (not adjoining), and I thought it would be ideal as it would be near the water supply. Alternatively have it the other side of the extension which is closer to the bathroom above and to the side.

My responses regards 40mm pur insulation (previous post) were concerning the interior lining of a solid 9" wall and not to do with cavity construction.

It seems we may have our wires crossed.

noseall said:

My responses regards 40mm pur insulation (previous post) were concerning the interior lining of a solid 9" wall and not to do with cavity construction.

It seems we may have our wires crossed.

Click to expand...

Ah, I see

So what thickness would you suggest