External finishes, insulation and vapour/condensation

[Anonymoose]

Hi all

 

I’m looking for some advice/reassurance with respect to my renovation/extension.

 

The property is a c. 1920 cottage of cavity construction (bricks and 75mm cavity) with peddledash render. We have removed an extension that was added in 1970 and are re-building/improving it. The original cottage has been taken back to bare brick. There is a slate dpc which looks to be in good condition. Cavity seems pretty clear where inspectable, and galvanised wallties are in good shape. We will add new neoprene wall ties around new openings, and generally to improve.

 

For the original cottage I would like to install a EWI + brick slip system on the ground floor, and then timber clad the first floor. This will visually match the new extension. The build-up for each will be as follows:

 

Existing GF:

25mm brick slip

50mm EWI (XPS with brick slip guide lines)

50mm original cement based pebbledash render

100mm original brick with lime mortar

75mm cavity (to be filled?)

100mm original brick

25mm PIR, foil taped to form vapour line

25mm Batten for service zone

12.5mm plasterboard

3mm skim

 

Existing FF:

25mm timber cladding tbc

25mm horizontal batten

50mm EWI?

50mm original cement based pebbledash render

100mm original brick with lime mortar

75mm cavity (to be filled?)

100mm original brick

25mm PIR, foil taped to form vapour line

25mm Batten for service zone

12.5mm plasterboard

3mm skim

 

I have three questions:

1. Is it worth continuing the XPS insulation on the timber clad first floor? If so do I need any ventilation between timber and XPS?

2. Should I have any concerns about filling the cavity wall? We are in South Wiltshire, the filled walls will be east, south and west facing. We’re not particularly exposed but on the side of a hill so the wind does whip through on occasion.

3. How low should the XPS and brick slips be installed, all the way down to DPC, or all the way to the ground/underground? (DPC is 150mm above ground level - just thinking about any rising damp concerns - but XPS does not absorb water, right?)

4. Maybe determined by the answers for one and two, but I’ll ask anyway, should the cavity be considered as a completely sealed volume, or does it need to “breathe”? I think sealed, but want to check.

 

Many thanks in advance, sorry for the long post

 

[Redbeard]

- Why are you proposing XPS, which is completely impermeable, rather than EPS which has a degree of permeability?

- Definitely fill the cavity (suggest EPS bonded beads, NOT Rockwool). Yes, insulation should not be used as a draught-blocker, but if the alternative is a potential through-draught and complete thermal by-pass (EWI left 'out in the cold') then, unless you can absolutely guarantee no air movement, insulkate the cavity (and of coiurse you get the R value of 75mm EPS anyway. 50mm EWI is not that exciting. Is that limited by roof oversail?

 

[MikeGrahamT21]

Would definitely not recommend XPS boards and then PIR on inside, you’ll be trapping moisture. EWI is generally EPS above DPC and XPS below DPC and on an old building it’s recommended to carry this all the way down to the foundation.

 

have a look at my blog which may well give you some ideas

 

 

[Anonymoose]

Thanks both

 

the 50mm xps idea was actually a brick slip tracking system (see https://www.brickslip.co/product/xps-brick-slip-backing-board/). Thats where 50mm comes from. We are replacing the roof and all windows/doors so could go for more if it was sensible.

 

I now appreciate that EPS is a preferred material for permeability. I wonder why those slip panels come as XPS, a quick good for EPS alternatives hasn’t yielded much. Maybe 10mm spacers will do the job albeit slightly slower.

 

Will check out your blog Mike, thanks

[MikeGrahamT21]

23 hours ago, Anonymoose said:

Thanks both

 

the 50mm xps idea was actually a brick slip tracking system (see https://www.brickslip.co/product/xps-brick-slip-backing-board/). Thats where 50mm comes from. We are replacing the roof and all windows/doors so could go for more if it was sensible.

 

I now appreciate that EPS is a preferred material for permeability. I wonder why those slip panels come as XPS, a quick good for EPS alternatives hasn’t yielded much. Maybe 10mm spacers will do the job albeit slightly slower.

 

Will check out your blog Mike, thanks

if you’re wanting to use real brick slips I.e heavy ones as opposed to the ones I used which are resin, I have seen a system which used EPS insulation, and then a metal mesh which had spacers on for brick slips.

 

XPS isn’t an issue, it’s just more expensive for same thickness, and you have to ensure vapour permeability on the internal side, which your initial build up didn’t have. Consider wood fibre boards to the interior and lime plaster if you wish to keep the XPS outer

[Anonymoose]

I like the idea of vapour line being internal as it will make it easier to tie into the extension and keep one very well sealed envelope.

 

So that leads me to EPS on the outside. The EPS brick slip system I have seen has 20/50/100mm insulation for £36/65/96 per square metre (incl adhesive) or I can just buy plain EPS and use spacers which would be much cheaper 100mm at £10 per sqm.

 

the EPS brick slip system is euro brick V/X clad, they recommend max slip depth of 25mm.

[MikeGrahamT21]

3 hours ago, Anonymoose said:

I like the idea of vapour line being internal as it will make it easier to tie into the extension and keep one very well sealed envelope.

 

So that leads me to EPS on the outside. The EPS brick slip system I have seen has 20/50/100mm insulation for £36/65/96 per square metre (incl adhesive) or I can just buy plain EPS and use spacers which would be much cheaper 100mm at £10 per sqm.

 

the EPS brick slip system is euro brick V/X clad, they recommend max slip depth of 25mm.

 

I know which one i'd be chosing!!

 

This is the metal mesh one i was talking about: https://www.wbs-ltd.co.uk/systems-solutions/system-finishes/15mm-brick-slips/

 

And for supplier for the normal stuff, i've always used these: https://ewistore.co.uk/

[Gone West]

55 minutes ago, MikeGrahamT21 said:

This is the metal mesh one i was talking about: https://www.wbs-ltd.co.uk/systems-solutions/system-finishes/15mm-brick-slips/

I used that system for the plinth on my last house. It was really straightforward to install.