External & Cavity Wall Insulation - can you use PIR with membranes?

ggpowell2

Hi I am renovating a house and have to remove retrofit insulation from the cavity as there is damp.   I am, also, upgrading the wall ties so was thinking I could put PIR in the cavity and since I want to clad with brick slips or render could put some external wall insulation (either EPS or PIR) on in order to achieve the required U value.   What I don’t understand is why some PIR boards are not identified as for cavity or external use when their descriptions are the same, other than a difference in price.   Would standard PIR boards be okay to use for the applications I have stated if I use membranes between each layer to protect against condensation e.g. over brickwork then over external insulation before cladding?

FreeBear

Now that the cavity is clear of insulation, I'd not recommend putting anything back in, and certainly not a foam - It would be a right pig to remove if you develop further damp problems.

PIR and EPS are pretty much moisture resistant by themselves, so shouldn't need a DPM. Watched a building not too far from here being clad last year. They used regular Celotex/Kingspan foil faced boards mechanically fixed to the outside of the walls then clad with timber on the upper half. The bottom half, EPS boards with a thin coat render over the top.

In terms of thermal performance, PIR is a little better than EPS. Regardless of which one you go for, use plenty of mechanical fixings, and pay attention to sealing around window/door openings. If you are rendering, use stainless steel or plastic beadings, not the zinc coated stuff - They will eventually rust and cause the render to fail.

Jeepers_Creepers

I'm struggling to understand how external insulation can work when you have a cavity. As in - I just truly don't know, so if someone could explain I'd be very grateful. Surely cavities are ventilated, so always draughty? So how can insulating the external skin help the inner house?

GG, how could you get PIR physically into the cavity? If you could, tho', and if it's clamped tight against the inner skin, leaving a cavity free behind it, then that is surely FAR better than insulating the outside? (It's what our new extension had - thermal blocks on the inside, with PIR clamped tight against it in the cavity (with plastic collars slid along the wall ties), so it must be to the latest standards.

Am I missing something? I can't see external insulation working to a significant degree unless it's a solid or single-skinned wall.

ggpowell2

Jeepers_Creepers said:

I'm struggling to understand how external insulation can work when you have a cavity. As in - I just truly don't know, so if someone could explain I'd be very grateful. Surely cavities are ventilated, so always draughty? So how can insulating the external skin help the inner house?

GG, how could you get PIR physically into the cavity? If you could, tho', and if it's clamped tight against the inner skin, leaving a cavity free behind it, then that is surely FAR better than insulating the outside? (It's what our new extension had - thermal blocks on the inside, with PIR clamped tight against it in the cavity (with plastic collars slid along the wall ties), so it must be to the latest standards.

Am I missing something? I can't see external insulation working to a significant degree unless it's a solid or single-skinned wall.

Because it is being renovated, the cavities are open around windows etc. so once the ties are removed (which will be done section by section) I can slide boards in, I have used a U-value calculator and doing this to my cavity alone is not enough, I need more either externally or internally and I don't want to lose room inside plus with cladding anyway it won't hurt me to put external on at the same time hence my thinking

ggpowell2

FreeBear said:

Now that the cavity is clear of insulation, I'd not recommend putting anything back in, and certainly not a foam - It would be a right pig to remove if you develop further damp problems.

PIR and EPS are pretty much moisture resistant by themselves, so shouldn't need a DPM. Watched a building not too far from here being clad last year. They used regular Celotex/Kingspan foil faced boards mechanically fixed to the outside of the walls then clad with timber on the upper half. The bottom half, EPS boards with a thin coat render over the top.

In terms of thermal performance, PIR is a little better than EPS. Regardless of which one you go for, use plenty of mechanical fixings, and pay attention to sealing around window/door openings. If you are rendering, use stainless steel or plastic beadings, not the zinc coated stuff - They will eventually rust and cause the render to fail.

Why not if the wall will be protected with external insulation and cladding to stop the cavity getting damp again? If I just do external it means a much greater thickness and then I worry about the weight of the cladding which is something else I cannot find good info on the internet about how to secure it if I went down that route...

stuart45

How wide is your cavity, and what size boards were you going to stick in.

FreeBear

ggpowell2 said: Because it is being renovated ...

I have used a U-value calculator and doing this to my cavity alone is not enough, I need more either externally or internally and I don't want to lose room inside plus with cladding anyway it won't hurt me to put external on at the same time hence my thinking

https://www.changeplan.co.uk/u_value_calculator.php

50mm of PU foam in the cavity will give you a corrected U-value of approx 0.35 assuming 100mm brick on inner & outer leaf. Put another 25mm of PIR on the outside, and you get down to 0.26 which under the target of 0.30 for refurbishments.

Without insulation in the cavity, you'd need 60mm on the exterior to get a U-value of 0.29. 60mm isn't a huge amount but you'd still need to extend the window sills and do some jiggling with downpipes (as you would with 25mm).

Or you could put 50mm in the cavity and use 38mm insulated plasterboard (25mm PIR bonded to 12.5mm PB) internally.

Up to you which route to take - Run the numbers by Building Control (assuming you are going for building regs sign off) and see what they say. You may be able to fudge the numbers by doing other thermal improvements if the pay back time for wall insulation exceeds 15 years.

oniongirl

stuart45 said:

How wide is your cavity, and what size boards were you going to stick in.

Steady 

Jeepers_Creepers

FreeBear said:

https://www.changeplan.co.uk/u_value_calculator.php

50mm of PU foam in the cavity will give you a corrected U-value of approx 0.35 assuming 100mm brick on inner & outer leaf. Put another 25mm of PIR on the outside, and you get down to 0.26 which under the target of 0.30 for refurbishments.

Without insulation in the cavity, you'd need 60mm on the exterior to get a U-value of 0.29. 60mm isn't a huge amount but you'd still need to extend the window sills and do some jiggling with downpipes (as you would with 25mm).

Or you could put 50mm in the cavity and use 38mm insulated plasterboard (25mm PIR bonded to 12.5mm PB) internally.

Up to you which route to take - Run the numbers by Building Control (assuming you are going for building regs sign off) and see what they say. You may be able to fudge the numbers by doing other thermal improvements if the pay back time for wall insulation exceeds 15 years.

Helpful post, FB.

Now't wrong with PIR in the cavity, but it should be clamped against the inner skin, and still leave a gap behind it tobesuretobesure.

"Or you could put 50mm in the cavity and use 38mm insulated plasterboard (25mm PIR bonded to 12.5mm PB) internally." to my mind the solution most guaranteed to have a significant and noticeable effect (since you can't have it all on the inside). 35mm internal insulation (if 9.5mm board) is not a lot of space loss for it's gain. 

I'm still struggling with the guaranteed benefits of external insulation - how can a draughty cavity (I know, I know) not make its effectiveness a bit of a lottery?

stuart45

There are full fill PIR boards available now, but you can't use the standard ones for full fill.

ggpowell2

stuart45 said:

There are full fill PIR boards available now, but you can't use the standard ones for full fill.

This was part of my original question, why not if there is going to be membrane and cladding on the outside to protect it from damp?