Green Building Forum - 50mm cavity wall - would adding 50mm EWI be worth it?

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Tue, 28 Mar 2023 13:48:35 +0100

My house has 50mm empty cavities, between brick leaves, the outer of which is pebbledash (1920's build). I am thinking of filling the cavities with bonded eps beads. I would like to get rid of/cover the pebbledash, as well as improve insulation if possible.

Would it be worth adding 50mm EPS EWI (ie get a substantial insulation improvement)?

I realise the pebbledash would need smoothing for good EWI attachment. More than 50mm EWI would be difficult for various reason.

Thanks

50mm cavity wall - would adding 50mm EWI be worth it?

Peter_in_Hungary — Tue, 28 Mar 2023 18:33:39 +0100

Adding EWI on to a cavity wall only works if the cavity is filled fully and without any air flow through it. Not that easy to achieve.

Adding 50mm of EPS EWI will improves your wall from a u value of 0.5 to 0.3 Putting on 100mm EWI improves it to 0.21 Guesstimate based on a brick both sides cavity wall with 50mm EPS inside. Your original wall structure may differ from my assumption.

The difference in cost between 50mm and 100mm EWI is just the extra 50mm of EPS, other costs remain the same.

You can play around with different insulation options at
https://www.ubakus.de/en/r-value-calculator/?
Use the demo version.

You can put EWI on to pebbledash without a 'smoothing coat' if you attach the EPS with a fillet of adhesive all the way around the edge of the EPS sheet and 4 dollops in the centre. The EPS sheet is then pushed on to the wall firmly to seat the adhesive. This method uses a bit more adhesive but saves the cost of a smoothing render coat. Of course the pebbledash needs to be in good condition to take the EWI with no hollow patches. (if there are a few hollow patches then knock off the pebbledash and attach the EPS as above with perhaps a thicker fillet to account for no pebbledash)

To get a dust free pebbledash surface for EWI a pressure washer is a good option.

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Wed, 29 Mar 2023 09:13:06 +0100

Thanks Peter.
Yes I appreciate 100mm ewi would be better than 50 (and 150mm better still) but there are some practicalities that may not allow it.

50mm cavity wall - would adding 50mm EWI be worth it?

Jeff B — Wed, 29 Mar 2023 18:20:19 +0100

Osprey - I too considered EWI on our cavity walls but for the same reason Peter gave above, i.e. air movement within the cavity, I decided against it. It is one of those imponderables, very difficult to prove or disprove.

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Wed, 29 Mar 2023 19:00:02 +0100

Jeff
I am pretty sure bonded eps beads injected into the cavity would pretty much eliminate air movement there, or am I missing something?

50mm cavity wall - would adding 50mm EWI be worth it?

Peter_in_Hungary — Wed, 29 Mar 2023 20:19:45 +0100

Posted By: OspreyJeff
I am pretty sure bonded eps beads injected into the cavity would pretty much eliminate air movement there, or am I missing something?

Depends totally upon the workmanship of the person doing the injection and unfortunately there is no easy way to check the results of the work.

50mm cavity wall - would adding 50mm EWI be worth it?

Jeff B — Wed, 29 Mar 2023 23:04:54 +0100

Posted By: OspreyJeff
I am pretty sure bonded eps beads injected into the cavity would pretty much eliminate air movement there, or am I missing something?

Sorry I missed reading the "bonded" bit! Maybe it'd be ok, I wouldn't like to say definitely. I must admit we have Rockwool fibres in our cavity walls so the risk of air movement maybe quite high. The only way to be 100% sure would be to seal the tops of the cavity which in our case was not possible access-wise.

50mm cavity wall - would adding 50mm EWI be worth it?

fostertom — Thu, 30 Mar 2023 08:01:47 +0100

Posted By: OspreyI appreciate 100mm ewi would be better than 50 (and 150mm better still) but there are some practicalities that may not allow it.

Can you outline those practicalities? might make a difference. On a project with 200EPS EWI on cavity walls with unknown-quality previous cavity fill, it's made a huge difference, as you'd hope with 200 EWI. No attempt to seal top of cavity wall.

50mm cavity wall - would adding 50mm EWI be worth it?

RobL — Thu, 30 Mar 2023 10:39:19 +0100

What about foam filling a cavity - it would be a good insulation technique to also stop convection.

We have pro fitted fluff filled narrow cavity walls, and 150mm diy EWI. It has been a battle to stop air leakage via the cavity - there were no cavity closers at the top, lots of holes all over - massive ones hidden by PVC T&G panelling. Big holes you could fit your hand into between floors! If you can't see it, it will be wrong...

50mm cavity wall - would adding 50mm EWI be worth it?

fostertom — Thu, 30 Mar 2023 14:06:22 +0100

Sure, unfilled cavities allow lateral convection from warmer spots on the cavity face of the inner skin to cooler spots on the cavity face of the outer skin (aka bypass), but do they really introduce outside air to inboard of the EWI? Where does the outside air come in?

Yes, you can't rely on rendered EWI as 'airtight layer' but it's still quite good at it. If that EWI is applied on extg render, then that's also quite good at it, as long as you fill render cracks before EWIing. Ditto internal plaster, as long as you deal with gaps in it - inter-floors, elect boxes etc.

Can add up to a fair wind-barrier, even if there's no formal 'airtight layer'. So where's the big 'draw' of outside air?

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 30 Mar 2023 17:05:39 +0100

Posted By: fostertom
Posted By: OspreyI appreciate 100mm ewi would be better than 50 (and 150mm better still) but there are some practicalities that may not allow it.
Can you outline those practicalities? might make a difference. On a project with 200EPS EWI on cavity walls with unknown-quality previous cavity fill, it's made a huge difference, as you'd hope with 200 EWI. No attempt to seal top of cavity wall.

Narrow driveway is one thing, if existing drainpipes were moved out to accommodate EWI. I guess EWI could be fitted around drain/sewage pipes without losing TOO much insulation (as wall cavities would be filled) but may look a bit odd.

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 30 Mar 2023 17:08:28 +0100

Posted By: fostertomSure, unfilled cavities allow lateral convection from warmer spots on the cavity face of the inner skin to cooler spots on the cavity face of the outer skin (aka bypass), but do they really introduce outside air to inboard of the EWI? Where does the outside air come in?

My cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 30 Mar 2023 17:11:00 +0100

Posted By: Jeff B

Sorry I missed reading the "bonded" bit! Maybe it'd be ok, I wouldn't like to say definitely. I must admit we have Rockwool fibres in our cavity walls so the risk of air movement maybe quite high. The only way to be 100% sure would be to seal the tops of the cavity which in our case was not possible access-wise.

My house cavity tops are accessible from the loft. If fact they provide the sole loft ventilation, which will have to be addressed when I do the walls..

50mm cavity wall - would adding 50mm EWI be worth it?

Peter_in_Hungary — Thu, 30 Mar 2023 18:20:17 +0100

Posted By: OspreyMy cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

Posted By: OspreyMy house cavity tops are accessible from the loft. If fact they provide the sole loft ventilation, which will have to be addressed when I do the walls..

How wonderful - outside air flowing through the cavity to ventilate your loft with air that will have been warmed by the inner leaf of the cavity wall-----------Hmmm

BTW if you select the Html button at the bottom of the comments box then quotes come out in blue, which makes the post easier to read.

50mm cavity wall - would adding 50mm EWI be worth it?

fostertom — Thu, 30 Mar 2023 18:31:47 +0100

Posted By: OspreyMy cavities are vented to the underfloor air bricks

but I guess you're looking at insulating, even full-filling your u/floor, as per much discussion on here? Then those air bricks can be sealed.

Posted By: Ospreyand and also have a couple of their own airbricks

Bung them up too - the EWI will do that anyway.

Posted By: OspreyI guess EWI could be fitted around drain/sewage pipes without losing TOO much insulation (as wall cavities would be filled) but may look a bit odd

Should be possible - tragic to half-measure your EWI investment just for a want of a bit of creative design!

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 30 Mar 2023 21:22:37 +0100

Posted By: Peter_in_Hungary
Posted By: OspreyMy cavities are vented to the underfloor air bricks and and also have a couplemof their own airbricks.

Posted By: OspreyMy house cavity tops are accessible from the loft. If fact they provide the sole loft ventilation, which will have to be addressed when I do the walls..

How wonderful - outside air flowing through the cavity to ventilate your loft with air that will have been warmed by the inner leaf of the cavity wall-----------Hmmm

BTW if you select the Html button at the bottom of the comments box then quotes come out in blue, which makes the post easier to read.

Agreed, but loft, and everywhere else is very dry, and in good condition for 100year old house. Not the warmest though.
(Thanks for html hint)

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 30 Mar 2023 21:25:53 +0100

Posted By: fostertom
Posted By: OspreyMy cavities are vented to the underfloor air bricks
but I guess you're looking at insulating, even full-filling your u/floor, as per much discussion on here? Then those air bricks can be sealed.
Posted By: Ospreyand and also have a couple of their own airbricks
Bung them up too - the EWI will do that anyway.
Posted By: OspreyI guess EWI could be fitted around drain/sewage pipes without losing TOO much insulation (as wall cavities would be filled) but may look a bit odd
Should be possible - tragic to half-measure your EWI investment just for a want of a bit of creative design!

Re airbricks - yes just stating current arrangement. Proceeding with under floor insulation soon, as part of which will be isolating airbricks from cavity. Still needed for underfloor, imo.

50mm cavity wall - would adding 50mm EWI be worth it?

fostertom — Fri, 31 Mar 2023 09:11:09 +0100

Posted By: OspreyStill needed for underfloor, imo

AFAIK, the consensus here was no, an old myth, not needed, provided any water entering from outside has been fixed.

Do these comments help allay worries of outside air blowing around in the cavity? What you say, Peter, Rob and Jeff?

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Fri, 31 Mar 2023 11:59:48 +0100

Posted By: fostertom
Posted By: OspreyStill needed for underfloor, imo
AFAIK, the consensus here was no, an old myth, not needed, provided any water entering from outside has been fixed.

Do these comments help allay worries of outside air blowing around in the cavity? What you say, Peter, Rob and Jeff?

I have seen a discussion whereby, if the underfloor void is filled with insulation, no ventilation is required. However, with my setup, the joist 'bottoms' would be exposed.

50mm cavity wall - would adding 50mm EWI be worth it?

wookey — Fri, 26 May 2023 15:16:02 +0100

I too have EWI (160-200mm woodfibre) outside a 55mm cavity, filled with blown fibreglass. It works very well indeed, but like RobL (very similar house) I've spent a long time finding and bunging up holes into the cavity, particularly between floors (lots of bricks missing for wiring and I taped round all the joists), and around windows. The top of the cavity has been sealed on half the house, but not the other side (yet). That was done later, when scaffolding was available.

So yes, as others have said, 50mm EWI is a very good idea, but only if you also fill the cavity. Filling it in more airtight ways will likely work better. And sealing off as many holes as you can find will also improve it. All of this is quite hard to quantify (i.e just filling the cavity will probably allow the EWI to work 'adequately', but the more hole-bunging the better, espescially the bottom-vents as those will generate significant stack ventilation).

In general the continuous EWI layer will be better insulation than the cavity layer as that often ends up with gaps and is bridged by wall ties and cement spillage and usually about 300mm of crap at the bottom. So I'd expect it to be _more_ than twice as good as just filling the cavity. You could also use PIR insulation for better U-values, given that your thickness is restricted to 50mm. There is a system for that (weber maybe?)

I would strongly advise spending the extra time/money to move your drainage goods outside the EWI so as to have a continuous layer, and it just looks way better. Yes, to some degree the fact that you also have cavity insulation means you don't _need_ to do this from a thermal point of view, but it still looks cheap if the insulation just stops around plumbing goods, and it makes rendering difficult and is likely to let water in.

50mm cavity wall - would adding 50mm EWI be worth it?

tony — Sat, 27 May 2023 08:35:37 +0100

For me 50mm of wall insulation is insufficient - hundreds of millimetres are needed

50mm cavity wall - would adding 50mm EWI be worth it?

wookey — Tue, 30 May 2023 15:17:25 +0100

Agreed. But 50mm EWI plus 50mm of cavity is 'not terrible'. U=0.27

One approach where 'narrow driveway/passage' is the problem is to step it out above head height so the mediocre area is minimised. And the option remains for some IWI too one day to compensate.

50mm cavity wall - would adding 50mm EWI be worth it?

tony — Wed, 31 May 2023 17:56:20 +0100

Would it pass building regs?

50mm cavity wall - would adding 50mm EWI be worth it?

dathi — Thu, 01 Jun 2023 08:35:10 +0100

have any of you used kingspan k5 phenolic boards their Thermal Conductivity: 0.021W/mK

50mm cavity wall - would adding 50mm EWI be worth it?

Peter_in_Hungary — Thu, 01 Jun 2023 08:47:58 +0100

Posted By: tonyWould it pass building regs?

Is there still the get-out clause where insulation limits can be ignored if it is not practical or too expensive (pay back within a certain time) to fit?

50mm cavity wall - would adding 50mm EWI be worth it?

WillInAberdeen — Thu, 01 Jun 2023 11:20:45 +0100

The proposal would comply with the building regulations limits for renovated walls (U=0.3), no problems there.

There is a further get-out that insulation should be "technically and functionally feasible and have a simple payback no more than 15 years", in which case U= 0.7 would also be acceptable.

50mm cavity wall - would adding 50mm EWI be worth it?

Osprey — Thu, 15 Jun 2023 08:55:42 +0100

Thanks for more recent replies to my OP.
I suppose I could have 50mm ewi under the pipework (in addition to cwi), and 100mm elsewhere: may be impractical though.
Stepping the sewer pipe out above charge height might work, and notmlook TOO bad. Will investigate.
(I found one website where the manufacturer (from diagrams anyway) had come up with a 3 sided profile to handle sewer pipes, with high performance insulation behind the pipe. Website seems defunct, and I can't find it now)

Edit: found it! It is called Thermo-pro from Enviroform