Green Building Forum - Insulating render for a filled cavity wall

Insulating render for a filled cavity wall

Jenna — Thu, 20 May 2021 18:33:20 +0100

Does anyone have experience of using insulating render on a 1950s house? I'm aware of products such as diathonite and bauwer for use on solid wall properties but I was wondering if another product exists which is suitable for use on a cavity wall which doesn't need to be breathable (obviously as long as it's properly installed).

The property is semi detached and has a narrow overhang so the cost of adding EWI is likely to be rather expensive and arguably not worth the cost in terms of the potential u-value improvement.

Thanks all.

Insulating render for a filled cavity wall

Nick Parsons — Fri, 21 May 2021 07:21:54 +0100

Is the cavity insulated? I am not sure that sheet EWI would come more expensive than Diathonite, from what I hear, but I have never used Diathonite myself.

If the cavity is insulated then I'd have thought that condensation might occur on the inner face of the outer skin (a Condensation Risk Assessment will tell you better), making breathability desirable in the outer skin.

I think Ty Mawr do an insulating plaster, but I don't know if it is suitable for external use. Haven't got time to search this morning, sorry!

Insulating render for a filled cavity wall

Peter_in_Hungary — Sat, 22 May 2021 20:16:58 +0100

Jenna - If you have CWI then first make sure that the CWI is done properly. - first it won't work if it is full of gaps and wind and second if it is not done properly it will negate any further insulation put on outboard if it.

IMO if you are going to re-render any way then cost out the benefits of the various options of ordinary render, insulating render and EWI - even 50 mm EWI makes a good difference. If you are not going to re-render then the cost/benefit of any sort of added insulation on the outside is probably marginal.

Insulating render for a filled cavity wall

Jenna — Mon, 24 May 2021 14:41:19 +0100

Thanks both. Yes the cavity is already insulated but I don't know how well - that's a good point.
The external walls need to be re rendered anyway so yes I think it would be a good idea to compare costs.

Insulating render for a filled cavity wall

Dickie_in_Yorkshire — Sun, 06 Jun 2021 13:36:43 +0100

How do you check if your CWI has been done properly? I also have a 1950s pebble-dashed semi-detached with very limited overhang on the gable end (I guess <10mm). The pebbledash has come away on the other walls due to overflowing gutters (which need to be fixed before re-rendering), so we know we will have to render - we've been thinking about EWI but omitting the gable end (would this risk condensation on that wall?).

The roof joists rest on the outer leaf of the wall at ceiling joist level, so I can't peek down the cavity. I think the CWI was done with little white polystyrene balls maybe 15-20 years ago, and the extension (including the gable) was done with fluffy rectangular stuff in 2012. I guess it's the little white balls I need to check, and I guess they don't go all the way to the top of the cavity.

I'm making assumptions due to the condition of the building when we moved in - such as the wiring that ran diagonally across the chimney breast between sockets and wall lights!

Insulating render for a filled cavity wall

jms452 — Sun, 06 Jun 2021 15:01:12 +0100

Posted By: Dickie_in_YorkshireHow do you check if your CWI has been done properly?

Probably the easiest answer to your question is 'hire a thermal camera when it's cold' and do a survey first thing in the morning.

Poly balls are one of the better CWI materials as they flow pretty well and are not as susceptible to slump. The quality of the cavity is the big variable.

Insulating render for a filled cavity wall

Peter_in_Hungary — Sun, 06 Jun 2021 21:19:52 +0100

Posted By: jms452Probably the easiest answer to your question is 'hire a thermal camera when it's cold' and do a survey first thing in the morning.

+1
but crank the heating up for a couple of hours first