I’m looking at an interwar semi where the facade needs re-rendering. It seems that this is an opportunity to render with an insulating render recognising that this will contribute only a minimal amount in terms of insulating the walls. Do members have any experience of using such products? In theory as an insulation measure zero rate VAT might be applicable.
I have been looking at Insurend as I believe they got a mention through People Powered Retrofit. Does anyone have any experience of using them for this type of measure?
After a day, no response. Maybe I should be more patient but I will try adding something.
The insulated render used at the top end of the retrofit industry is Diathonite, which is cork and lime based. I doubt that Insurend are using that or they would be keen to give it prominence.
I checked the Insurend web site and found nothing about the render they use, so here are some questions to ask.
Is the render airtight? Render often acquires hairline cracks that allow air to circulate through and behind it, causing a chill effect. Generally 2 cm of new render can be considered airtight but it does no harm to ask.
Is the render vapour open/vapour permeable? It should be.
What is the U value of the render at normal (2 or 3 cm) thickness?
What is the U value at 4 or 5 cm thickness? You could look into thicker than normal render to reduce the combined U value of the walls.
Can the render be applied below the damp proof course and/or below ground level? You want to avoid cold bridges and ideally want to go down to the footings.
Do they only render to the soffit or do they go up to the top of the final course of bricks? Normally render stops at the soffit but if insulated that would cause a cold bridge at top floor ceiling height, making the corners of the ceilings liable to get black mould.
What is the insulating material in the render? You may have preferences for natural low carbon material if there is a choice.
Remember that the render will almost certainly be damaged if the windows or doors are replaced, so you may want to do them first.
Reminder: low U value is good, high U value is bad.