Top tips for insulating extension on a budget

Hello, I am due to have an extension in the spring, a single story kitchen diner at the back of my bungalow 1920s bungalow and a loft conversion. I am having a meeting with my builder and architect on Wednesday morning.

The builder is a lovely guy and very experienced but I know he is going to want to use traditional materials. I am on a tight budget, but would like to be as environmentally friendly as I can afford to be. I know I can’t afford an air source heat pump, but I imagine there will be some things I could do.

I would love your top tips on for what to push for, and where to buy from.

Oh and I think I once saw part of a video on environmentally friendly render that could go on the outside of the walls, did I dream that?

many thanks

Helen in Bingley

Depending on the shape of your existing bungalow and the proposed overall shape you may be able to improve your form factor. The lower the form factor the easier it will be to achieve energy efficiency. Unfortunately bungalows do suffer from poor form factor.

The energy ideal for an extension is that is more thermally efficient than the wall it replaces, so that your energy usage goes down with the bigger floor area and not up. The likelihood of that happening is low, but it is a target. Build the extension to as near to Passivhaus standard as you can. You can still use traditional materials to achieve that but with great care.

Lime can be applied outside as a render or inside as plaster (or in a damp kitchen or bathroom, as an internal render) there is an insulated version containing cork granules and called Diathonite. Diathonite is a certified Passivhaus component. As Diathonite is pre coloured you can omit the wall decoration stage. Fresh plaster or render shouldn’t be decorated for a while anyway. If insulating the outsides of the walls remember to fit soffit boards after insulating and not before, allowing the insulation to reach your roof insulation.

For an explanation of Form Factor see

Thank you so much Tim. Very useful indeed.

There are a few different versions of Diathonite, what are the differences between them ?

Many thanks


The latest one, Thermactive, has an improved U value and is for internal use only. The older version, Evolution, is for either, or both, side(s) of the wall. Both can be used for ceilings. Deumix is the same as Evolution with an additional salt inhibitor and is presumably for retrofitting damp premises.

There is also a Diathonite screed. Presumably not for use over under floor heating. I hadn’t realised they did screed until I read the Ecological Building Systems web page just now.

Finally there is Acoustix, which is formulated as a sound absorber rather than thermal insulation.

Brilliant thanks Tim


@Helen_Stone, any update? I hope your architect understood the Form Factor issue. That is a major point to optimise.