Home Forums General Discussion Curtains – Thermal performance

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    Miranda Struthers

    Hi All

    I’m looking for some definitive advice about curtain performance on a couple of points.

    1. Can anyone advise if “thermal” (rubber-backed) curtains have any significantly greater thermal performance than single layer non-thermal- though I know it does depend on fabric thickness and weave also. AM I correct in thinking that the rubberised backing creates a barrier so the air cannot easily pass through the fabric?

    2. How should I best describe the thermal properties of lined curtains? My understanding from HPA training is that air having to pass through 4 surfaces (curtain + lining) creates thermal resistance. Others I know speak of air pocket between the two layers being trapped…..what is the best way to describe in laypersons terms?

    Any advice gratefully received, many thanks.


    Ian Mayes

    Hi Miranda,
    “thermal backing” , doesn’t add anything thermally to a single layer of material, as long as that material had a close weave originally. Thickness is not that relevant, unless you got to something like velvet , that has air held ” in it”. I guess it would slow down air movement, but if it is air flowing through your curtains that is causing your heat loss, then you have a great big draught problem that curtains won’t help.
    It is not about air passing through the curtain, but heat energy passing through. The layers of air that are stuck to both sides of the material slow down the movement of heat. And the extra pocket of air you get when there are 2 layers of material helps in the same way. The same way that all bulk insulation works, lots of pockets of air. And that’s how I’d describe it to the client. The more air pockets, the better. So really the curtain is just a surface for air to stick too and when you add layers , you add pockets of air.
    Hope that helps.

    Miranda Struthers

    Many thanks Ian,

    I know it’s not an exact science so your description is really helpful to explain to clients. I’ve also had a conversation with techies at BRANZ and am looking at getting some curtain testing done to see if we can get a better scientific understanding of the thermal performance of lined (double-layered) vs single layered curtains, floorlength vs short, pelmets vs not etc. Will keep you all posted as that project develops but any ideas you (or other HPAs) want to feed in for a testing scenario would be great, thanks.



    I and I am sure others would be really interested in the findings if you do manage to get BRANZ to do some testing.


    Have you seen the consumer work?


    Fred Braxton

    Thanks Alex. I was thrilled to read the Consumer article. It’s the best summary of this issue that I’ve read. It should be required reading for all EDAs and HPAs: in particular the summary “Tips” at the end.

    The curtain industry would love everyone to think that effective window treatments are all about the material, and the more expensive the better. But The Fit Is The Thing. One of the highest-performing scenarios is a net curtain close to the glass. Film secondary glazing is also highly effective. Both are within the financial reach of most people. I’d be very surprised if BRANZ research shows any different result.

    But none of this will count for much without dealing to another common elephant in the room: opening timber-framed windows nearly always have air leaks due to warping. This is usually obvious only on windy days. On calm days you can often find these gaps by listening carefully for the places where outside noise is more obvious. V-seal is usually a good solution.


    Wow, this was surprising reading, netts & fans can be benifical. Who would have thought.

    That gives another thumbs up for heat pumps as they have a fan, but I wonder how significant the placement would be.

    Problem with netts is householders don’t open them during the day to allow good radiant heat in. Mainly due to the fact they are on plastic coated wire instead of free flowing rails.

    Tim Bishop

    That consumer article is great! But why don’t they present the results in units that mean something, like R-value or U-value?

    Back when I was doing my MSc at Otago, I experimentally determined the in situ benefits of curtains and film secondary glazing, and estimated costs. How do the costs stand up today?

    The approximate R-value results are:

    Timber framed window, single glazed (baseline)
    Timber frame, single glazed, with drapes and pelmet
    R0.3 $0-$150/m2
    Timber frame, single glazed, with window film
    R0.4 $5/m2
    Alu frame, thermal break, double glazed, low-e
    R0.4 $450/m2

    More info at: http://www.shac.org.nz/2015/09/26/thermal-performance-of-curtains/


    Ian Mayes

    Thanks Miranda,
    I would be very keen to input on any curtain study. The one thing I do know is, there is huge variation in performance between really well fitted, pelmeted, full width and right on to the floor and with at least 2 layers ,of course, curtains and what I see in most houses. So testing needs to be done on a range of curtain scenarios.

    And great conversations and comments and lovely to see the hub being used.

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