Home Forums General Discussion Secondary Glazing

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    Jo Wills

    Is there a recommended thickness for secondary glazing?

    Norman Smith

    Hi Jo, do you mean the thickness of the glass or the size of the gap between the two panes of glass and therefore the overall dimension of the double glazing system? The latter is more important in determining optimal performance. Norman

    Nik Gregg

    Hi Norman, your feedback is interesting, is there a formula or method around the impacts of glass thickness, vs the size of gap.  For simplicity – assuming standard glass and just good old household air in between the gap.

    I realise that the window frame may dictate what will work and what won’t, but is there a formulation or rules of thumb that guide thickness of glass and gap impacts on performance?


    Norman Smith

    Hi Nik, this is an area which has been subject to considerable research and would be available in the literature in topics fields such as WERS – window energy rating schemes. Sites such as ashttp://windows.lbl.gov/adv_Sys/hi_R_insert/GapWidths.html offer an introduction to the subject – Optimal gap width for double and triple glazing systems.


    Here in NZ I found the report referenced below highly relevant, not the least because it demonstrates to households that (shock horror) the DIY retrofit film double glazing performs as well as, and sometimes better, that some ‘proper’ DG at a fraction of the cost. I can make this report available to anyone who is interested, also speak from my own experience.

    This rasies the issue around how to resource topic areas when more information is required than available within the Hub. Some years back I was funded by the electricity sector to provide an information service to answer “all” questions by members of the public. In the case of such an enquiry as this I believe a combination of desk-based research and talking to people at BRANZ and others would be required to provide a reasonable answer, and that would take me at least an hour.

    Cheers, Norman

    SB10 Conference Paper Number: 60
    Scientific Paper, Student Paper
    Houses with single glazing represent a large majority of the New Zealand housing stock. With the
    recent changes to the NZ Building Code Clause H1 Energy Efficiency, new houses require higher
    glazing thermal performance. This will lead to an increased need for cost effective methods to
    improve window thermal performance in existing single glazed houses without completely replacing
    the windows.
    There are several secondary glazing options available including ‘stick-on’ plastic glazing as well as
    aluminium framed glass solutions that are installed inside the existing joinery. Secondary glazing is
    marketed as a cost effective alternative to insulated glazing units, providing both improved acoustic
    and thermal insulation to existing windows. There is little information regarding the in-use
    performance and cost benefits of secondary glazing in New Zealand. This paper will explore the
    efficacy of the secondary glazing products when installed in existing single pane frames.
    A guarded hotbox was used to make thermal transmittance measurements on a typical single glazed
    aluminium window. Four common secondary glazing systems were retrofitted into the window – (1)
    thin plastic film; (2) magnetically-attached acrylic sheet; (3) aluminium framed secondary glazing;
    and (4) aluminium framed low emissivity (low-E) secondary glazing. Models of ‘typical’ New
    Zealand houses created in the ALF building thermal simulation programme were used to explore the
    heating energy savings and cost benefits provided by the different secondary glazing systems in a
    range of locations.
    Of the tested products, the low-E secondary glazing produces the largest cost-benefits. Secondary
    glazing was found to not be a financially viable solution in warmer climates such as Auckland. In
    cooler climates such as Christchurch and Dunedin, secondary glazing was found to be a cost effective
    retrofit alternative for existing single glazed households.


    Hi Norman, do you have a pdf or link to pdf of the above paper. This is just the sort of thing we’d like to include in the best practice library.



    Norman Smith

    Hi Sally, have the pdf file, and its a serious piece of research (see below) but I can’t honestly sign off to the requirement – By uploading files you acknowledge you have the right to share the file and agree to it being shared among the Hub community” Pretty sure it would be ok but don’t want to go there just in case, and wouldn’t want to compromise the Hub. However, I can make it available directly to anyone who asks. Norman

    Nigel Isaacs was primary supervisor and John Burgess the secondary supervisor. We would like to
    thank Building Research for the scholarship supporting Nick Smith, and to Beacon Pathway for
    funding the laboratory testing. We would like to thank BRANZ for the use of their test facilities, and
    Ian-Cox Smith, Mark Hearfield and Roger Stanford of BRANZ for their assistance in the assembly and testing. [The work of one A. Pollard of BRANZ is referenced.]

    Norman Smith

    Hi again Sally, not sure where to report this interesting, albeit harmless, glitch but I have a Doppelgänger in the form of Jo Wills; every time I post something the system reports Jo has done the same thing at the same time.



    Thanks Norman

    Jo Wills

    Thanks for your response Norman, I am keen to see the PDF so if you could send it on to me that would be great.

    Chris Freear

    Hi Norman

    I too would love a copy chris.freear@epc-nz.com



    Eion Scott

    Hi all,

    Nick Smith presented on this research at the 201o Eco Design Advisor Conference. You can see his presentation here http://www.ecodesignadvisor.org.nz/assets/Secondary-Glazing.pdf



    Norman Smith

    Thank Eion, Hi All, a helpful summary.

    I thought I’d lrovided this comment (which followe here) but it must have been somewhere else.

    In my experience Nick’s research failed to draw out the real benefits of retrofit film because he assumed, perhaps not unreasonably, it had a life of one year. True I don’t have young children or leaping dogs but the film on a window wall in my hallway (four 1m x 750mm windows) has been installed for eight years and is going strong. A tighten up with the hir dryer every couple of years is all it has needed. Because the nature of the construction made it possible I had some fun by installing additional 15mm beading, to add another layer of film to give me triple glazing. It worked, must do the calcs some time. If you can expect window film to last several years the economics blows everything else out of the water; I wonder how many double glazing companies guarantee their products for that long? Have got photos but then – there’s nothing to see!!

    Cheers, Norman

    Nik Gregg

    Hi Norman

    Thanks for the abstract and the web site detail.  I would be interested in a copy of the pdf also.  I have down loaded from the site Eion provided, so have an overview, but the full report would be interesting.

    Thanks again.



    Hi Norman, re: permission to upload a document into the Best Practice Library. Out of interest is the research you’re talking about in the public domain? (i.e. available on a website). I’m going to follow this up so it would be good to know if its online or you just have the pdf.



    Vicki Cowan

    Hi guys

    all the SB10 and SB07 papers are still available on the BRANZ website.  Here’s the link: http://www.branz.co.nz/cms_display.php?sn=177&st=1&pg=7050

    There was a lot of really good research presented at those conferences so I highly recommend people have a browse amoung the papers.  The authors and titles are listed on the page, and then you can click through to the papers.

    I have to echo Norman’s comments about the life of the plastic shrink wrap secondary glazing.  Dogs and small boys live in my household and I installed it in the first winter we lived in Gisborne – 5 years later most of it is still up, though I have to admit I take it down for summer in our main living room.




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