Doubling up: Secondary glazing case studies

A friend helped Jasper Lee with the installation of DIY secondary glazing—5 mm acrylic sheets cut to size by a local plastics wholesaler. The brown film is a protective layer that was removed after installation.
We hear from a variety of householders about their window upgrades using secondary glazing and retrofitted films.

Film + DIY secondary glazing

by Jasper Lee

My wife Melissa and I purchased a three-storey 1980s double-brick townhouse in the inner eastern suburbs of Adelaide back in the middle of 2016. We were new to Adelaide, but quickly became aware of the climate extremes during summer and winter. As Melissa works at home, and we have a toddler at home as well, thermal comfort was important for us, and we wanted to achieve this in a sustainable manner. We had made some basic DIY draughtproofing upgrades at our last property, a rental, with the permission of our landlord, but really wanted to make major improvements now we owned our own home.

Prior to our purchase, the house had been rented out for several years and little had been done to improve its energy efficiency. We had a six-month overlap while we were still renting, so we had time to plan and execute our retrofit upgrades. We started with the low-hanging fruit first: draughtproofing doors, windows and skirting boards. We also took advantage of the support from REES, the SA government energy efficiency program, to upgrade all lighting from halogens to LEDs. We also discovered that the cathedral-style ceilings were missing any form of insulation, so improved this with blow-in Rockwool insulation.

The next things we tackled were the windows. We took a bit of a mixed approach, based on the window aspect and usage, and we staggered the upgrades over the time until we moved in. Our approach was also governed by cost. Replacing the windows or changing their sizes/position would have set us back in excess of $20,000, compared to the $2000 we spent on upgrading 14 window panes with window film and secondary glazing. We did replace a poorly functioning back door with an argon-filled uPVC double-glazed sliding door, because it needed to be replaced anyway; this cost $3300.

Our lounge room is south-facing and summer temperatures are not a problem due to its shaded aspect and the stack effect that moves hot air up into our attic room. However, in winter, retaining heat while also maximising natural light during the day was a priority, due to its regular usage by the family at home. Replacing the lounge room windows with double glazing or secondary glazing would have been quite costly, due to their total size of 3000 mm x 2300 mm. At the time, I wasn’t confident of doing the secondary glazing DIY, due partly to the window’s size.

Instead, the Enerlogic films got our attention due to their performance claims (both insulating U-value and heat reduction SHGC) that put them in a different league to other films advertised at the time. It appeared to be a solution to improve the winter performance of our lounge room with minimal time/cost. As installer, we chose the Energy Efficiency Centre in Adelaide due to a discount promotion they were running at the time. This was a simple three-hour installation done by one of their tradespeople.

We also wanted to improve the performance of our bedroom windows. The bedrooms are south-facing, but on summer evenings we get late afternoon sun coming from the south-west which came in through the windows and made the rooms quite hot during the evening. The most common, and cheapest, window films are the reflective tints aimed at reducing solar heat gain, similar to the window tinting used on cars. They have little or no insulating effect, but can be effective where you want to reduce solar heat gain. We decided to use these for our bedroom windows (10 window panes over three windows in three bedrooms).

We chose bedroom coatings to reflect external light to reduce heat gain and the lounge ones to reflect internal heat to prevent heat loss in winter.

In the bedrooms, I coupled the reflective film with DIY secondary glazing with 5 mm acrylic custom-cut by a local plastics wholesaler. I installed these with the help of a handyman by inserting them into the grooves of the window frames, fastening them and then silicone sealing around the plastic, allowing an expansion gap for summer. This process took about two days to install 10 panels and wasn’t technically difficult, just time-consuming.

The window film products we used were:

  • lounge room: Enerlogic 70 window film, provided by the Energy Efficiency Centre. Cost: $850 for four window panes (price lower due to a promotion at that time)
  • bedrooms: Suntek DRDS-35 window film, provided by Adelaide Premium Tinting. Cost: $775 for 10 window panes.

It’s difficult to separate out the window film performance from other upgrades we did. In the lounge room we installed the Enerlogic film and blockout curtains and in the kitchen we replaced the back door with a new door with uPVC double glazing. In the bedrooms, we installed the basic reflective tint films, DIY secondary glazing and blockout curtains.

I did take some temperature measurements to see if there was any effect from the window upgrades. There did seem to be a small difference: in the lounge room, the early morning temperature in August went from 16 degrees before installation to 16.5 degrees after. In the bedrooms, evening temperatures in February went from 32 degrees before installation to 31 degrees after. It’s not a huge difference, but perhaps it has helped exclude some sun.

When we did the windows, the improvement in thermal performance would have been somewhat compromised by the lack of wall insulation. We further improved thermal performance with brick cavity blow-in Rockwool insulation nine months later, and have found there are now no noticeable hot spots on the walls after two to four hot days above 30 °C. The maximum internal temperature is now 30 °C.


Left: Four years on, Alan Cotterill is pleased with the performance of his DIY acrylic secondary glazing. Right: Jasper and Melissa chose Enerlogic film for this large window in their living room. It improves the performance of the glazing without noticeably impeding the view.

Update on DIY secondary glazing

by Alan Cotterill

In ReNew 135 in early 2016, I wrote about my DIY installation of secondary glazing in my four-bedroom brick-veneer home in Wagga Wagga, using cast sheet acrylic.

Not including the two sliding glass doors, I installed 34 acrylic window panels into my sliding powder-coated aluminium windows.

I had to find a solution to two major issues. The first was that silicone sealants did not bond to sheet acrylic; apparent adhesion was more due to a suction effect. I solved this by etching the edges of the sheets with a thin film of a co-polymer sealant (Parfix Maxi-clear). The silicone sealant then bonded strongly with these edges and the window frames.

The second problem was that the degree expansion of acrylic is four times that of aluminium. I managed this by using a highly flexible silicone sealant (from Parfix) between the aluminium and the etched edges. The sliding window panels required a tight-fitting acrylic sheet to sit inside the frames in order to use the existing rubber inserts (with a generous amount of silicone sealant) as spacers; loosening the two screws in each corner of the sliders allowed for expansion of the sheets and for the frames to truly square up and better accept the sheets.

There has been no decline in the visual clarity looking out. I have been careful to only use warm water with a few drops of washing up concentrate to clean the acrylic panels, which has done an adequate job. One external acrylic sheet has deep scratch marks from a visiting Bernese mountain dog desperate to gain entry. Our own dogs and cat have caused no damage. My early efforts at applying a bead of silicone sealant inside the frames of the sliders were a bit messy, but the normally fitted insect screen frames usually fully shields this from view.

As stated in my original article, I found the acrylic sheet panels were detaching until I worked out that etching the sheet edges with the co-polymer sealant would give a strong bond. Only one sheet has partly detached since edge etching. I think this was due to the co-polymer bead being too thin and not spread before it dried, so there was not enough etched surface area.

I have kept unsightly insects from getting between the glass and acrylic by sealing off any drainage holes at the bottom of the frames that might allow entry. There are 1.5 mm holes drilled into a top corner of each acrylic sheet to stop ‘ballooning’ of the panels in very hot weather, but insects haven’t gained entry through these. I don’t expect these holes to significantly reduce the efficiency of the double glazing.

I keep direct sunlight off the double glazing in the warmer months by hanging a double layer of shadecloth off my guttering using bulldog clips—a quick chore to put up and to remove. Subjectively, I feel our house has been cooler in our very hot summers, though our evaporative cooler has only one speed on those extreme days! In Wagga Wagga, the cold winter is where energy usage is at its highest. I expect that our gas central heating bill and winter comfort benefit the most from this project, but I don’t have hard data on this.


DIY warning

As always, take care with any DIY project. Wear protective clothing and eyewear when working with windows. We recommend anyone doing DIY secondary glazing to use cast acrylic or another polymer glazing material, such as polycarbonate sheeting, rather than glass. If using glass, you must take extreme care when handling glass sheets. See the extended warning box on p.78 in the article ‘New glass is greener’ in this issue for more information.

Magnetite secondary glazing

by Kevin Cato

Over several years, my partner Jenny and I turned an uncomfortable warehouse apartment into a cosy home in winter and a cooler home in summer. In the process it also became more sustainable.

Our apartment features a mezzanine with two bedrooms and a bathroom; a kitchen, laundry and study underneath; and a full-height living room with a glazed wall and double doors leading to a semi-outdoor atrium behind the original facade.

We moved in during the summer and soon realised there was no through ventilation to allow the hot air to escape from upstairs. We replaced three basic skylights with double-glazed opening roof windows with blinds. These could be opened at night allowing the heat to escape. We got a shock to find there was no ceiling insulation, so after freezing through a winter and baking in a couple of summers we decided to remove the roof cladding and put in a double layer of insulation to R4.

Kevin and Jenny's apartment features a glazed wall - 5m hight at its hightest point - opening onto an atrium behind the original facade of the warehouse. Installing Magnetite secondary glazing contributed to a dramatic improvement in the thermal performance of the apartment. Ceiling insulation and painting the roof with heat-reflective paint also helped,

Next we addressed windows. The glass wall at the end of our apartment is 5 m high at its highest point and 4.5 m wide. Needless to say, the heat gain in summer and in particular heat loss in winter were tremendous. We could not sit within 1.5 m of the glass wall in the winter. The panel heaters were creating rising hot air which hit the cold glass then dropped, creating a cold draught. The apartment took forever to heat and the moment the heating was turned off the heat would quickly be lost through the glass.

We came across a company called Magnetite, which installs secondary glazing where your existing windows are left as they are and clear panels are magnetically attached to a slim frame on the inside. The end result has a negligible visual impact, which makes it perfect for any place with an owners corporation, as you do not need to seek permission. Optical-grade acrylic is used, which is a better insulator than glass. As a bonus, the Magnetite can also reduce the amount of outside noise and reduces condensation. We had the entire glazed wall fitted with Magnetite, along with the smaller office window at the other end of the apartment which looks out onto the internal common area.

With the secondary glazing and ceiling insulation, the temperature seldom falls below 18 °C and with heating doesn’t take long to get to 21 °C, whereas before it struggled to get to 20 °C. The temperature is also far more constant and even throughout the apartment—before, when you walked towards the glass it got noticeably colder. We also put in external blinds to keep the direct sun off these windows, which has meant a big improvement in summer.

Even after the roof insulation was installed we still felt it got too warm upstairs on hot days. That was when I came across Solacoat heat-reflective paint. Unusually for a strata apartment, we own our own roof, enabling us to use it as we see fit—within reason of course! I was able to paint the roof myself; we have good access to the roof, and it’s not very steep, making it safer to work on. It took me the best part of a day. We found the paint made a big difference: the roof went from being untouchable on even a mid-20s day to just being warm. We now cope well with just ceiling fans. For me, it’s the best $350 I have ever spent!

My biggest takeout from this journey is that an efficient house is a comfortable home and you cannot put a price on that.

Disclaimer: Kevin liked their product so much that he now works for Magnetite doing installations.

 Magnetite info

According to Magnetite, their product provides good insulative benefits, reducing the U-value of a standard timber-framed single-glazed window from around 5.5 to around 2.6. For a standard aluminium-framed window, the U-value can be improved from around 7.4 to 2.9.

As the optical-grade acrylic they use is so clear, the effect of installing Magnetite on the SHGC (solar heat gain coefficient) of the window is only small, but the company can provide low-e coatings to reduce solar heat gain if this is an issue.

For current pricing, please contact your local Magnetite office. Magnetite is currently not available for DIY.

The EcoGlaze system uses an unobtrusive frame (in this case white) to hold a wheet of acrylic in place. Richard notes that one advantage of secondary glazing for period homes is that it means you can retain the character of the existing windows.

Energy savings with EcoGlaze

by Richard Keech

I fitted secondary glazing to my home in early 2010. My period home in suburban Melbourne has timber windows—a mixture of double-hung sash, fixed and awning windows. I chose EcoGlaze from EcoMaster, which suits my type of glazing. I had the EcoGlaze fitted to all the windows (about 50 glazing elements) at a cost of about $12,000 at the time. In hindsight I probably need not have fitted it in the laundry, toilet and bathroom because these spaces have very low occupancy and aren’t actively heated or cooled.

I know that in 2009 my home used 42 GJ of gross energy (gas and electric) and in 2010 this was down 5 GJ or 12%. The glazing was the only substantial change in that time, and the home occupancy was the same over both years. So 12% is probably a reasonable estimate of the impact on overall energy consumption. Heating and cooling is currently about 45% of overall energy consumption, but at the time it was probably more like two-thirds of energy consumption— about 28 GJ in 2009—because we had not yet

upgraded our heating and cooling systems. So the reduction of 5 GJ translates to 18% reduction in overall heating and cooling energy, attributable to the upgraded glazing.

The glazing system uses acrylic sheets and the air gap is not completely sealed. Because of this the sheets are removable to deal with any fogging up that may occur. Since installation, I think we may have needed to disassemble one window for fogging. Another factor is that acrylic is slightly softer than glass, so it’s slightly more prone to scratching. Even so, we’ve not had any concerns about scratching and the windows still look great.

A lot of the character of older homes is carried by the original windows. I’ve seen the character of period homes altered badly by fitting modern windows. So, it has occurred to me since installing EcoGlaze that, for retrofits, in one important respect, secondary glazing is actually better than new double glazing because the character and charm of the period windows is preserved.

The EcoGlaze secondary glazing system has fully lived up to expectations and I’ve been very happy with the result.

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