Browse Forums Windows & Doors 1 Jul 04, 2015 3:39 pm Apologies if this question has been asked previously, but could anyone offer help on how to compare different u-values of windows from the WERS site? e.g. if a double glazed aluminium awning window +frame has a u value of 3.0 and another suppliers u-value is 4.5, what does this difference actually mean? In other words, does this mean the 4.5 rated window is 50 % worse in terms of retaining solar heat in winter ? Many thanks Re: How to compare u-values of windows e.g. 3.0 vs 4.5 2Jul 04, 2015 10:01 pm Basically the U Value is the number of watts lost per sqm of glass per degree c So if you want the internal temperature of 22 degrees and it is 5 degrees outside (17 degrees difference) The window with a value of 3 will lose 17 x 3 watts of heat = 51 watts for every Im of window. The calculation for the 4.5 window will be 17 x 4.5 = 75 watts It's not just about keeping heat in in winter its also about keeping heat out in summer. See this link for more information; http://www.anewhouse.com.au/2012/02/ins ... s-windows/ The Harder You Try - the Luckier You Get ! Web site http://www.anewhouse.com.au Informative, Amusing, and Opinionated Blog - Over 600 posts on all aspects of building a new house. Re: How to compare u-values of windows e.g. 3.0 vs 4.5 4Jul 05, 2015 11:14 am Bashworth has explained it well, but I'll try to work through an example of how it translates to costs.
The window coverings will also affect the overall U-value. If you're going to have thick curtains and pelmets, they have their own u-value of around 7 themselves, so the combined U-value for the 4.5 window is u=1/(1/7+1/4.5)=2.74, and for the 3.0 it's 2.1. Now, going by our house as an example, the combined window area in the thermally regulated zone is around 18m2, and as a rough guide, here we have around 60 active cooling/heating days a year, where the a/c would run against around 12 hours across a 15 degree inside/outside differential. So, in my example, the conductive heat load from the windows is U4.5: 2.74*15*18 = 739 Watts (U * temp diff * area) U3.0: 2.10*15*18 = 567 Watts Across 12 hours a day for 60 days, that's U4.5: 739*12*60 = 533kWh U3.0: 567*12*60 = 408kWh For an Air-conditioner of EER 3.5 using electricity at a cost of $0.32/kWh for electricity, that's an A/C cost for window heat loads of U4.5: 533/3.5*.32 = $48.70 per year U3.0: 408/3.5*.32 = $37.33 per year So, in this example, going to the U3.0 windows saves $11 a year in A/C running costs. This is for Adelaide though, where there is almost no heating requirement in winter, and summer loads are just across hot weather periods. In more extreme climates, and where you want to regulate the temperature more closely, the cost difference would be higher, but you should be able to adjust the above numbers to suit. With no curtains, the costs come to $80 and $53, making a $27 difference. With the better windows, you also get more comfort during the times when you have the windows closed, but not running the A/C. Re: How to compare u-values of windows e.g. 3.0 vs 4.5 5Jul 05, 2015 11:29 am That is very generous of your time to run through those facts and figures on the subject. It is appreciated. I didn't think blinds could be equated into such an accurate money saving figure. Now to try and get honeycomb blinds into my wife's radar for choice in looks. I like the look of honeycomb and if fitted inside the window frame, all the added benefits of energy savings without need for pelmets. I can see all the benefits. Colour choices are good and if the block out material is used the best R value is obtained too. All good in my books. My wife will have the final say on blinds though. I am convinced. Thanks again. Re: How to compare u-values of windows e.g. 3.0 vs 4.5 7Jul 07, 2015 11:27 am Quote: The window coverings will also affect the overall U-value. If you're going to have thick curtains and pelmets, they have their own u-value of around 7 themselves, so the combined U-value for the 4.5 window is u=1/(1/7+1/4.5)=2.74, and for the 3.0 it's 2.1. Thanks onejohn. I knew curtains contribute significantly to the thermal efficiency of external windows and doors but I was unsure of the figure. Stewie There is evidence of water on the head reveals. That could have happened during the build, or may be happening currently. There are water stains above a couple of windows… 2 3813 Personally, considering your layout (study/work desks in bedrooms), I don't think you have any other option but to leave NW windows and make them as big as possible e.g.… 7 10415 Hi, I'm planning to build a new house and wanted to get some feedback on the different window designs for a master bedroom. Our bedroom is north facing and the bed will… 0 4538 |