High performance residential windows

A question from someone on Salt Spring elegantly captures the many questions we receive about residential windows.

I just spoke with the window expert at one of the two building suppliers on Salt Spring. He had not heard of ESTAR windows and was not familiar with the BC Hydro rebate program. When I asked him about low-E plus argon, he said that the window suppliers are telling their retailers that argon is no good because it leaks out, forget the argon and go for E-squared instead.

Now I have not seen any technical papers that confirm the leakage argument; about 6% over five years is what I remember, but you will have more current info. And how  E2, when the coatings are applied to only one surface, will out-perform low E plus argon is beyond me. If it does, I would sure like to hear about it.

Anyway, these suppliers (they are big) are presumably giving the same message to retailers across BC. But, maybe they know something that I don't???

SBC response:

Having first alerted BC Hydro about this potential misunderstanding I went to one of the pre-eminent green building services engineers certainly in BC but probably in the country. His response is as follows:

You have to separate the sales hype from the laws of physics.
 
1.  Argon fills:  yes, argon can and will leak out at a certain rate due to it’s small molecule size.  The rate is dependent on the quality of the window seal.  Good seal= low leakage, likely less than 2.5% per year. Conventional residential/low cost commercial window seal= likely higher than 3% and could be as high as 6%-7%. Obviously, with better seals (such as those in EnergyStar rated windows), then the argon wouldn't leak out nearly as fast.

2.  Low-e coatings are a selective reflecting coating that is intended to reflect back the long wave infrared wavelengths to provide an “enhanced” thermal performance.  The reality is that there is no effect on the actual window surface temperatures and the “overall” thermal performance is only improved slightly.  The benefit of the low-e coating is that it does reflect heat back into the interior (when the low-e coat is on the inner pane of glass), and will reflect out some amount of solar infrared radiation in summer, when the low-e coat is on the outer pane of glass.

3.  Nothing beats air spaces as the best basic practical way to improve window performance- triple glazing or suspended film glazing units are all better performers without having to resort to argon and low-e coatings. 

And then the next step is to pay attention to the framing and the installation details to avoid thermal bridging.