I would consider Toronto to be cold enough to necessitate the better U-Factor window, but not without more data on the windows in consideration. If that U-Factor of 0.23 is the result of a much darker range of surface 4 Low-e, I wouldn't take that over a standard dual pane.

I was wondering if the .23 was triple pane and .27 was dual pane.

As WoW pointed out, there are a number of factors to consider besides just the U value (factor)

I am surprised that Toronto would allow .27. I have that in NC.

How much of a difference would certainly depend on how much window area you have.

The math isn't that difficult.

In the end, it probably is a 5-8% difference in the thermal performance of your house. Not huge but significant. There is a slight comfort difference of course with sitting near the window.

Thank you all! Both options are dual pane. The one with the higher U factor has low E 272 glass with argon and the one with the lower u factor has evolv low E glass with argon. @WoW- When you refer to a “darker range of surface 4 low E” are you talking about a tinted window? I am not sure what you mean.

@David Cary: can you elaborate on how you calculated the 5-8% difference in thermal performance? This is our first time buying windows

What does your contractor recommend (at your price point)?

Inline Windows makes very good fiberglass windows in Ontario. Here. Triple pane would be on my list to check out. U-factor is listed Here.10 pages under casements. The triples are at the end.

Surface 4 is the interior surface (i.e. facing the home). Many dual pane IGUs are getting some of those lower U-Factor numbers because of an applied surface 4 coating that, unfortunately, drives the condensation resistance (CR) down a good bit.

BTW...if you are going to compare the items in question with regards to percentages, I would compare them directly. So, in this case, the window with the U-Factor of 0.23 is approximately 15% more efficient than the window at 0.27.

While it depends a lot on glazing area, typical is about 12%. That would roughly represent 30% of your heat loss. Very roughly.

Then your windows are about 20% off or 1/5 of the 30% - so 6%.

But it depends greatly on your wall R-values, roof R-values and ACH50 scores. As those get better, the window choice makes a bigger difference. Typically, in northern (and progressive) areas, the requirements for roof/wall/ACH are higher so windows become a bigger variable.

This is why in higher performance homes, triple pane becomes more common. Not in NC mind you, but certainly in the northern US.

In my climate, solar is significantly more cost effective than triple pane windows. In your area, hydro is so cheap that triple pane may not be worth it. As I check Ontario code, it is very lax which is somewhat surprising. Except for the ACH which is quite strict.

Thank you @WoW and @David Cary for your explanations! Much appreciated!

Read up on the warranty period for argon in a window...

"Read up on the warranty period for argon in a window..."

Where are you going with that Gargamel? I did not know that argon gas had a warranty?

It tells you how long the argon stays between the glazing ...3 years (?). I don’t know if “warranty” is the right word...it's in the fine print. Argon escapes, which I guess doesn’t really matter, but why pay more for a window with argon. Just my opinion

If argon leaks, part of the insulation is lost from the window. It would then have lower performance. This fact has been a knock on argon for years. I find it one of those arguments against a new efficient technology - which sometimes have merit. But this is not like filing your tires with Nitrogen which is often done by auto enthusiasts. It makes a tiny difference - Argon is more significant. And it is also ubiquitous in new windows so the debate is fairly irrelevant.

When seals leak, the panes get condensation between and look cloudy. Then the window gets replaced - typically under warranty which is often lifetime for original owners.

(This is my understanding but I am no pro).

I suspect that it leaks sometimes before you notice and that is a downside.

You will find reports that the leak rate is 1% per year but I don't think that is really hard data.

My last house we built. The windows were all wood and we painted them dark. We had about 40 windows and replaced about 10 before we left in 8 years. I blame the dark color since it was mostly West facing windows and the sun caused them greater heat stress because of the darker color. The window manufacturer didn't agree. We had about 12 West windows and I think 8 of our 10 were on that side, the others were East. We sadly didn't have any South windows.

Argon leaks yes, is it warrantied, no. As far as how fast the opinions are all over the map.

"When seals leak, the panes get condensation between and look cloudy. Then the window gets replaced - typically under warranty which is often lifetime for original owners."

Typically 10 years s the seal failure guarantee and glass only, on you to have the glass swapped out.

Any company worth a anything is going to have glass seal failure warranties of 20 years at a minimum.

My warranty with a company that got bought by Plygem got me new sashes which are super easy DIY to swap. I did have to paint them and I did have one fixed window that was a bit harder to swap.

And seal failures happen regardless of Argon of course.

Yeah Eric? I thought everyone was on 10 now, even Andersen? I know commercially all the IGU's we buy and sell are 10 but residential must be different.

I thought that was 10 years on the frame materials from the wood guys? I still thought glass was 20 years? Its been a while since I have looked at warranties.

Cardinal IGU's carry a 20 year, 1.5x original cost, unconditional warranty (related to materials and workmanship) to the window manufacturers. What is offered to the window customer is up to the window company.

Something close to about 95% of wood windows in North America use Cardinal IG units, so I would expect most if not all wood guys would offer a 20 year warranty when using Cardinal IG's. As far as I recall, most do.

A smaller percentage of other-than-wood window manufacturers also use Cardinal IG's and I would expect that any company that uses Cardinal IG's, regardless of frame material, would also offer 20 year warranty on the IGU...why not, it costs them nothing if a failure occurs, even if that failure is 19 years and 11 months after the date of manufacture.

Since Cardinal introduced the XL stainless spacer in 1992, their IG failure rate has been consistently at about .2% of all units produced, even though Cardinal predicted .25% over 20 years (and .5% over 50 years) when they originally introduced the spacer.

Cardinal's newest stainless steel spacer, Endur, is anticipated to be very similar (and a little better) in longevity performance when compared to the XL.

Also, since Cardinal manufactures the glass and the coatings, and puts the whole thing together in a Cardinal IG plant, the warranty is on the entire IGU system, not just on component parts.