R-value ratings measure a material's ability to resist the flow of heat. A single pane glazed window has an approximate R-value of 0.85, while a double pane glazed window has a value of 1.5 - 2.0.


You also get less condensation at the windows which can cause all kinds if trouble with wood work and mold. I don't think you would get the higher R with the old frames, with new installation methods I would go all the way to new construction windows, vinyl.

 

Double pane window will Low-e will have an R-Value of about 4 as a complete assembly.

 

Hi pman, you said: "I've been curious how a well sealed single pane window with new weatherstripping does versus a completely new retrofit dual pane window siliconed into the existing metal window frame, with that drainage air gap at the bottom letting air seep in."

That doesn't sound like a retrofit I would expect to see, having to silicone the new glass into place. Even if it is an actual sealed double pane the existing metal frame kills it. And an air gap at the bottom I don't understand but probably not good.

Is your intent (description) to order sealed double pane glass and replace just the existing single piece of glass?

Bud

 

Bud, my only option for this old house is retrofit windows.

If you look on youtube how retrofit windows are installed, the new windows sit inside the old aluminum window's frame.
The flanges are then siliconed to the outside wall, screwed through the side, some spray foam insulation for the gaps, with 2 silicone gaps at the bottom to allow trapped water to flow out.

I've been told by this one house inspector that I would not save much energy by doing a retrofit window, but many people buy retrofit windows because they look better than their old silver aluminum windows.

I want real proof these retrofit windows are worth the money

 

Most every window will leak sooner or later. Every few years we have window outfits going door to door selling retro fit. I am not sure if the old windows leaked but the leaks sure show up about a year after the new, I have re done more than a few with real windows. .

 

I confess I'm not familiar with a retrofit for aluminum framed windows so your explanation helps.

But generating any number would only be a wild guess. Assuming you are only concerned about electric bills during the cooling season, those bills are only part air conditioning. And the glass you want to replace is only a small part of all of the surfaces that gain heat that the ac had to remove. And your improvement would be at best a 50% reduction of what the glass is responsible for.

My wild guess, I'd be surprised if you saved $20 a month during peak cooling season.

From my energy auditor work I suspect there are other places to better put those efforts and dollars. But since replacing those windows with full modern units is off the table and you will be doing all of the work, it will save some.

Bud

 

From what I read some time ago, there is not that much savings from going double pane. It was a study and it found that well gasketed single pane window with a storm windows were as good as new double panes. If there is heating under that window, even comfort would'nt be a factor I think. Gas filled double pane, also, not a high tech solution because the gas eventually leaks out.:smile:

 

Gas doesn't leak out of the window is well made. Cardinal has verified testing of gas leakage rates of less than 1% per annum. The window is garbage before the gas is at a level where it isn't working.

 

It really takes some number crunching to answer the question. A complete heat loss calculation needs to be done on the entire house with your existing windows first to establish a base line. From there the calculation can be adjusted for double pane glass. The difference of BTU loss can then be translated into real heating and cooling dollars.

So if you are up to the due-diligence, here is a basic on Manual-J heat loss calculator.
http://www.loadcalc.net/


Anyone who says "my windows will save you X amount on your utilities" without doing the heatloss calcs is blowing smoke.

 

Yoda, I'm still not clear as to what he wants to compare. The typical comparison between single pane windows and double pane includes the contribution of the frame, not just the center glass. In his case he has and would be keeping the aluminum frames. Basically I see those frames as killing most of whatever benefits a new double pane might give.

Maybe Manual J asks the right questions?

Bud

 

Sounds like he wants to buy double pane glass panels and replace the glass only. I agree, doesn't really solve the old leaky frame issue and money would be better spent replacing the entire window.


Adding storms might be a better option than retrofitting glass.

 

....with that drainage air gap at the bottom letting air seep in.

A picture of the existing window would help. Up here, an old house might have aluminum storm windows on the outside, with drain holes at the bottom, and then wooden double hung towards the inside of the house. But in your area, I think you possibly just have horizontal sliders in aluminum track. But its a little surprising to me you have a drain channel between inside and outside.

Your old windows probably leak like sieves. Assuming its installed properly, I would expect you will have considerably better insulated windows with new retrofit double pane, as mentioned previously. But on the other hand, what are your annual heating and cooling costs? Probably not much, and saving 20% (to pick a number out of thin air) on not much is even less not much. Keep in mind however, that if you have a number of South or West facing windows that get a lot of sun, you can get windows with glass with particularly low solar heat gain which will make living in there much more bearable in the summer.