Should I double glaze steel basement windows?
rasputin0
| Posted in Energy Efficiency and Durability on
My 1927 tudor revival has its original steel basement windows. Each window has 3 panes of glass and they’re about 19″ x 28″ for the whole unit. I’ve hired someone to strip them down, repaint them, and generally restore them to peak shape. Obviously these things are _never_ going to be energy efficient, but perhaps some improvements can be made.
I’ve attached a pic, you can see the interior storm in place. Once the window is refinished I’ll smooth out the spots where the storm meets the wall.
Is it worth installing double glazing?
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Replies
rasputino,
What are your along term plans for the basement? It looks like it is uninsulated now, so the windows probably don't make an appreciable difference to the energy it loses. I would probably leave them alone, and if you decide to insulate later, replace them with more efficient ones.
Malcolm, I'm going to quibble a bit.
It depends on what the goal is. If the goal is saving energy, if those windows pass 50 BTU/hr per square foot today and you can get that down to 25 BTU/hr with double glazing, you get that savings regardless of what the surrounding walls are like. To answer the question, "Is it worth installing double glazing?" we need to know a few more details, like what the double-glazing would cost, what kind of heat there is and what it costs. If it makes financial sense, again it makes sense regardless of what the surrounding windows are like.
A related, but independent, question is whether insulating the walls would save more energy at a lower cost. If so, it's a better investment, and if funds are limited they should be devoted that way. The reason I say it's independent is that the presence of a better investment doesn't make the alternative a bad investment.
However, if the goal is comfort, it does get a little more complicated. If the basement is cold and uncomfortable because it is poorly insulated and drafty, it will probably still be cold and uncomfortable with double-glazed windows, and upgrading the windows won't achieve the goal of increasing comfort.
What complicates the analysis is that it's very difficult to either measure or estimate the performance of a window like this. I'll add that sometimes it's worth making improvements that don't pay off in strict economic terms of energy savings because they make a space more comfortable, which is hard to put a price on.
I don't currently have plans to finish the basement. I like having a workshop. I'm also not entirely certain if it would be possible to finish it, code concerns, etc.
I could talk to my window guy about how much it might cost.
Finishing is not the same thing as insulating. Even if you don't finish the space it's a good idea to insulate it.
Any particular insulation techniques worth considering? I'm curious
You just insulate as if you're going to finish, but don't finish.
Actually, you already have double glazing with your the original window plus the interior storm. Not what you were thinking of when you asked your question, but that's what you have.
In order to upgrade the glazing in your existing steel framed window, you would need to add three separate dual pane IGU's to the original window as part of the rehab, and while that would seem to be an improvement over what you have now, you would still be dealing with the high conductivity of the steel frame and steel muntin bars which could negate much of the potential thermal advantage of adding new IG units.
An IGU is thermally divided into two different areas, edge-of-glass, which is the glass area within 2-1/2" of the edge, and center-of-glass, which is the glass area not within 2-1/2" of the edge. Even with the best IGU on the market, a very cold steel frame that is in contact with the glass will result in cold EoG which is going to affect thermal performance of that IGU. And since each individual IGU is not very big the percentage of EoG vs CoG tends to be significant - EoG is colder, therefore the average temperature of the glass in the IGU is colder - basically the potential thermal improvement of the multiple IGU's over the single panes of glass in your specific application might not be looking as good as it might seem.
On the other hand, the glass in the storm panel isn't touching the steel muntin bars since there is a layer of air between the inside glass and the original window that's acting as a thermal break between steel and glass. Also since the inside storm is a single glass lite and not three separate panes there is more CoG and less EoG, and that's a good thing, thermally-speaking.
Best case would be to upgrade the internal storm to a dual pane unit with a LowE coating on the glass, but if that isn't a viable option, then just make sure that the internal storm is as tight as you can get it in the opening, avoiding air leaks as much as possible, and if at all possible use an internal storm that has a less conductive frame surrounding the glass.
A double glazed storm never occurred to me. I wonder if such a thing is on the market. Obviously I could just install two storms if I really needed to.
Your long explanation largely confirmed what I suspected - double glazing probably just isn't worth the effort and expense.
Dual glazed storm windows are available, but since a number of long time storm window manufacturers are no longer producing storm windows, I am not sure how hard they are to find.
I also think that ensuring that the existing storm us tightly sealed to the opening, plus an upgrade of the existing window is the most cost effective solution.
I think carefully air sealing the inner storm will have the most effect for the least money. Unless you get lucky and find an assembled window that fits in the spot, a custom storm or window replacement probably won't improve enough to notice. If you ever get to finish and insulate the walls then the windows will once again be a larger percentage of loss.