Can you put multiple windows back to back in a thick wall assembly?
Let’s say you are building a 16″ wall. Windows are the weak spot by a huge margin.
I have done some reading on the use of internal and external shutters that can be used at night but it seems that the technology is fairly immature and there can be problems with condensation and/or thermal shock (cracking).
What if you did this?
– Buy two good windows.
– Install one window normally (near the outside edge of the wall, fully flashed, etc.)
– Leave a 1-2″ gap. For all intents and purposes, that gap would share the same air as the inside of the wall assembly. It would have a window sill but would also have vents into the assembly. In other words, you would not try to make this an air-tight/vapor-tight space.
– Install another window.
– You would then have a normal size window sill to the inside.
The R value of this window assembly vs a single window should be a little greater than 2x. If your normal window is R4. This would be a little higher than R8.
Potential problems:
– You could try to keep the gap between the two windows air-tight and vapor-tight but that would be hard and you risk running into issues with the space becoming pressurized/depressurized and distorting the glass during hot and cold temperatures.
– On the other hand, by allowing that space to be equivalent to the space inside of the wall, you may decrease the effectiveness of the wall assembly since the entire space inside the assembly now has a shortcut to the outside.
– Perhaps the space in the gap could be air-tight and hooked up to some very small ducts from a dehumidifier to allow the pressure to be regulated with dry air. That is getting fairly complicated.
Has anyone seen windows installed like this?
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Not sure if this would work as you expect, but even if it did it's nothing practical. You could get legitimate R-8 windows for less than the cost of two R-4 windows, and not have to deal with the massive amount of extra work and hassle. I guess you could ask the same question about two R-8 windows. I have a feeling it's going to be pretty problematic.
What part do you think will be the least practical?
If we assume for a minute that a window manufacturer made a window that was intended to be mounted in this fashion... Would the main problem be with the air that exists in the gap between the two windows?
As Trevor points out, cost is a problem. I'd also be afraid that the view out the window would be limited and/ or distorted. Why not do a mock up and report your results?
This was discussed a few months ago, but for the life of me I can't think of a search that brings the thread up.
To make it a little more concrete, this is the wall I am working with. (It's not actually that thick but I could get rid of the interior window sills entirely). As you can imagine, that wall loses a lot of energy.
I seem to remember an article, either on this site or on Fine Homebuilding about a deep energy retrofit where the builder sprayed foam on the outside of the house, right over the siding, trim, etc. and added new siding, trim, and windows, and I think he or she left the existing windows in place. We have too many articles on deep energy retrofits for me to wade through them all looking for it, but I've asked Martin. So, if I'm remembering right, he'll know what I'm talking about.
Have you done a cost analysis of your proposed idea vs. just buying better windows?
Also, why not install one window and make custom storm windows from insulated glass and a nice weatherstripping detail. This may not get you the same R-value, but you can take the storms off for the summer and you won't have such a clunky situation when it comes to functionality. Here are some articles about the efficacy of good storm windows
https://www.greenbuildingadvisor.com/article/low-e-storm-windows-are-big-energy-savers
https://www.greenbuildingadvisor.com/article/save-energy-with-storm-windows
https://www.greenbuildingadvisor.com/article/four-affordable-ways-to-improve-the-energy-efficiency-of-old-windows
Well, Martin found the article, but it doesn't discuss the dual windows from a performance perspective. It turns out the original windows were single-glazed and were kept for their character. Still and interesting approach to a deep energy retrofit. Here it is:
https://www.greenbuildingadvisor.com/green-homes/brand-new-appearance-and-performance-for-an-older-duplex
Brian,
There was a Q&A about the same topic on GBA within the last year. It discussed the potential problems of the air-space between the two units, the complexity of installation, and concerns around egress in bedrooms.
The technology of internal and external shutters that can be used at night is not necessarily
immature - if you are willing to use standard insulated exterior doors as shutters. They are
inexpensive, easy to install, very robust and will add an additional R value of 6 or higher to
the glass at nights - the downside is the necessity to have windows sized to fit standard door
size openings. But if you are designing a house this becomes trivial.
Have you seen a project where this has actually been done?
If your asking about using exterior doors as shutters _ yes! I designed
and lived in a house with 16" thick walls and used exterior doors as
shutters (both interior & Exterior shutters) on every window in Flagstaff
Az. (climate zone 6a). Lived in it for about 6 years - no condensation or
any other problems - was able to use standard size sliding door glass
(the most inexpensive glass you can get) for the fixed glass windows - you
can't beat this for cost!
I'd love to see pictures to it if you have any.
Thanks for the responses and links everyone. Very interesting reading.
I did this in a home I built near a military airfield . The wall was only 6 inch plus resilient bars .
I used budget jen weld windows - a pair far cheaper than one super efficient noise reducing one . The occupants say it works so well they are not bothered by the jets which pass right overhead at under 200 feet . ( outlying field Coupeville ) . In theory it far exceeds the U value and noise reduction of any single window - Quad pain with 2 x 1/2 inch gap and one wapping 5 inch one . Each single window did meet the then code for high noise area . Hoping to do the same in my next house there - a 12 inch ICF wall . Found this when researching the new energy code to see if this option complies . It does if you consider the double windows as a single unit . Hope the county thinks the same . I can not really afford the hyper expensive windows that the new WA energy code requires . Hence doubling up !
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