Window "Airlock"?

[IronRanger]

I'm researching alternative building and one book I've ordered is on strawbale construction.  I'm wondering if anyone's tried a window "airlock"?  ie, a window on the exterior and one on the interior.  If one used less expensive windows, it wouldn't increase the costs much.

Is there a reason, other than cost, that a person shouldn't try it?  The main thing I can think of is condensation. 

A bit off-tangent...anyways, the more I research, the more I find that superinsulation's the way to go here in northern Minnesota (cold and moist).

I've ordered Mike Oehler's book and Rob Roy's book on underground and earth-sheltered homes, but permitting concerns me (even on the Reservation which I live).

[MountainDon]

When an airspace that is intended to insulate thermally becomes too wide or deep it looses some of it's effectiveness. Convection currents set in and move the heat around more. Off hand I don't recall the limits, but straw bales get you into that area.

The good thing would be that the window frames themselves would have a great thermal break, a problem with some cheap windows.

[IronRanger]

Hmm...I wonder if there's enough room to frame an inner and outer window in 2x6 framing?

I'm looking into staggered stud framing too, so 2x8-2x10 would be something to think about.

I found this (relating to convection):

http://74.125.95.132/search?q=cache:3aP_xL-IXYYJ:www.2hsc.com/residential/manufacturers/tvm/tvm_physics.pdf+heat+loss+by+convection+currents&cd=14&hl=en&ct=clnk&gl=us&client=firefox-a

"Reflection and emissivity by surfaces can ONLY occur in SPACE. The ideal space is any
dimension 3/4" or more. Smaller spaces are also effective, but decreasingly so. Where there is no air
space, we have conduction through solids...

HEAT LOSS THROUGH AIR

There is no such thing as a "dead" air space as far as heat transfer is concerned, even in the case
of a perfectly air-tight compartment such as a thermos bottle. Convection currents are inevitable with
differences in temperature between surfaces when air or some other gas is present inside. Since air has
some density, there will be some heat transfer by conduction if any surface of a so called "dead" air space
is heated."

[n74tg]

Back in the 1970's, my house in South Dakota had single pane wood frame windows.  We had two sets of (wood frame) window screens.  One set had screen, the other glass.  In the winter we changed out screen for glass, with noticeable improvement in house warmth.  I think the winter frames fit tighter than the screen frames, for less air leakage.

Now, as for your other question.  My house has 2x6 walls and I am using 5/8" exterior sheathing; and I will use probably 5/8" sheetrock on the inside (my studs are on 24" centers).  This gives me 6-3/4" total wall thickness to install windows in.  I am using 3.25" frame thickness replacement windows, so I have almost 3-1/2" of unused wall thickness available (that you could install your second window in).  

If you use (double pane) replacement windows on the inside and new construction (double pane) windows on the outside I think you could approach the insulation values being advertised by the superinsulated windows at seriouswindows.com.  In fact, since their windows are only triple glazed (triple pane), I think you might exceed their performance.  It won't leave you much room for trim on the inside, but it will definitely be an interesting approach if you choose to do it.

If you choose to go this direction, keep us posted, and take lots of pics.

good luck.

[IronRanger]

If you choose to go this direction, keep us posted, and take lots of pics.

I will, but I'm several years away from starting a build.

I looked at seriouswindows.com and I didn't find any prices, but I'm assuming a type of air lock would be far cheaper than a single window with a high R-value.

Who knows...a few years from now windows might be highly insulating and affordable.

If you use (double pane) replacement windows on the inside and new construction (double pane) windows on the outside

That's definitely an idea.  Thanks.