Expert: Ted Barnhill (Principal, Design45 LLC) - 3/14/2008

Question
hi,please bear with me on this lol  im building a small cabin
ab 16 x 28 using old railroad ties for footing and 2x6 walls
on 16 in centers and r 30 insulation.
now this is where it gets tricky  a friend of mine is going
to be replacing the windows in his apartment complex, they
are old wooden 2 pane windows
he's putting in these e rated windows and is giving me the old ones  now what i was thinking was to use 2 or 3 of these
double panes for my windows in each opening , so i would get
the added r value.
do U think it might work ? maybe go to 2x 8 walls  ??
thank you Patrick

Answer
Patrick,

I applaud your creative building style, and ability to find reuse opportunities.  Your idea raises several questions for me:

- How old are the windows?  (I'm guessing 15+ years)
- Do they leak?  (seems likely)
- What type are they?  (double-hung?  fixed/picture?)

It is true that more panes, if properly sealed, increase R-value, but there is a point of diminishing returns.  Triple-glazed windows are logical in very cold climates; quadruple-glazed not as much.

By far more energy is lost via air infiltration in leaky windows than is lost through the glass itself.  In other words, a tight new window (even single-pane) is better than multiple layers of leaky windows.  

You should also think about the purpose of the windows - are they for light?  views?  Each pane of glass lets less then 80% of light through (when spotlessly clean), so a double-glazed window lets 65% or so of the light in, and two of these windows (4 panes) would let in only 40%, and similarly obscure views out.  As your double-double setup would not be factory-sealed, it would behoove you to install them in a way that allows cleaning of all four surfaces.

By the way, you mention R-30 and 2x6 studs -- this can be achieved with R-19 fiberglass batts and a 2" layer of rigid foam on the outside of the studs, or by spray-foaming the stud cavity (R-value varies by foam type).  Do not compress R-30 fiberglass into a 2x6 cavity, as this defeats the purpose of the insulation and is worse than just using R-19 batts.

Good luck with your interesting cabin.  -Ted