[Greenbuilding] foam-core wood window frames in Passive House(!)

[Bob Korves]

----- Original Message ----- 
From: "Stephen Thwaites" <stephen at thermotechfiberglass.com>
(snip)
> The Passiv Houses by any standard are very efficient energy efficient
> houses - heating energy is less than 15 kWh/m^2 per year!!
> The program requires the windows have a U of  0.8 W/m^2K or about 0.14
> Btu/ft^2F -- or R 7.1 ft^2F/Btu!!
>
> Any window that achieves that kind of insulating value needs to have
> insulation in the frame - hence the rise of the thermally broken wood 
> window
> in Germany and neighbouring countries
>
> But a couple of things for North Americans to rememeber before bemoaning 
> the
> fact we are 'behind again':
> - the climate here, with just a few exceptions, is sunnier than in 
> Northern
> Europe
>  -- so passive solar gain is even more important here than there
>  -- so the big bulky light blocking frames of the German Tilt and turn
> windows aren't a problem there, but represent a lost opportunity here
>  -- in other words you can use a slimmer framed window that isn't R 7.1 
> and
> get a lower heating bill than you would with a bulky, thermally broken 
> wood
> frame
> - European insulating values are calculated to a slightly different
> standard, so their window U values are a smidge ?5% ?10%?not exactly sure?
> lower than ours for the same assembly
> (North American stnds will eventually reference the same ISO stnd as the
> Europeans, but standards change very very very very slowly)
> - people in Europe are willing to pay alot more for windows than North
> Americans - even a non thermally broken wood Tilt & Turn window is about
> twice the price of the most popular North American clad windows - so you 
> can
> imagine the price spread when you start talking thermally broken wood
> frames.....(one reason they can afford better windows is that they live in
> smaller houses, but i digress even further....)
>
>
> To answer the question about the impact of an insulated frame:
>
> - insulated frame or not makes no discernable difference in a double 
> glazed
> window
> - my 'back of envelope' calculations say that in a triple glazed window an
> insulated frame could make about a 2 W/m^2 of window area difference over
> the heating season
>  -- so up here (Canuckistan) w/ a 200 day heating season, a 60 m^2 house 
> w/
> 15% of its floor area in windows, insulated frames on a triple glazed 
> window
> makes a (200 days x 24 hrs/day x 9 m^2 of windows x 2 W/m^2) 86 kWh
> difference -- if my math is right
>  -- Recall if you're building  a 60 m^2  Passive House you've got a 
> heating
> energy budget of  900 kWh
>  -- BUT a passive house has a shorter heating season so......insulated
> frames might make a  ?60 kWh? difference
>
> Does that help answer the question?
>
>
> Stephen Thwaites
> Thermotech Fiberglass Fenestration
> Ottawa Ontario Canada

In 'Movable Insulation' (1980) there is a Technical Appendix showing heat 
loss(gain) for windows facing north, south, east, and west for various 
American cities for each month of the heating season and for the season 
total for various types of windows -- single, double, and triple pane and 
also for each of those three with R-5 movable insulation applied 14 hours 
per night.

In every case the windows with movable insulation are more effective, which 
is no surprise, but it also shows that for equator facing orientation that 
the most effective window for minimizing total heat loss while maximizing 
total heat gain (with specified shading coefficients) is either single pane 
with movable insulation or double pane with movable insulation.  In most 
cases the two are about equal and are far above the other four choices. 
Triple pane was worse in every case.

The reason has to do with better transmittance of the sun's energy through 
fewer panes.  The book was printed in 1980 and there are lots of new windows 
since then, but for heat gaining, equator facing orientations all those 
special coatings likely do not increase heat gain, but rather only reduce 
loss.  An simple and effective movable insulation with single glazed windows 
can be much less expensive than premium windows.  Properly designed movable 
insulation can also make the efficiencies of sashes and frames much less 
important.

Note that this is only for equator facing windows that the sun shines on. 
And, of course, the movable insulation must be diligently applied and 
removed.
-Bob Korves