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

Sat Dec 1 15:38:27 EST 2007

A couple (or more) points, before answering the question.

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

----- Original Message ----- 
From: "Reuben Deumling" <9watts at gmail.com>
To: <greenbuilding at listserv.repp.org>
Sent: Thursday, November 29, 2007 4:34 PM
Subject: [Greenbuilding] foam-core wood window frames in Passive House(!)

> So here's a German fellow who made his own wood windows in a manner
> intended to minimize thermal bridging across the frames.
> <http://www.lrz-muenchen.de/~Passivhaus/3fenster.htm>  It is
> inspiring, but also appears to be a great deal of work. I'm curious
> whether any of you (Nick?) might be able to weigh in on the thermal
> significance of window frames. In other words, how to contextualize
> the effort required to do something along these lines in light of
> plausible btu/hr savings? Would the same thermally optimized house
> with and without these special window frames be very different in
> terms of heat loss?
> More photos at: <http://www.lrz-muenchen.de/~Passivhaus/3fenste1.htm>
>
> For purposes of calculation we could assume:
> 4,000 HDD/yr;
> 1,100 square feet of exterior wall at R-40 - single storey;
> 660 square feet of floor at R-27;
> 660 square feet of ceiling at R-60;
> 130 square feet of window with the best glazing available;
> --have I forgotten anything?
>
> Thanks.
>
> Reuben Deumling
>
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