[Greenbuilding] windows - flush vs recessed

Mon Jan 30 12:05:49 CST 2012

Hi John,

I think the biggest thing to take from the PH mantra of 'windows in the
mid-insulation plane' is just as you've pointed out, don't be stupid and
put the windows outside of the insulation plane. However, it's still quite
common to do this 'stupid' thing. Even some of our net zero energy homes
here in Edmonton have mounted windows on the concrete layer of a
foundation wall for example.

You might call that stupid, but if it's still common practice then it's an
issue for education. I don't think you'd argue that the BEST practice from
a thermal loss point of view is to place the windows in the mid insulation
plane. Just knowing what is Best Practice is important, and if you're
trying to squeeze everything you can out of an envelope, then you do
what's necessary.

You're right, moving the windows slightly off the mid insulation plane
will only slightly degrade the thermal performance of the assembly. Do you
care? Depends on the project.

The PH folks are only presenting what is thermally optimal, it's up to us
to balance everything else into the equation. I fully agree that the
written PH material is crap and their example assemblies are about 5%
useful in North America. But the science is sound, and I'd prefer to laud
them for showing us best practice in efficiency than slag them for being
too anal or for losing things in translation across the ocean.

Cheers,

Stuart Fix, P.Eng., LEED® AP
PHI Certified Passive House Designer
MASc. Building Science
Mechanical Engineer
ReNü Building Science Inc.

#206, 506B St. Albert Trail | St. Albert, Alberta | T8N 5Z1 | C.
780.554.8192 | sfix at renubuildings.com

-----Original Message-----
From: greenbuilding-bounces at lists.bioenergylists.org
[mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of John
Straube
Sent: Monday, January 30, 2012 10:17 AM
To: Green Building
Subject: Re: [Greenbuilding] windows - flush vs recessed

I have heard numerous sources of "window must be in middle", but page 5 of
your reference is an excellent example. Thanks for pointing it to me John.
The presentation is full of interesting and useful information, but also
some obviously incorrect stuff.

The difference is U=0.85 vs 0.83 W/m2K for the whole window. This is R6.68
vs R6.84 or R0.16.
This is beleivable, although the THERM model is not realistic for the
first two models (no space around window) and dangerous (how will water
drain out at head? how will it drain at sill?)

On page 6 the author leaps to a 22% difference.  Clearly not possible with
the kind of numbers she presents on page 5. Essentially unbeleivable
unless there is a large window area and a massive thermal bridge. Or
calculation errors.

On page 7 she shows another therm analysis. Again, one that traps water,
but likely buildable.  No numbers given. Likely the window detail she
planned on using

On page 8 there is a bunch of gobbly gook.  I can see the she is using
292 meter (3000 feet!) of window edge. This is about ten times or more
what a house of the size she is looking at would be.  The addition seems
odd. U is a measure of heat flow per unit area, you cant multiply by
length, L!  You can add psi (linear heat loss) to U (area heat loss), and
this is normal.  Of course, the answer is a simple UA(wall) + UA(window)
+psi (length of frame-wall intersection). Very worrisome.

pp 9-11 have good examples which one rarely buildings (covering the frame
with insulation on top and not the bottom makes the window look odd to
most folks, and is challenging to build and maintain) but notwithstanding
some subtlety about details, all those curves show a typical and expected
result:
-the thermal bridging impact is small and not far from optimal if you
avoid installing window in uninsulated part of wall.
- locating window near the middle of the insulation is usually best
location and may reduce heat flow by as much as 2-3% versus near the out
edge or inner edge of the insulation.

p 12. Finally a real window with flashing.  Good warning: using a chunk of
steel that penetrates is bad.  Use plastic or tar-based products when you
penetrate the insulation layer. Oh, note there is no insulation attached
to the face of the window frame.

Equation on page 16 is ridiculous. If only it were so easy. It could be
correct for a certain type of building with a specific window area in a
specific climate, but almost certainly wrong for most of the US and
Canada.
Pg 18 ignores the overheating during warm spring and fall days that are
sunny.  SHGC=0.6 can work in many cold to cool climate homes, but only if
the window area is tightly constrained or operable exterior shade is
deployed. In Zones 4A or less, choosing this SHGC will result in very
small windows or cooking in the swing seasons.

So, this is further evidence that middle of wall is not critical, but may
be best by a couple percent. Windows installed outside the insulation
layer can be significant thermal shorts, cause comfort and condensation
and must be avoided.

While all the details matter, the big heat losses matter more, and
designing for the last 100 Watts of heat loss is somewhat misplaced when
there are so many other important building details that need concern
(health, safety, durability affordability, maintenance etc) Even heat loss
up the plumbing drain stack is likely more significant than a "decent"
window install.

On 12-01-30 10:48 AM, John O'Brien wrote:
> On Mon, Jan 30, 2012 at 8:47 AM, John Straube<jfstraube at gmail.com>
wrote:
>> Hiya George. Of course I know it is per unit length, as I quoted the
>> units of W/mK So if I have a 5' by 4' window (1.5 x 1.2m) the
>> additional heat loss of 0.01 W/mK is 0.054 W/mK or in 0 F weather, 2
>> Watts. Lets say we allow us 0.03 W/mK difference between the ideal
>> location and the location preferred by the owner, or builder, or the
>> window durability.  Now we are up to 6 Watts/window. In a house with
>> 12 windows, this is 72 Watts, at DESIGN CONDITIONS for Zone 5, and we
>> have 2000 W flowing through the windows themselves.  The prescence of
>> absence of overhangs and neighboring buildings will likely have this
>> much ipact on heat loss.
>> The real reason to worry about this stuff are the examples of when it
>> is done horribly, and you have a 0.2 W/mK (which works out to
>> 41W/window or 500W for 12 windows at design) detail and you start to
>> risk condensation on the window edge.
>>
>
> http://passivehouse.us/passiveHouse/2010_Passive_House_Conference_Pres
> entations,_November_5_files/2010%20Conference-Windows%20Roundtable-Bro
> nwyn%20Barry.pdf
>
> Where most of the 'window should be in the middle' has been coming
> from. Anyone want to take a stab at her numbers?
>
> She claims "By moving the windows to the center of the wall the Annual
> Heat Demand was reduced by 22%."
>
> Seems somewhat not insignificant.
>
> J
>
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--
Prof. John Straube, Ph.D., P.Eng.
Dept of Civil Engineering / School of Architecture www.buildingscience.com

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