[Greenbuilding] windows - flush vs recessed

Mon Jan 30 15:02:19 CST 2012

Agree completely. Window in the middle of insulation is likely (not proven to my satisfaction yet) the best location. Certainly we can call that "best practise".  I not only believe we need lots of education, I do run around both Canada and the US speaking to groups about just this stuff.  And it is remarkably common to see windows installed in masonry or steel stud walls completing missing the exterior insulation.

The problem is, various "best practise" ideas from Passiv Haus have become dogma (0.6ACH at 50 is THE number, U=0.8 windows, 0.3 ACH min ventilation, etc ), and then we get people who insist there is something magical about one aspect or another and that it is based on science.

As long as people are aware of the constraints, "window in the middle of insulation" is a great statement.

On 12-01-30 1:05 PM, Stuart Fix wrote:
> 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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-- 
Prof. John Straube, Ph.D., P.Eng.
Dept of Civil Engineering / School of Architecture
www.buildingscience.com