[Greenbuilding] windows - flush vs recessed

[Stuart Fix]

Hi all,



I was forwarded this thread and decided to join the conversation. What I’ve
read below about PH recommending windows to be installed in the middle of
the wall isn’t quite correct, the recommendation is to install the window
centered over the mid-Rvalue plane of the wall. So that’s the middle of a
double stud wall, or further to the exterior of a REMOTE style wall. The
whole point is to keep the isotherms as parallel as possible through the
transition between window & wall, or in non-geek speak it means to minimize
the temperature differences between window & wall components.



Overinsulation on the exterior is quite common with inward-opening European
windows, but it’s quite tricky to do with outward opening North American
windows. Interior insulation on North American windows is also tricky since
it can cause condensation to form on the interior side of the frames. North
American windows tend to have slim frames anyways, which leaves less of an
area to over-insulate and makes the pain less worth the gains.



Regardless though, I agree with the caution about ensuring proper
installation design details. PH-level design is all about detail control,
from both thermal and moisture points of view.



Cheers,



Stuart Fix, P.Eng., LEED® AP
PHI Certified Passive House Designer
MASc. Building Science
Mechanical Engineer
ReNü Building Science Inc. <http://www.renubuildingscience.com>

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



*From:* greenbuilding-bounces at lists.bioenergylists.org [mailto:
greenbuilding-bounces at lists.bioenergylists.org] *On Behalf Of *Bill Allen
*Sent:* Monday, January 16, 2012 3:28 PM
*To:* jfstraube at uwaterloo.ca; alan at abramsdesignbuild.com
*Cc:* greenbuilding at bioenergylists.org;
greenbuilding at lists.bioenergylists.org
*Subject:* Re: [Greenbuilding] windows - flush vs recessed



John,

Might you have a similar sill detail for the case where you are
retrofitting external insulation and not changing the original location of
the window?  That's what I'm looking into.

Thanks

Bill





On 01/16/12, John Straube<jfstraube at uwaterloo.ca> wrote:



The most common detail I have seen (mostly in PH details, but is one which
uses an undrained EIFS over the window.
The flashing you are talking about is just a rain shedding / sun shield /
esthetic finish. Water leaks past those, and as a result leaking windows
are the number one moisture problem in buildings. The flanges and grooves
are not far enough in to deal with rain leakage in most exposed locations,
they are just the rainscreen.
The drainage plane must come under the window (aka subsill flashing - see
attached simple sketch for one mid-performance solution), regardless if it
is flanged (most common in US residential windows and a lot of Canada) or
not (most commercial, all European, many Canadian residential windows).

The low cost solution is often to make the the face of the exterior foam
the drainage plane. This works well with exterior flanged windows, but can
also work with plywod bucks.
The higher performance solution is to place the drainage plane behind the
foam (which can be rockwool sheathing in this arrangement) but outboard of
the sheathing and framing (which are the components that need protecting).

We have taken flanged windows and attached them instead with metal tabs, as
sometimes this installation method is desirable.

I dont have the NIST house drawings on my laptop, but will try to get them.
Have lots of site photos: see peel and stick membrane air and water barrier
wrapping inside the window opening. When you add R25 on the outside of
this, you get a very low U-value enclosure.





On 12-01-16 10:26 AM, Alan Abrams wrote:
>
> One design to avoid at all costs is the one which covers the window
> frame with exterior insulation, therbey trapping rain leaks in the
> wall (but reducing building heat loss by as much as 1%).
>
> John
>
>
>
> what this suggests then is that the drainage plane is outside the
> exterior insulation, right? I'm trying to recall the arrangement in
> the NIST net zero test house, which has a conventional frame wall
> with a membrane applied to the structural sheathing, and then two
> layers of foil faced foam, then furring and siding. but (having
> loaned the drawings to someone who did not return them) I can't
> recall where the windows occurred in the section, and if the membrane
> was the primary drainage plane.
>
> wrt to protecting inner layers of wall when windows are set in from
> the exterior plane--note that conventional US clad windows, such as
> Weathershield, have a groove to receive metal flashing in the
> outermost edge of the projecting part of the extrusion.
>
> conversely, in a passive house under construction in Arlington VA,
> which I visited yesterday, the windows had no flanges, but were
> attached with tabs that projected toward the inside edge of the wall.
> integration with the rainscreen was not complete yet, but seemed to
> rely heavily on butyl tape--whereas the PH detail manual--and other
> photos I've seen--feature metal extensions at least at to cover the
> sill. (btw, the Arlington windows were untreated vinyl by Intus,
> rated at +/- U-0.14)
>
> -a

-- 
Prof. John Straube, Ph.D., P.Eng.
Dept of Civil Engineering / School of Architecture
www.buildingscience.com
------------------------------


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