Construction - Window Installation

Most of the north and west exterior walls of the house are earth sheltered and the garage abuts the east wall.

Only 12 second windows were built instead of the 14
shown on the plans.  The first story windows stayed
true to the plans, making a total of 19 large and 1 small
 south-facing windows 

  Therefore, we would have been hard pressed to fit glazing into any of these walls.  But, no loss, we intentionally designed the house with all south-facing glazing (except for one small east window under the entry portico).  It took twelve 3' x 5' windows on the second story, four 3' x 5' and three 3' x 6' windows on the first story to meet minimum code for the ratio of glass-to-floor area.  The interior of the house is laid out so that all spaces except the bathrooms and the TV viewing area have windows.  The living room has no ground level windows but, since there are no second story rooms above it, it is generously lit by the second story clerestory windows.

 Green Features 

All the windows and the two exterior doors are designed for maximum sustainability.

     -  Energy efficient glass                -  Recessed profile               

     -  Energy efficient frames             -  Overhangs                

     -  Fixed vs.Operable                     -  Angle of incidence 
     -  Swing closure                            -  Peripheral seal            

     -  Translucent (frosted) glass        -  Glass-to-floor ratio

(Click on any photo to enlarge it for better viewing.)
    
Energy Efficient Glass
Our windows and doors have all of the bells and whistles,

like double glass and low-e coating, that make them first quality.  But, at +/-R-4, they are like holes in the wall compared to our fifteen inch, R-60 walls.  According to Daniiel Chiras in his book The Solar House; Passive Heating and Cooling, the problem is not just the glass -- about a fourth of heat gain or loss occurs through the frames, making it imperative that both the glass and the frames be energy efficient.

Energy Efficient Frames
A fiberglass frame has the same coefficient of thermal expansion as window glass unlike wood and vinyl frames (vinyl is by far the biggest mismatch).  When the frame and glass expand and contract at the same rate, air leakage between the frame and glass that might compromise the seal of the double panes or admit air into the living space becomes moot.  Unlike vinyl, fiberglass is resistant to UV rays, is much stiffer and accepts paint.  And it is made from sand, the most abundant substance on earth, rather than petroleum.

Fixed vs. Operable 
The top label at the right contains a map of the US with shading to indicate the area for which the window is designed.  Notice that it encompasses all but the southern reaches of the country.  The second label shows a reverse picture; only the southern reaches are shaded.  Fortunately, both labels cover St Louis.

Each label has a series of four numbers near the top.  The first number is the U-value which is an inverse expression of R-value in that the lower the number, the higher the R-value.  Notice that the U-value for the top label is lower than for the bottom label.

The two labels were photographed on side-by-side windows.  The top label is on a fixed unit and the bottom on an operable unit.  The fact that the maps and the U-values show an energy rating that is better for the fixed glass window validates our decision to limit the number of operable windows.  Since we will be depending on the energy recovery ventilator for most of our fresh air, having only 8 of 19 windows that are operable will be sufficient for additional ventilation when it is needed or desired.

Swing Closure
Windows and doors that are hinged prevent air infiltration better than sliding doors and windows.  To say it another way, casement, hopper and awning windows and hinged doors are more energy efficient than double- or single-hung windows or doors and windows that slide horizontally.  The reason is that swing doors and windows can be locked tight against a gasket-like seal while sliding units need a looser fit to be able to move up and down or side-to-side.  While the fit of modern sliding units are infinitely better than their ancestors, their air sealing still falls short of the gasket-type.

Frosted Glass
Greenhouses utilize the solar advantage of translucent  glass.  The glass in old-time greenhouses was painted with whitewash.  Newer greenhouses use "clear" sheet plastic that is, in reality, translucent in that you can't see clear images through it or they utilize translucent corrugated fiberglass panels.  Accordingly, we have 19 south-facing windows at least 3' x 5' in size of which more than half have translucent (frosted) glass instead of clear glass. 

The solar performance of the two types of glass differ in this way.  Sunshine through clear glass warms whatever it shines on.  If "it" is thermal mass, like concrete overlaid with ceramic or porcelain tile and of an appropriate color, the solar heat is absorbed into the mass without overheating its surface, which is a good thing.  If "it" lacks thermal mass (think wood or carpet floors, furniture and even drywall), "it" overheats and warms the air when it is not needed.  In a sense, precious heat is wasted.  

Frosted glass, on the other hand, diffuses (scatters) the sun's rays.  The heat rides the air currents within the dwelling until it either finds thermal mass or it conserves heat in the thermal mass by providing heat that would otherwise be coming from the thermal mass.  In our case, the thermal mass is the downstairs floor, the lower 10' of the long concrete back wall of the house and the soil under and behind them as well as the soil  beneath the insulation-watershed umbrella adjacent to the house.  Consequently, all of our first story windows are transparent because of their proximity to the thermal mass whereas all but two of the second story windows, where mass is missing, are frosted.

Recessed Profile
Today's new construction windows have nailing fins that position

the surface of the glass flush with the outside surface of the wall.  While this design eliminates air leakage between the window and the wall framing, it is not as energy efficient as it could be if the glass were not flush with the wall.  Here's why.

The rate at which heat is lost through the glass is dependent on the rate at which heat is removed from the outside surface of the glass.  If the heat can pass through the glass and linger for awhile on the outside surface of the glass, it slows the transfer of more heat.  What determines the amount of lingering?  Air movement.  Wind can remove the heat as fast as it forms and, since heat seeks cold, the rate at which heat is drawn through the glass accelerates, a phenomenon called "wind washing".

Our windows, by being positioned about 10" inside the plane of the wall and being south-facing, are sheltered from the prevailing north and west winter winds.  As such, the amount of wind-washing is about as minimal as it gets.  The red arrows in the nearby photo bracket the 15" thickness of the wall; the green arrows delineate the area on which the window rests; the blue arrows show the distance from the window to the outer surface of the wall -- about 10" after the trim boards are in place.

The distance between the top of the windows and the 2'
overhangs is greater for the first story than for the second
story but the difference in the amount of sunshine reaching
 the floors inside is negligible even for the taller windows
 in the foreground.  (The sheathing is protected by lumber
wraps and sheet plastic which will be replaced with proper
house-wrap and metal siding eventually.)

Overhangs
Sunshine through the windows is a blessing during the cooler months of the year but very unwelcome from May through September.  At our St Louis latitude, it takes a two-foot overhanging soffet to block the midday sun during the hot summer months.  

Early morning and late afternoon sun can shine under the overhang but, with the windows recessed, the adjacent wall offers shade in a way not possible with flush-mounted windows.  However, after the end of May and until the first of September, the sun angle increases enough that sunlight is completely blocked all day, even during the early and late hours. With the windows recessed, though, the sun that does reach the floor during late spring and early fall covers an area a third less than would be the case with flush-mounted windows.  As long as the overhang is reasonably close to the top of the windows, the height of the windows

Even with tall windows and an overhang a good
 distance above the tops of the windows, very little
 midday sunshine reaches the floor in early May;
 later in May none will.

is not critical. The nearby photos show the extent to which the sun reaches the floor during the first week of May through windows 6' tall and an overhang +/-2' above the tops of the windows.

Angle of Incidence

The same windows in late afternoon after the sun has
dropped low enough to shine under the overhang; the
amount of sunlight reaching the floor is restricted by
the recessed profile.

Another important concept in understanding passive solar is the angle of incidence, the angle at which sunlight strikes the glass in windows or the roof of a greenhouse.  The more perpendicular the sunshine is to the glass, the more energy absorbed by the glass.  The steeper the angle of the sunshine, the more energy reflected by the glass.  The practical take-away is that at least 90% of the sun's energy reaching a vertical window during the summer months does so at such a high angle that it is reflected rather than raising the temperature inside the house.  By the same token, the amount of energy absorbed through vertical windows at winter solstice time is at least 90% of what could be expected through glass that is tilted toward perpendicularity with the winter sun, as often seen in pictures of passive solar homes and greenhouses.  But tilted glass is not a good idea because it absorbs too much unwanted heat in summer unless it is covered with an exaggerated overhang, which is rare in the pictures I've seen.

In summary, our passive solar design utilizes three things to maximize solar gain when it is needed and minimize solar gain when it is not..  First, our windows are vertical rather than tilted for the reasons just discussed.  Second, we have overhangs designed for our latitude that provide adequate overhead shading.  And, third, our windows are recessed for lateral shading.

Peripheral Seal
The decision to recess the windows meant using "replacement" rather than "new-construction" windows.  No doubt about it, the latter with their nailing fins are better at blocking air leakage between the windows and the framing and sheathing that surrounds them unless the air sealing around replacement windows is done with extreme care.  

Sealing our windows was complicated by the fact that I built the housing for them as premade wall sections using custom jigs several years in advance of even ordering the windows. I deliberately made the window openings larger side-to-side and top-to-bottom than the window manufacturer specified because I knew that it would be extremely difficult to enlarge the openings once the units were in a wall.  As a result, the gap on either side of the windows averaged 1/2" and the gap at the top of the windows more like 3/4".  

All three sizes of backer rod --  3/8", 1/2" and 5/8" -- were
 needed in various combinations to close the gaps 
between the window frames and the rough openings on
 the exterior side 

At first glance, it would seem that all one would have to do for fool-proof air sealing is to fill the gaps with spray foam but even minimal expanding foam that is designed for this purpose is capable of distorting window frames if used in excess.  Rather the window manufacturer recommends that the foam be limited to a 1" strip located about 1" into the space between the window frame and the rough opening on the interior side.  On the exterior side, it recommends a course of properly-sized backer rod just inside the outer edge of the frame and, in doing so, leaving enough room for a bead of caulk.  Since our windows are inset, the caulk will be added when the jam extension between the window frame and the trim boards around the window opening are installed.  In this manner then, the spray foam provides the air sealing and the backer rod and caulk provide weather sealing.  The wide 3/4" gaps at the tops of windows were partially filled with plywood ahead of the foam and backer rod.

Glass-to-floor Ratio
In his definitive book, The Passive Solar Energy Book; A Complete Guide to Passive Solar Home, Greenhouse and Building Design, Edward Mazria recommends for our temperate climate (average winter temperatures of 35 to 45 degrees) a ratio of 0.11 to 0.25 sq ft of south-facing glass for 1.0 sq ft of floor area "in order to keep the space at an average temperature of 65 to 70 degrees during most of the winter".  Only our concrete first floor can be counted as floor area for calculating the ratio, so 2,800 sq ft of floor area to 354 sq ft of south-facing glass gives us ratio of 0.13, which falls at the lower end of Mazria's recommended range.  

Such a low ratio could be discouraging for us older folks who do better with temperatures in the mid-70s and the numbers get even worse because Mazria's figures for the temperate zone do not differentiate for cloud cover.  Here in the St Louis region, according to Daniel Chiras, only 47% of possible sunshine during the months of December, January and February actually reaches the glass.  Therefore, a conventional winter-centric passive solar house would definitely require supplemental (CO2-emitting) heat.  However, when heat coming through our windows during winter is added to the heat generated by our Annualize GeoSolar summer-centric system, it is almost certain that our thermal mass will maintain a comfortable year-round temperature hovering around our preferred 74 degrees without any supplemental heat.  Don Stephens, the father of AGS warns that too much heat could actually be a problem eventually that would necessitate mothballing part of the solar collector.  (For an understanding of AGS, click on "Featured Post" in the left column above.)

Installation
Not much needs to be said about the actual installation of the windows except for some considerations dictated by working alone.  Most of the windows were 3 x 5' and about, I would guess, 60-70 lb.  Knowing that I would be working alone, I requested that the double windows be shipped as individual windows rather than joined together with a factory-installed mullion.  Even then it was a stretch to hoist the second story windows to place without using a ledger on which to set them part way up.  By using temporary vertical stops against which to set the window, I could lift it into the opening without its falling outward and, by installing the stops exactly plumb and at the exact location of the exterior surface of the window in the rough opening, the window ended up automatically in the position it needed to be in except for plumbing in a left-right direction. 

The additional work required for the site-made mullions is at least partially compensated by the knowledge that they are more energy efficient than factory-made mullions would have been since they are filled with foam board insulation.