Even with our tight budget, we intend not to compromise on window
and door quality because they are the weak link in heat transfer in and out of
the building envelope. For example, the
R-value of double pane glass is a paltry 1.5 - 2 while the R-value for our
exterior walls will be over 45. The best way we can
maximize the thermal performance of the windows and doors is by controlling air infiltration between the wall framing and the window.and between the window frame and the window sash. Then we need to control thermal bridging through the window frames and through the glass itself.
Air Infiltration
The two ways air infiltrates windows is between the window and the framing and between the window sash and the frame. In order to hold the window back from the plane of the exterior wall (see below), we will have to use replacement windows that do not have nailing flanges like that of new construction windows. Nailing flanges are the first line of defense against air leakage between the window frame and the framing. We will have to compensate for the lack of flanges with caulking and minimal expanding foam insulation and precise fitting of the inside and outside trim.
Air infiltration between the frame and the sash is easier to fix. Instead of sliding windows
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| Casement window |
Thermal bridging, a form of conductive heat transfer, occurs through both the glass and the
frames. Double glass marginally improves the R-factor of single pane glass by providing a dead air space between the panes. An R-2 is not much to write home about. Since windows are essentially poorly plugged holes in the wall, a better strategy is to limit the number and size of windows, particularly on the north and west sides of the house (US Midwest). Except for two or three small windows on the east, two of which are in the garage, all of our windows will face south.
The addition of argon gas between panes of glass also helps to reduce thermal bridging by impeding convection currents in the space between panes. The literature in my library and online information lead me to believe that argon is more hype than help because it escapes within a few years and therefore is not a good investment. Recently, the sales rep for the window company that will supply our windows convinced me otherwise. The argon in his windows is guaranteed to be 80% effective for the first 20 years and costs only about $10 per window. That sounds like a good investment after all.
Thermal bridging through the frame is easier to control than bridging through the glass by choosing materials that have low thermal conductivity. Fiberglass and vinyl are better insulators than wood and wood is better than metal so, if our budget allows, we will opt for fiberglass for both windows and doors. If not, vinyl will be a reluctant second choice at least for windows. Nevertheless, choosing vinyl would be a double-edged sword from a green building standpoint; while it minimizes thermal bridging it comes largely from petroleum. Fiberglass is a green choice for three additional reasons: (a) it has the same coefficient of thermal expansion
as glass so the seal between the panes and between the panes and the frame lasts longer, (b) it is many times stronger than
vinyl and (c) it is greener because it is made from the most abundant resource on
earth -- sand. Also, we will specify
warm edge spacers between the panes, which, at nominal cost, reduces thermal
bridging through the edges of the glass.
However, we will not opt for fully insulated frames because the payback
is not as quick here as in northern climates.
The addition of argon gas between panes of glass also helps to reduce thermal bridging by impeding convection currents in the space between panes. The literature in my library and online information lead me to believe that argon is more hype than help because it escapes within a few years and therefore is not a good investment. Recently, the sales rep for the window company that will supply our windows convinced me otherwise. The argon in his windows is guaranteed to be 80% effective for the first 20 years and costs only about $10 per window. That sounds like a good investment after all.
Thermal bridging through the frame is easier to control than bridging through the glass by choosing materials that have low thermal conductivity. Fiberglass and vinyl are better insulators than wood and wood is better than metal so, if our budget allows, we will opt for fiberglass for both windows and doors. If not, vinyl will be a reluctant second choice at least for windows. Nevertheless, choosing vinyl would be a double-edged sword from a green building standpoint; while it minimizes thermal bridging it comes largely from petroleum. Fiberglass is a green choice for three additional reasons: (a) it has the same coefficient of thermal expansion
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| Proprietary graphic comparing coefficient of thermal expansion |
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| Another proprietary graphic showing the strength of fiberglass relative to vinyl |
Winter Winds
Glass that directly faces winter winds is considerably more conductive than glass on the leeward side of the house because the wind removes the thin layer of insulating air on the exterior surface of the glass (wind-washing), thereby accelerating the loss of "fresh" heat through the glass. We plan no windows directly facing the prevailing winter winds. Our exterior walls will be 15+ inches thick which will allow us to recess the south-facing windows into the wall as much as 6" thus sheltering them from wind-washing more than if they were mounted flush with the wall.
The fact that the house will be earth sheltered on the north side and half of the west side will pretty much neutralize cold winter winds. Nevertheless, soon after purchasing our property years ago, we began establishing a shelter belt north and west of the future house as another buffer. And the eastern red cedars that we planted are native to our area and very beneficial for wildlife.
Tinting and Low-E
Exterior overhangs will shade the
windows from the summer sun but will allow winter solar gain. The gain will be maximized by using clear glass instead of tinted glass. However, we will specify
a low-E coating in order to slow heat loss out through the glass in winter. Admittedly, low-E will diminish slightly the amount of
solar gain in winter but our AGS system is the principal heat source and gain
through the windows will be welcome but not essential.
That said, we may leave off the second level overhangs for the first one or two years. It will probably take that long for the AGS system to charge the thermal mass with enough heat for a year-round constant floating temperature at a comfortable level. Accepting some summer solar gain to augment the output from the solar collector might be a good strategy.
Also, after the AGS system reaches equilibrium, we will assess the need for thermal window shades for nighttime and gray day use. If the system provides plenty of heat, they would be superfluous, if not, they can be added. Their function would be to keep moving air (convection) away from the glass and thereby slow conductive heat loss through the glass. Secondarily, less convection would make for a more comfortable living environment. In order to be successful, the shades would need to be sealed as tightly as possible on all four edges. Heavy drapes, regular window shades or louvered blinds help some but are not the same thing and, in some cases, actually enhance convection.
That said, we may leave off the second level overhangs for the first one or two years. It will probably take that long for the AGS system to charge the thermal mass with enough heat for a year-round constant floating temperature at a comfortable level. Accepting some summer solar gain to augment the output from the solar collector might be a good strategy.
Also, after the AGS system reaches equilibrium, we will assess the need for thermal window shades for nighttime and gray day use. If the system provides plenty of heat, they would be superfluous, if not, they can be added. Their function would be to keep moving air (convection) away from the glass and thereby slow conductive heat loss through the glass. Secondarily, less convection would make for a more comfortable living environment. In order to be successful, the shades would need to be sealed as tightly as possible on all four edges. Heavy drapes, regular window shades or louvered blinds help some but are not the same thing and, in some cases, actually enhance convection.
