windows & thermo-mass

glass-to-mass ratio measures the interior mass (floors, walls, furniture) against south facing solar glazing. Designers must balance a interior mass with window area or risk overheating. Summer sun can be controlled to some extent by with sun awnings or screens (and if lucky, deciduous foliage), but only to a point. Winter temperature swings between mid-day and late night is a different matter.

Dan Chiras, an environmentalist and Colorado university teacher, explains how the balance between glass area and interior mass is calculated in his clearly written, comprensive book. (
The Solar House; Chelsea Green Publishing). The first 7% - 12% of glazing is balanced by the usual building material and furnishing mass such as drywall, flooring, timbers and heavy furniture (7 % is usual). If the south glazing area goes beyond 7% of total floor area, the house requires additional mass to avoid overheating. (

Beyond 7%, each square foot of south glazing needs 5.5 sq ft (0,51 square meter) of directly lit floor mass or 8.3 sq ft (7.71 square meter) of wall mass indirectly or directly illuminated. Clearly, well insulated buildings with masonry interiors and uncovered masonry floors can accommodate more glazing.

As to glazing areas on the remaining three sides of the house, Chiras gives these percentages: east - 4% maximum; north - 4% maximum; west - 2% maximum. Just bear in mind that solar heat retention is necessary to lesson temperature swings between sunny days and and night time or cloudy days, and that increasing the south window exposure alone will yield uncomfortable results. There must be sufficient glazing to let in sun light and enough mass to absorb the resulting heat.

Window construction is also critical. The important considerations are sash, glass and frame thermal conductivity. Aluminum and steel are thermal bridges and conduct out almost all heat and shouldn't be used. Not only are they as cold to the touch as the exterior, they cause condensation. Low-emmissivity, ie, low-e, coated glass is necessary and in some cases double or triple glazing cavities filled with inert gas (though the gases will escape at higher altitude). Vinyl and polyester windows perform well in most cases, though deteriorates and discolors during prolonged exposure to the sun. they also expand and contract with temperature swings causing seals to leak. Wood windows and aluminum-clad wood windows are the best as long as interior glass spacers are non-conductive.