What Windows Where

A few general rules can be applied based on your home’s orientation. In most climates, south-facing windows should allow as much winter sun in as possible and have a high SHGC paired with the lowest U-factor you can find. Properly sized overhangs or adjustable exterior awnings, shutters, or other structures help mitigate summer heat gain. Insulated shades can also reduce cooling loads, although they aren’t as effective as blocking the sun before it enters the house.

In general, east- and west-facing windows should block as much sun as possible, so you’ll be shopping for the windows with relatively low SHGCs and U-factors, although windows with SHGC less than 0.35 may appear too dark. In places with high summer cooling loads, east, west, and even north windows would ideally also block as much sun as possible, as early morning and late afternoon solar gain can be substantial in mid-summer. In most climates, northern windows should be limited and as insulating as possible (lowest U-factors).

The guidelines for installing highly efficient windows (such as triple-pane units) are the same as installing insulation elsewhere in your home. If you live in a climate with high heating and/or cooling loads, then the purchase will pay back quickly in reduced energy costs. For many locations, investing in additional insulation is the best way to reduce carbon emissions compared with other renewable energy purchases, but also to yield the highest annual financial return for your investment. Knowing your climate’s total degree days (TDD), a combination of heating and cooling degree days (HDD and CDD) is essential. Homeowners who live in climates with TDDs above 4,000 should invest in the windows with the lowest U-value possible.

Windows on the north, east, and west walls should be minimized to achieve maximum thermal performance.	Huge expanses of south-facing glass can result in too much nighttime heat loss. Instead, a well-placed area of low U-factor and high SHGC glazing does the job.