· Michael Blair
A sunroom is a glass box, and glass admits solar heat efficiently. In Middle Tennessee that is a benefit for a few months and a serious problem for several others, and orientation is the biggest single factor.
Orientation decides most of it
South-facing gives good winter sun at a high summer angle that overhangs can block. North-facing is the coolest and gets the least direct gain.
West is the difficult one: low afternoon sun straight through the glass in the hottest part of a Nashville day, and overhangs cannot block a low angle.
Glazing specification
The number that matters here is solar heat gain coefficient, not just U-factor. Our cooling season is long, so rejecting solar gain matters more than in a heating-dominated climate.
Low-E coatings tuned for solar control make a large difference on west and south glass, and roof glazing should be specified more conservatively still.
- Prefer north or east where the site allows
- Low SHGC glazing on west and south elevations
- Insulated roof rather than glass roof in this climate
- Overhangs and exterior shading beat interior blinds
- A ceiling fan makes a substantial difference
The glass roof question
A glass roof is the most dramatic option and the hardest to live with here. Roof glazing takes the sun at its most intense angle in summer and there is no practical way to shade it from outside.
An insulated solid roof with skylights gives much of the light with a fraction of the heat.
Making an existing one usable
Exterior shading — an awning, a pergola, planting — works far better than interior blinds, because it stops the heat before it is through the glass.
Applied window film reduces gain measurably and is the cheapest retrofit. A ceiling fan and a dedicated mini-split make the difference between a room used sometimes and one used often.