· Michael Blair
Solar heat gain through glass is a large share of the cooling load in a Nashville house, and window treatments vary enormously in how much of it they actually stop.
Interior versus exterior
Once sunlight is through the glass, most of its energy is already in the room — an interior blind absorbs it and re-radiates it as heat. Exterior shading stops it before it arrives.
That is why an awning or a shade sail outperforms any interior treatment, and why exterior solar screens are the most effective retrofit.
What works inside
Cellular shades trap air in their cells and are the best-performing interior option, particularly triple-cell products. Fitted close to the glass with side channels they perform substantially better still.
Reflective backings help by sending energy back out through the glass rather than absorbing it.
- Exterior shading always beats interior
- Cellular shades are the best interior performer
- Side channels and a tight fit matter a great deal
- Reflective backing reflects rather than absorbs
- Low-E film is a cheap effective retrofit on west glass
The west-facing problem
Low afternoon sun comes in almost horizontally, which is why overhangs do nothing for it and why west rooms overheat in a Nashville August.
Vertical shading — exterior screens, a well-placed deciduous tree, or heavy interior treatment used consistently — is what addresses it.
Consistency matters
A treatment only works when it is closed, and a west-facing room needs its shading closed through the afternoon whether anyone is in it or not.
Motorisation on a timer or a sun sensor is genuinely useful here rather than a luxury, because it removes the requirement to remember.