How to Use Thermal Trombe Walls Properly

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As with all passive solar design options, Trombe walls are a great addition to your home design: they retain a lot of heat, helping to reduce your nighttime heating costs throughout the cool period of the year. However, again like all other options, they need to be used correctly or they won't do you any good.

  • Only use Trombe walls facing directly true south or within a 10° angle.
  • Make sure the wall material can hold a lot of heat: brick and concrete are ideal. Concrete blocks filled with sand also work well. Rammed earth or adobe are fine, but not as good.
  • Use the best possible glazing: ideally use a double pane of high-performance glass.
  • Only use single glazing in the mildest climates.
  • In colder climates, target very low U-value glass with low-e coatings and, if you can afford it, noble gas filling in the glazing air space.
  • Make sure the glazing frame is thermally separated from the wall, or you will get heat loss from the wall via the frame and glass. Wood is good for this, though it will take a serious hammering from the intense heat in the air space (150°-190°F, or 65°-87°C). Metal is a bad choice.
  • Use the best caulk and sealants you can get in Trombe wall construction. They must be able to withstand significant expansion and contraction.
  • Use a selective material on the exterior surface of the wall to absorb all the energy possible and thus release less into the air space (and give better solar gain). This is especially important in very cold climates, and reduces the need for external insulation panels.
  • If you can't afford a selective surface, choose a paint that can resist high temperatures.
  • Don't cover the interior side of the wall more than necessary: Earthen, gypsum or lime plaster work well for heat transfer. Concrete and stucco are good, too. Drywall only works if it's fitted tight against the wall and not furred. Note that these products outgas potentially toxic chemicals, except earthen and lime plaster, and that lime plaster is dangerous to work with.
  • Don't skimp on insulation. It's a must in almost every case.
  • Go for rigid foam panels on the outside or internal insulation between the glass and the wall. They are both effective, though the former is tougher to install than the latter. Internal insulation is more complicated because it must not affect the wall's operation.