Insulation

Published on October 19th, 2012 | by Henri

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Ventilation of a finished attic

Q.    We are finishing off our third-floor walk-up attic. My concerns are ventilation in the summer and heating in the winter. I was considering a roof-mount power vent, but our contractor does not think it would make much of a difference and the ridge vent we had installed should do the trick. I was also considering installing an AC/ heating unit to avoid the expense of bringing up the steam heat. I would appreciate your thoughts.

A.    I wish you had told me where you live; knowing the geographical location is often very helpful for giving more accurate advice. I can, however, provide very general advice. I agree with your contractor that a roof fan is not the thing to do; most of the time roof fans have a negative effect on both heat and air conditioning because there usually is not enough make-up air in the attic itself to satisfy the demand of the fan.

The best way to ensure comfort in your case is to have effective ventilation from the soffits to the existing ridge vent (ridge vents alone are not effective), a high level of insulation between and possibly under the rafters, and a very tight air barrier to prevent any convection of air from the finished attic into the rafter spaces. An alternative is to spray closed-cell polyurethane foam (do not use open-cell foam) between the rafters, but the lack of ventilation may affect the warranty and longevity of the roof covering.

A combination heating and air conditioning unit – or heat pump – is a good idea for the finished attic and it may be fine if your climate is not too cold.

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