Roof vents

Published on March 1st, 2016 | by Henri

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Roof ventilation

Q.    We are planning to replace our roof shingles (removing all original material) so I wanted to take this opportunity to improve the attic ventilation.

We currently do not have any moisture problems that we know of but the builder installed two types of ventilation systems, powered and passive. I’ve heard that only one type should be used, so I would like to improve one and eliminate the other. In addition, several of the current plastic louvers leak and need replacement.

My wife is questioning my plan to remove the 8 louvers and add another power ventilator and additional soffit vents. The bottom line is she wants me to run my plan by you first. What do you think?

A.    You are smart to listen to your wife; she is correct in questioning your plan. What I will recommend should save you money now and over time. Your detailed e-mail is very helpful; it has been edited because of its length.

The best ventilation system is passive, and consists of continuous soffit venting (so that every rafter space is ventilated), unobstructed air space to the ridge and an externally-baffled ridge vent.

Normal heat loss from the house into the attic, and attic air, heated by the sun on the roof, activate air movement from the soffits to the ridge.

Do not use any of the ever-popular roll-on ridge vents. The reason for insisting on an externally-baffled ridge vent (such as Shinglevent II) is because, when the wind blows, the baffles direct the wind over the ridge vent. This creates a suction that increases the ventilation of the attic – the Bernoulli Principle. In the case of unbaffled ridge vents, all ventilation is stopped because the wind enters the ridge vent and blocks the flow from the soffit vents (with some unbaffled ridge vents, snow and rain can also enter the attic.)

If you use the whole-house fan to cool the house in summer, be sure to cover it with an insulated cover in winter, as it is responsible for a considerable heat loss.

Consider replacing all existing plastic vents in the soffits with off-the-shelf metal soffit vent strips for the entire length of the soffits and getting rid of the plastic roof vents. You should also get rid of the power vents on the roof; in summer and winter, they draw conditioned air from the house, increasing your energy costs.

You will find that the recommended combination will lower the attic temperature significantly. Add several inches of blown-in cellulose to the attic floor; this will keep the rooms below cooler.

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