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Venting alone can't keep upstairs cool

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Question: My wife and I own a 21/2-storey house in the West End, and we need to replace the roof in the next couple of months because it is leaking. The fir planks underneath are in good condition, according to an inspector who looked at them from the inside, so only spot repairs will be made to specific boards that may be rotting. We plan to have all the shingles removed, a proper waterproof vapour barrier laid down, and then if we can afford it, have a metal roof installed directly onto the vapour barrier surface. There will be no strapping and therefore no gap between the metal and the sheathing or vapour barrier. My question has to do with venting and keeping the third floor cool.

I had an estimator from a local company come out, and he suggested that I didn't need anything apart from the ridge vent on a metal roof, and that we may not need any roof vents if we only go with shingles. The reason being that we plan to gut the entire third floor and do a good layer of spray-foam insulation inside before finishing it with two bedrooms and a full bathroom. I am far more concerned about keeping the third floor cool in the summer rather than warm in the winter. Right now, with the insulation being sawdust stuffed up behind the plaster walls between the roof joists, it is an absolute sauna in the summer. I would have assumed we'd need to vent like crazy to stop the buildup of hot air, and we even considered putting bathroom exhaust fans, vented out to the exterior via roof vents, in all rooms on the third floor just to give us a way to clear out the hot air.

The finished third storey will have a three-foot knee wall on the exterior wall of each room, but it won't be a sealed wall because we plan to build drawers and shelves into the knee wall.

Thanks for your help, Bjorn Radstrom.

 

Answer: Many homeowners confuse attic ventilation with proper air flow through the interior of a house. Both have some similar functions in summer, but also differ in fundamentals during the heating season. I will expand on this and suggest the proper methods for insulation of the upper floor of your home.

Most attics in our climate can be effectively ventilated with a combination of passive vents. These vents on a typical hip or gable-style roof may be located either on the peak or the roof, in the soffits or at gable ends. Passive vents normally work by a combination of convection and wind. In most cases, warm air that collects in the attic under a peaked roof will rise and vent out through roof vents. This will often be aided by cooler air that is drawn in through the soffits, which helps push out the warmer air as it rises near the peak. Gable vents can also aid in some houses by allowing wind to blow through attics, removing stale, damp air that may be stagnant if minimal soffit ventilation is available. All of these factors are important on very warm, summer days, but are critical in the cold winter to prevent condensation and moisture damage in the roof system.

While these attic vents will help cool a hot attic in the warmer months, this may have little effect on the interior air quality or temperature. Because warm air rises, the heat from a very warm attic will not heat up the upper floor, except for some possible radiant heat in a very poorly insulated ceiling. Adding more attic vents should have little cooling results for your upper floor. Adding high-density blown-in foam in the attic, roof system, and wall cavities may provide some more relief, but will not provide the results you desire. That is because warm air will also rise inside the home, just as it does in the attic. If your house walls are poorly insulated, and doors and windows older and leaky, warm air from outside will work its way into the living space. This air will heat up further from passive solar heating and indoor activities and will rise to the highest point in the home, the upper floor. Opening strategic windows to vent some of this warm air will help, but is not the final answer.

The only way to cool the upper floor of your home is with mechanical means. This will require the use of either window or central air conditioners, especially on the upper floor. Even with good insulation and ventilation on the upper floor and attic, interior air currents within the home may make the upper floor unbearable on hot summer days. Air conditioning on the upper floor will cool and dehumidify this warm air, allowing it to slowly fall to the lower floors. This cooler air may even help to reverse the normal convective currents within the home, making the entire building more comfortable on the hottest days.

Adding roof vents to the upper portion of your currently designed roof system will be important, but future needs will depend on where you are planning on installing the new foam insulation. If you leave the small, flat ceiling in the upper floor and insulate this area as well as the rest of the space in between the rafters, the vents can stay. But if you are planning on removing the ceiling and changing to an insulated, vaulted ceiling, there will be no need for vents as the entire roof cavity will be filled with foam.

While it may seem odd with a very old home such as yours, air conditioning may be the only effective way to make the upper floor cool enough for your liking in the heat of the summer months. Insulating the roof system with modern foam and adding attic vents may make sense for many reasons, but it will not make the upper floor much cooler in July and August. Only mechanical air conditioning will provide cooler, dryer indoor air for your living space.

 

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at www.trainedeye.ca.

trainedeye@iname.com

Republished from the Winnipeg Free Press print edition September 21, 2013 F10

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