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Homefix: Negative pressure for your home

Question: We had a problem with ice buildup on our roof this past winter and water damage inside on the first-floor ceiling. We had a roofer come in and remove some of the shingles on our roof and some siding. He installed an ice shield, because it was not installed correctly before. We spoke to an insulation company, and they want to blow foam insulation onto the roof deck on the second story and as much as they can reach on our room addition (first floor with a cathedral ceiling). They want to seal the attic roof completely, sealing all air vents, soffit vents and gable vents. They claim because it is foam and not fiberglass, we do not need ventilation in the attic.

Have you ever heard of this process? They will also move the venting from our bathroom fan out the side of the house. Currently, it is vented to the attic and is covered with fiberglass insulation. They will leave the fiberglass insulation in the floor of the attic, but pull off or cut the vapor barrier from one of the layers. Should we do something different in the attic? Is there something else we should do to correct the ice problem?

Answer: Attics require ventilation to remove moisture and heat. If the underside of the roof's decking is insulated with an expanding-foam insulation, the air in the attic space will be within a few degrees of the temperature of the rooms below the attic. When there is no significant temperature difference between the attic and the conditioned rooms, the moisture levels will be similar.

In other words, you do not have to ventilate a conditioned attic. Ideally, the loose-fill insulation on the attic floor should be removed so that the attic space, the attic floor and the ceiling of the rooms below the attic will be the same temperature. The bathroom fan and any other vent fans need to be vented to the exterior of the home. The purpose of the fans is to remove moisture and to create a negative pressure inside the home.

When the interior of the home has a negative pressure, fresh air will enter the home through voids around windows and doors, openings in the exterior walls at outlets and switches, poorly sealed wall framings, etc. It is important to supply a tightly sealed and insulated home with fresh air. Once the insulator has foamed the attic, I would strongly recommend that you have a blower door test to verify the home's ability to exchange the inside conditioned air with the outside (fresh) unconditioned air. A home that is insulated and sealed may require the installation of an air-to-air heat exchanger to provide fresh air to the home's occupants.

Question: I know you've addressed this issue before, so I apologize for having to ask again. Are crawl-space vents always supposed to be closed? Or does it depend on the season of the year?

Answer: Simple physics tell the story of the foundation-venting solution: 1) Heat moves from hot to cold. 2) Moisture moves, in the case of a crawl space, through the air from more to less. 3) Cold air cannot hold as much moisture as warm air; therefore, warm air is humid and cold air is dryer.

If you live in a warm, humid climate, the vents should be closed or eliminated completely because the outside air will always be more humid (moist) than the crawl-space air and the humid air will travel through the crawl-space vents in order to equalize the moisture content of the air. If you live in a cooler climate, the foundation vents should be open so that the cool, moist crawl-space air can move to the colder outside dryer air. If you live where there are four distinct seasons, you need to open the vents in the winter and close the vents in the summer.

No matter where your home is located, a better solution would be to eliminate the crawl-space vents by sealing them from the crawl-space area using a 2.5-inch-thick rigid foam insulation. Insulate the crawl-space foundation walls, the ends of the floor joists that are exposed to the exterior and the crawl-space entry door. Again, use a rigid foam insulation fastened to all of the exposed foundation walls. Cover the exposed crawl-space floor with a 6-mil-or-heavier vapor barrier, and seal all ductwork at joints and seams.

Finally, cut a small, 4-inch opening in the supply duct and cover the opening with a pest screen. This will allow the heating/cooling unit to condition the crawl space in order to control the humidity levels. You should consult with a heating and cooling contractor to make sure the HVAC system is adequate for the crawl-space ventilation.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors.