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?