Ventilation Issues In House
Proper ventilation in the attic:
- Keeps the house cool in the summer. (Attics radiate heat downward in the summer when they are hotter than the living area.)
- Keeps the attic cold in the winter, which can prevent ice dams.
- Allows moisture to escape from the house. Some warm, moist air will always find its way into the attic, so ventilation is essential. This is important year-round, for preventing mold and rot, but it is especially important in the winter, when the moisture is more likely to condense.
Be aware that increased ventilation decreases the effectiveness of any insulation that is a poor barrier to air infiltration, such as fiberglass batts. The increased ventilation will create low pressure areas, so that the house will push conditioned air through the insulation faster than it normally would.
With insufficient ventilation:
- Attic heat can penetrate into living areas during summer.
- There will be excessive humidity, which can cause mold and eventually rot.
- Water vapor can condense and collect on insulation, on rafters, and on the underside of roof sheathing. This will reduce the effectiveness of the insulation, and can greatly hasten the activity of mold and rot.
- Condensation and mold will also occur in the living area, especially on perimeter walls (because they are coolest) and where ventilation is poorest, such as in corners and around furniture.
You will need more ventilation than usual if:
- You live in a damp climate.
- Your house is in the shade.
- The crawlspace or basement has a dirt floor.
- There isn't much wind.
- You have a modern, super tight house.
- You have a solid masonry house.
- You have a house with impermeable siding such as vinyl or aluminum.
- You do not have sufficient (or any) vapor barriers.
Most houses treat the attic and basement as unconditioned space. You can think of unconditioned space as outdoor space, minus the rain and snow. The unconditioned space surrounding the living area shouldn’t be wide open, but it shouldn't be sealed shut either. A good compromise is to have two foundation vents in the basement and two different types of vents in the attic. Vents should always exist in pairs (but not necessarily two of the same type) to allow for cross-ventilation. In an attic, one member of the pair should be low on the roof, and the other member should be higher up, so that outside air is pulled through one and out the other. Natural attic ventilation through these vents is usually sufficient. Powered vents in the attic may interfere with proper furnace and fireplace venting.
Some ways to ventilate an attic naturally:
- Soffit vents.
- Ridge vent (you can cover the ridge vent with shingles).
- Gable vents.
Modern homes often incorporate all three types of attic vents, providing continuous cross-ventilation via multiple air pathways.
Make sure gable vents have screens to keep out insects and animals, and keep the screens clean to maintain proper ventilation.
Never close or block off the vents to a damp basement or crawlspace, except in extreme cold to prevent pipes from freezing. Closing the vents to a damp basement or crawlspace will cause mold, rot, and structural defects. Sometimes, a basement or crawlspace will look bone dry, but is transpiring moisture through the dirt floor at a rapid rate. To see if this is the case, lay down some clear plastic on the dirt floor for a few days and observe how much water collects on its underside.
If your basement or crawlspace is dry and has been dry for several years, you can:
- Close the foundation vents in the winter to conserve energy, and open them again in the warmer months, to allow interior moisture from the house to escape.
- Close the foundation vents permanently, install a polyethylene vapor barrier on the floor (just to be safe), insulate the basement or crawlspace walls, and part of the floor, if necessary, and include the basement or crawlspace as part of the conditioned space of the house. If you take this route, you don’t need to insulate the floor above the basement or crawlspace, but it doesn’t hurt if the floor is already insulated. Keep an eye on humidity. There will be less condensation on walls and pipes, but possibly greater humidity because of trapped air, requiring increased ventilation in the upper floors and attic to compensate.
Advantages of insulating a dry basement and crawlspace and making them part of the conditioned space of the house:
- Decreased condensation, because walls are closer in temperature to the air inside the house, and because cold pipes are not exposed to outdoor air during the warmer months.
- Reduced energy losses from ducts passing through the basement.
- Reduced risk of pipes freezing in winter.
