When winter hits, it seems crazy to let cold air into the house. But some parts need a fresh supply — warm or cold — circulating year round. Without it, attics and crawl spaces trap moisture escaping from living areas. That fosters mold growth, degrades insulation and eventually rots wood timbers and plywood. Circulation is key, even in Southern climates.
To prevent those problems, conditioned spaces (parts that are heated and cooled) should be surrounded by insulation but also by vapor barriers. Usually thin plastic sheeting and sometimes foil, the barriers come with perm ratings (for permeability) and are designed to keep moisture from leaching through. But even new houses draped in huge sheets have leaks because the barriers have so many perforations, like dozens of electrical switches and outlets.
Moisture leaks
As the temperature drops so does the humidity. So why is moisture a problem? Because a household of four people produces as much as 3 gallons of water vapor per day — half from simply exhaling, and the rest from activities like showering, cooking, washing clothes and even watering plants. Picture the effects of just one gallon of water sprayed onto your attic insulation every day, how the fluffy piles could become a soggy mess of mold spores that no longer prevents heat loss.
Moisture seeping into crawl spaces can create even more problems because most are partially underground, weeping water if not full-fledged leaks through the masonry walls, and already damp. That’s trouble, because insulation between floor joists just above a crawl space is topped with a vapor barrier, leaving the fluffy batts below exposed. Without ventilation — that counterintuitive step of opening foundation vents instead of closing them — a crawl space can become a chilly swamp.
Heat leaks
Because heat rises, temperature loss down through an insulated floor into a crawl space is inconsequential. Problems lie in the other direction — even with batts piled up in the attic. Some heat gets through, carrying moisture along with it, particularly at openings like pull-down attic stairs and recessed light fixtures that can’t be covered with insulation.
Without ventilation, like cracking a gable-end window in the attic an inch or two, the moisture can cause condensation problems and the heat can cause ice dams. Winter ventilation keeps the attic temperature closer to the roof temperature and stops the melting.
The main culprit
The most common problem in attics is insulation between the floor joists that extends onto the roof overhang and blocks the vents. In that tight triangular space, some installers give the end of the batt a shove and who knows where it lands. Loose insulation pumped into attics will block even more air unless baffles are installed to stop it.
More Top Picks Dash Deluxe Electric
With either type of insulation, careless installation can block vents — and makes no sense on the roof overhang because there is no heated space below to protect. If not enough fresh air gets in, it doesn’t matter that outlets like ridge or gable-end vents are open. One of two things will happen. Either airflow will stop, fostering mold, or the outlets will start pulling conditioned air from living spaces. To maintain ventilation, batts and loose fill should stop directly over the house wall.
Basic fixes
Leave sliding foundation vents at least partially open
style=”list-style: none”>
Reduce moisture, say by insulating water pipes that drip
style=”list-style: none”>
Seal vapor barriers around outlets and switches with barrier tape
style=”list-style: none”>
Insulate to keep conditioned air out of vented spaces
style=”list-style: none”>
Don’t block vents with insulation