This Old House - attic question

I was watching an episode of ask this old house. The house in question had a typical attic. It had exposed beams with insulation between the beams. It was not a “finished” attic. For the project on the show they were installing extra insulation. At the end of the episode the host recommended that since the attic did not have vents the homeowner should leave the windows under the eves open year round. My attic happens to have vents.

Here’s my question. Vents or no, why is it necessary to have your attic space be the same temperature as the outside air. While I understand it is unnecessary to heat an unfinished attic, wouldn’t your house leak less warm air if the attic vents were closed and the attic space acted as a buffer between the outside air and the heated home. What is the function of having those vents? Condensation maybe? Please satisfy my curiousity.

You want to keep the temperature under the roof as close to the same temperature as close as possible. It is to prevent ice from melting and refreezing under the shingles.

There are two concerns, with the first addressed by boytyperanma. However, one would think that “insulation between the beams” ( probably joists or rafters), intended to reduce heat transfer, would preclude that, at least to the extent of “R” value. If the show installed insulation between rafters ( under the roof surface) without soffit vents the roof will begin to self destruct.
The greater concern is transmission of water vapour from the warm side transferring and condensing on the cold side, degrading building materials, fostering mould growth and further lowering the rated insulation value.
Vapour barriers are tricky to get right on new builds and should be considered nonexistent on old construction.

Here’s a diagram of an ice dam that can form when the attic is warm and the outside air is below freezing. Can be a real problem.

Based on your question, I think you mean the insulation was being installed between the ceiling joists, which in a “typical attic” lie horizontally across the walls and form the ceiling support. The attic space in such a situation is uninsulated space, and in this case, unventilated space if there are no roof vents.

The problem is condensation. In the winter warm air from the home will penetrate into the attic because the there is not a good vapor barrier. When this warm and moist air hits a cold surface, the vapor will condense and the surface will become moist. If it’s cold enough, it will even form ice on the inside of the attic surface, usually on the rafters b/c the warm moist area is rising and that’s the first really cold surface. (The joists stay warm b/c they are contiguous with the room below.) Moist rafters are a problem–mold and mildew over time, and even rot–but ice forming on the inside is a disaster. It accumulates over time and when it eventually warms up enough to melt the ice, it will rain inside the attic.

This Old House should have recommended putting in proper roof ventilation. Eave vents–even gable vents for a large attic area–are not adequate insurance. I didn’t see the show so there may be more to it than I have inferred from your post. Proper ventilation allows the water-vapor-containing air to get outside before too much of the moisture condenses on the inside of the attic surfaces.

Here in Toronto, the building code requires at least one inch of air space between the insulating materials and the inner surface of the roof in order to prevent condensation on said inner surface. It seems counter-intuitive, but the logic behind it is sound.

When the insulation is put between the roof rafters (typically to get a cathedral effect) not only does there need to be a space between the insulation and the roof decking, said space should be open to the eave vent below and a roof ridge vent at the top so there is good flow of air for proper ventilation.

There are materials made to maintain this ventilation space and they should also be used so the the insulation material does not expand to fill the entire void. They are just a shallow trough of styrofoam or similar material that gets pressed or tacked in place between the rafters before the insulation is installed. Since insulation laborers are at the bottom of the food chain, subs get paid by the job and not the quality, builders don’t know or don’t care, and there is no easy way to inspect or figure out after the fact if a proper space was maintained, this is one of the commonest serious insulating flaws. The hapless homeowner wonders why her skylight is “leaking” when the real problem is condensation…

Yes this is what I meant. Horizontal ceiling joists with insulation in between. It wasn’t a cathedral or vaulted style ceiling. So this begs the question, how can you insulate a vaulted ceiling without having the condensation/ice problems?