Should I actively circulate air into conditioned attic?

Question

I'm almost certain that I'm going to seal my attic using closed cell foam and make it a conditioned space. I intend to have a professional spray closed cell foam to the underside of the roof deck. I will not be finishing it in any other way.

I want to do this because properly sealing the living space ceiling is a nightmare with so many and light penetrations, plus the cieling heights change throughout. There are currently laid Batts which are insufficient. Additionally there have been repeat pest invasions so I'm hoping address multiple problems here.

The roof is tile, vented with intermittent soffit vents and a few off ridge vents.

Once the attic is brought into the conditioned house envelope should I install systems to actively circulate its air with the rest of the living space below?

Answer 1

Here’s an interesting article on this subject by Joe Lstiburek of Building Sciences. In short, he says there has to be air circulation between the house and the attic in order to remove moisture from the attic. In older installations, leaky ducting in the attic provided enough circulation. In newer installations, ducting is often so well sealed that moisture is not removed from the attic and can lead to problems.

His recommended solution is to provide HVAC inlets and outlets in the attic which provide about 50 sfm per 1000 square feet, which is about 1/3 to 1/2 air changes per hour. However, there is a conflict with current building codes because spray foam is combustible and the codes do not allow vents in an attic with exposed foam insulation. His recommendation is to change the building code to allow this practice, as long as a smoke detector is installed in the return air duct. His article says 2018 is the earliest that change could be made - I don’t know where it stands.

I think the conclusion is that if your ducting is a little bit leaky you’ll be fine, but if it’s really well sealed it gets very complicated.

Answer 2

There is a new house under construction in my neighborhood in which the insulation envelope will be under the roof decking. The builder told me the attic temperature will be much cooler than the ambient on a hot day, but he DID NOT say the attic would be conditioned space. It did not occur to me it would be, but maybe it will have some small amount of conditioned air going into the attic to keep it dry and within some limits which would be much wider than for the living space.

I don't know how the attic is going to be vented. I had assumed it would have controlled venting from the outside, but maybe it is from the a/c and furnace in winter. There are no sofitt vents anywhere and I see no roof vents for the attic air. The only roof penetrations I see are sewer vents, tankless water heater vents (separate coaxial vent/combustion air for two tankless water heaters, each 180 kBTU/h), kitchen hood exhaust vent. The clothes dryer vent is not going out through the roof.

The builder told me that the insulation will be spray on/in cellulose. I assume this type of insulation will be open cell to allow drying to the attic so that water from small roof leaks will not be trapped under the roof underlayment in the OSB decking and around the roof rafters.