1) Closed and open cell foam respond differently to humidity – There’s a doc at Joe Lstiburek’s building science website that discusses sheathing rot attributed to foam moisture retention.
2) Actively conditioning a foamed attic may trigger additional fire protection code scrutiny…such as having to install drywall over the foam
3) No foam job is perfectly air tight – some outside air leakage is inevitable, especially in the case of retrofits
4) We are more and more often having to retrofit dehumidifiers into foamed attics, open cell in particular, especially in beachfront houses – they experience significantly higher average wind speeds.

Thanks Allison.

I believe we are going to close everything off and run electricity and plumbing for a dehumidifier should we have an issue – which we will monitor with a temperature and humidity monitor.

I did have an additional thought…a lot of the articles talk about the attic being conditioned because of duct leakage from HVAC systems in the attics. My thought pertains to homes without HVAC in the attic and no supply in that space either…

In theory, couldn’t you use a similar philosophy as exterior soffit vents as an interior solution to get air to pass freely between rooms into the unconditioned space?

Say for example you put a few of those vents covers in a hallway or bedroom(s) to allow air to get into the attic. Wouldn’t that with the already obvious leakage points (can lights, drywall seams, etc) provide sufficient air to condition the space…

The foam was so effective that we were quite lazy about closing the attic access folding stair assembly – we often left it down / open for weeks at a time.

As a precaution, I specified a smoke detector for that space.

@David Stockum wrote:
>If at this point we are locked into a spray attic ceiling…

Assume you meant spray attic roof deck…?

> Dehumidifier or introduce supply air

You definitely won’t need a dehumidifier with closed cell foam on the roof deck. The code does not require supply air in unvented attics where the roof deck is insulated with air impermeable insulation, and in any case, you definitely DON’T want a return vent in unfinished space.

If you seal potential ceiling leakage paths from bathrooms with a shower and any other high moisture areas, the attic shouldn’t need any supply air. If the attic has a walkable storage area, you may want to run a supply duct to that area for comfort. I can’t say how much supply air without knowing a lot more about the house, but you don’t need to heat or cool the area to the same temps as conditioned space. You can always add a damper or close the duct off if you find that you don’t need it.

We have a cross gable roof. The A section is a sandwich vault where joists and rafters are the same members.

If at this point we are locked into a spray attic ceiling, I’m wondering which path to take forward. Dehumidifier or introduce supply air. If we do introduce supply air, how much, and does it need a return vent or not?

Hard to do this without reviewing your plans, but I can answer a couple of your questions.

First, when I recommended against attic/roof fan, I’m referring to ANY powered fan that is designed to ‘cool’ a vented attic. They have been proven to cause more problems than any benefits.

Second, gable vents (at the top of the gable) are effective in lieu of ridge vents (but hip roofs do need ridge vents). However, without soffit vents, I’m not sure if your home would pass code. Depending on your home’s soffit design, it may or may not be practicable to add those vents. If not, you may be locked into an encapsulated attic. That’s not a bad thing, especially if you have a lot of light cans (which are hard, but not impossible to seal). But encapsulation is more costly than vented, all else being equal.

How you would vent the A vaulted ceilings depends on whether you have scissors trusses (i.e., ceilings are at a different angle than roof) vs a sandwich vault where joists and rafters are the same members.

Thanks for your response Dave. I just saw this after responding to Allison’s initial response.

I think I’m asking many followup questions that you touched on here in my response to him.

We have, unfortunate already added to canned lights (LED).

We do have the option of modifying our insulations plans as that works is still a few weeks away.

If I’m reading this correctly you are suggesting we leave the gables open, no attic/roof fan….but would you add a gable fan or no?

Then we would ensure the ceiling is tight and insulate the ceiling with blown-in fiberglass. All wall top plates should be sealed with spray foam.

Am I missing anything?

Sorry for all the questions. Trying to do the right thing and it seems everyone has an opinion.

Thank you for the response and followup, all.

I would not that at this point, the roof has just been finished and is sealed up. No ridge vent or soffit vents. We do currently have gables vents at the peak of the roof on either side that have yet to be closed.

The home is a ranch. Half the home has vaulted ceiling ( looks like the shape of an A) the other half is not vaulted and resembles the shape of a Triangle.

I would prefer not to go back and add a ridge vent or soffits.

Could we in theory leave the gables open and not spray foam the roof decking, instead using blow in insulation? …not sure how that would work in the Vaulted section (A).

If we do decide to go with an encapsulated attic, we close the gables and introduce supply air, do we also need a return vent?

Thank you all in advance!!!

Good points, David. In addition to a vented attic being more cost effective with no mechanicals in the attic, it’s also the safest. Here’s an article I wrote last summer based on a conversation I had with Kohta Ueno of Building Science Corporation:

The 3 Safest Ways to Insulate the Top of the House