@Danny, I was attempting to be responsive to Cajunjb1’s concern re: open return. As you know, I’m no fan of spray foam insulation (mostly based on economics), but you appear to be answering a different question: whether spray foam should ever be used in the first place.

In re foam risk, I am reminded of Peter Sandman who says,

“Risk = Hazard + Outrage”.

What’s more frightening to me is; we don’t really know as much as we pretend we know about the “hazards” of spray foam. Add to that, how little we know about human physiology and even a smaller fraction about human psychology (“outrage”).

The medical literature does seem to support a condition known as toxicant induced loss of tolerance. Everything is fine. Then one day the tolerance bucket fills up and the person can’t tolerate colgonge, deodorant or walk down the detergent aisle at the Walmart. Let’s face it, us Amuricans seem to love our chemicals.

Since our metric for “no problems” often relies on “no complaints”, the tolerance bucket can be a game changer. Our anecdotal assumptions can be quickly turned on their head and we now have exponential liability exposure. More like,

Risk = (Hazard x 100) + (Outrage x 1,000) or more if a child is involved.

David, absolutely, there would be no difference in the separation layer. Would not wish upon anyone to go through a botched foam prep.

Yeah, latex paint provides some vapor retarding properties, probably depending on the particular paint, number of coats, sheen, etc. Still, in my view, it would be like living next to a brownfield site and hoping that some barrier will hold. The outgassed substance would have its own vapor pressure and would seek equilibrium.

To any DIY homeowners, or owners dealing with “adventurous” contractors: (1) just follow building codes resisting temptation to think “eh, stretching the code here or there makes sense to me so I’ll do it!”; and (2) run instead of walking away from any vendors, or installers who swear by their products explaining that their outgassing effects are no worse than a baby aspirin. Combine (1) and (2) with sound building science advice and you will probably end up with a successful return plenum.

@Paul, yeah, it seems like that’s what I was saying. But since drywall is an effective air barrier, I’d be curious to know to what extent any outgassing from improperly cured polyurethane insulation will pass through drywall. In any case, how is that any different from having dedicated returns if there’s outgassing in the attic that’s separated from living space by only the ceiling drywall?

David, I need to disagree about covering with drywall making an improperly cured foam safe, what you essentially stated. Drywall is quite permeable.

@Cajunjb1: In general, the IRC doesn’t require dedicated return ducting (and as I noted in my previous comment, this is my preferred approach). But as Danny noted, the IRC has provisions (see IRC Section R316 Foam Plastic) that might preclude what the builder recommended re: open return in semi-conditioned or conditioned attic with exposed spray form. Also, I agree with Danny that potential VOC outgassing from improperly cured foam is a risk not worth taking. OTOH, if foam insulation is full covered with drywall, there’s a ductless return isn’t a problem on either count.

I’m not sure if there’s any support to suggest pathogenic contamination. Chemicals and VOCs may be an issue.

Please know that spray foam is an on-site chemistry experiment. If the applicator doesn’t understand the chemistry, mixing ratios, proportioner and hose temps and substrate conditions, you might end up with a big problem. Even with properly applied foam, a chemically sensitive person can still have an adverse reaction.

The bigger issue is compliance with the fire protection requirements for foam plastics in the Residential Code. All foam must be separated from the living space with a thermal barrier. (Drywall or alternatives with code acceptable criteria). If the attic is only entered for service of utilities, an ignition barrier is permitted. But this gets really complicated when the attic becomes a return plenum. Not just for foam, but also for electrical wiring and plumbing.

I’ve heard people claim they do it. But the liability risk seems overwhelming to me. Especially since litigation these days is more about which lawyer is the best actor.

No. Return air should have a dedicated duct system. Using the attic space, even if encapsulated, will potentially expose your return air to pathogens, VOCs and who knows what else. The purported savings are not worth the risk.

I have a builder recommend an open return system on a spray foam house. Open grilles to the attic throughout the home. No duct for return system. Does this sound like its code compliant?

Minisplit is constant air filtering and QUIET, not the drone of horrible billowing ducts. Forced air in most residential cases temp is up and down as the stuff is on and off …mini splits are super QUIET inside and out, much less energy used for equivalent heating and cooling, most residential setting have connected spaces that one indoor unit can easily handle, and just like duct-ed systems open the door for the other rooms, it works great. And each area is controlled to a desired temperature, ducted is one temp setting for all, very inconvenient, especially with the mostly dismally installed in homes, one room ice cold, other too warm in both winter and summer. Mini splits are the way to go, million in use around the world, far more than ducted systems like the u.s. uses, it’s a big world out there, minisplits are used in more places than our style of heating and cooling