Son in the process of planning stage of building a home. Two builders are wanting to foam attic and walls. I have my doubts about foam due to off gassing and sick build syndrome. Am I out of touch or the old school passive ventilization to remove the water vapor still the tired and true method and use fiberglass insulation with a radiant barrier.

Recently did a deep dive into the pros and cons of the various insulation types. I'm in PA, put a decent size addition on my house. In the home stretch of completion. I ended up with tried and true fiberglass and rock wool depending on the location. R-49 in ceilings and R-21 in walls, that is our code here.

Reasons:
1. Was doing it myself, so labor cost was a strong factor. Spray foam would have been 3-4x the cost just for the material, plus the labor if having to get someone to do it.
2. Difficulty doing any modifications after foam is done; wiring, other.
3. Out-gassing was a concern. Wife has asthma.
4. Related to number one, payback period of energy savings was upward of 30-something years.
5. If not done properly (better contractor = more money), spray foam can be a disaster with moisture issues.

A few minor things on this project. My HVAC system is heat pumps/mini splits so each room has its own climate control. I insulated in between each and every wall and floor for both sound and thermal, so I can adjust and limit the amount of energy expended on controlling less used rooms. Plus the exterior walls are all 2x6 construction so allowed me to use R-21.

You just opened a can of worms with strong opinions both sides waging war, I'll bet.
Both side are "right" in their own fashion, so I will limit my opinion/advice to only a tiny piece of this puzzle.

ATTICS
Metal roof = foam rafters, seal attic (closed cell more ideal than open)
Other roofs = rockwool attic floor & vent it ridge/soffits

I'd be much more concerned with spray foam leading moisture issues than off gassing

I cannot speak to the other issues, but one thing that I have issue with in foamed houses is playing the "Where is the leak coming from?" game. If it is not an obvious roof penetration, roof leaks are make much more difficult to trace with spray foam. Might not be a big deal, but it can be the difference between replacing a small area of roof vs. replacing the entire slope. Just something to think on.

I will say working in a spray foamed attic is the bee's knees. Moisture concerns should be mitigated by using a proper HVAC contractor that runs Man J and Man S calcs. But that should be the case no matter the insulation format.

With a new build, I would avoid spray foam.

5yo "new build" w/spray foam

Had to retrofit HVAC w/whole house dehum in the attic.

Prob solved, happy with the foam

Jason_Roofer said:

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I will say working in a spray foamed attic is the bee's knees. Moisture concerns should be mitigated by using a proper HVAC contractor that runs Man J and Man S calcs. But that should be the case no matter the insulation format.

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I assume no vapor barrier on the top side of the roof decking? You don't want a vapor barrier (closed cell foam) on the underside of the plywood and a vapor barrier (underlayment) in the top side...I would think that's asking for moisture damage.

We love our spray foam. Hope to never be without it. I was just up in the attic.

But highly recommend a whole home dehumidifier.

Generally speaking IF the new build is properly designed in all of its systems for the foam, it seems to work well. I am complet let against foam retrofits and there is a pretty big leap of faith in the "designed properly for it" part. Also wonder about potential long term issues. Humidity is definitely a concern.


They seem really efficient, but what I have never seen is a comparison of two exact houses - one with foam and one traditional built to modern efficiency code. My gut says that if that comparison occurred it would not be a significant difference and it may not justify the additional expense and unknown of the foam.