So, I'm out of my weight on this one, and could use some advice...

 

Good friend of mine with a lot of construction experience is now in Reno, NV, and is remodeling a house there. His new CPDS for applying two part foam is so much more efficient and controllable (compared to two-part canister foam) that he has some significant material left over from air sealing the crawl space. He's considering using the product by air sealing the attic and then blowing in a layer of foam.

There's a couple of inches of FG batt on top of drywall in the attic. In my climate zone (Wisconsin) I'd be sure to invest all the labor to move the batts off the ceiling and lay the foam directly on top of the drywall, to assure that the drying potential is all in the right order from warm to cold. My gut tells me that in Reno the batts do NOT need to be moved out of the way to apply the foam directly to the drywall, because there's so little moisture risk in his climate.

I only visit him during the summer, so I always think about Reno as a hot/dry climate. I looked it up, and I see that it IS dry -- they do get only a dozen inches of moisture a year -- 8 inches and rain and 25 inches of snow. But Reno has 5,600 HDD and only 700 CDD.  The data says that about four months of the year, low temps get just below freezing. So now I'm not so sure that the easiest way to foam this attic(and the way least likely to expose the crew to dust, mouse droppings, etc.) is going to be that smart.

Do these batts need to come off the ceiling?? Or is there so little risk of moisture damage that he can just blow a couple of inches of two-part foam right over the top of the batts??  

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I'm not a fan of this proposed approach. I am in attics all the time, and a clean looking job is a rare find. If you foam on top of the batts then you trap any mouse dropping, dead mice, etc. in the conditioned zones. Also, if anyone has to get into the attic, there is a good chance they will screw up some of the foam, etc.

Spend the time and money (as I'm sure your gut is telling you), and foam on top of the gyp board, and then lay those batts back down (the right way, and you know what I mean).

Just my take on it!

Redefine the thermal boundary (from attic floor to attic ceiling), or remove the batts and foam the floor.  In either case, make sure you roll over the perimeter wall top plates.    

Definitely not. Removal of the existing insulation when moving the thermal barrier to roof is required by code. Sorry I do not have time to find the exact code reference, I would be thankful if someone provides a link for my future reference.

You do not want to trap all the old stuff with no way to vent its various odors it built up over years.

Remove the batts.   Stretch out the foam to cover as much as possible but all top plates for sure.  Then put the batts back and blow another foot of cellulose on top of that.    Great System.      R&R those batts isn't that much work.    They dont need to come all the way out of the attic.   Just pile them up, do some work and put them back and finish where the pile was.   Definitely worth the effort.

I definitely second that, as long as the budget permits.

I assume you are just laying down a few inches of foam on the drywall only for an air barrier? The only thing I don't like about this is when you have ducts in attic space above bottom chord. You lose the biggest advantage of bringing them into conditioned space. I mean you could coat those too but I think that is a bit shabby. I also like the serviceability of going to the roof deck and removal of old.

In the ideal world you are dead-on Craig, but in the world I find myself most days money is the deciding factor.

You hit the roof deck with closed-cell and you're looking at a multi-thousand dollar project. Not to mention that you are now throwing away a good deal of batt insulation. 

If the concern is the ducts in the attic, then just seal the sons of guns up and cover them up with cellulose.

Just the way I see it...not preaching the Gospel here.

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