Replies

1. Expert Member
   NICK KEENAN | Jun 24, 2021 11:31pm | #1

   I can only think of two cases where closed cell is better than cellulose: 1. if the roof is unvented; 2. if the roof assembly is too thin to support the R-value you want, foam has a higher r-value per inch than cellulose.

   Tell us more about your attic and what's in it now.

2. 
   creativedestruction | Jun 25, 2021 12:22am | #2

   There are no health benefits to closed cell foam installation. If anyone in your family has chemical sensitivity, quite the contrary actually.

   The margin for error in calculating ROI on a situation like this is extreme. If it achieves what you need it to achieve, e.g. improved comfort from reduced infiltration, less condensation on ductwork, then do what needs to be done. If you're foaming the roof deck then it will certainly tighten things up but you'll also be adding more conditioned volume which adds back some energy use). If it gets things too tight (blasphemy!) then you may need to install a ventilator -- what's the ROI on that? Too many variables.

3. Expert Member
   BILL WICHERS | Jun 25, 2021 10:24am | #3

   There are no health benefits, spray foam is probably a little higher risk than cellulose in that regard. There MAY be comfort benefits, but not because spray foam is better -- the benefits come from the fact that spray foam is good for air sealing. There are other, cheaper, ways to do the air sealing though. I would estimate the payback for a project like this will exceed the useful life of the structure in most cases, assuming your current insulation is reasonably well installed.

   If you want to improve your attic insulation, and assuming you have a vented attic with no mechanicals (furnace, air conditioner, etc.) in it, and assuming you have a leaking attic floor (which can be easily verified with a blower door test), I would remove the current cellulose, do a good job of air sealing the "old fashioned way" with caulk and canned foam, then install a new layer of cellulose at least to code minimum R value. I would also make sure your attic is properly vented with soffit and ridge vents, and I'd install baffles at the eaves prior to instaling the new layer of cellulose.

   To provide more detailed advice, you really need to provide some more details about your attic.

   BTW, regarding spray foam, you're going to find a lot of "don't use it because it's not green" info on here. I'm probably one of the most pro-spray foam people on this forum. I recommend spray foam ONLY for a few niche locations, which are unvented cathedral ceiling assemblies, irregular crawl space and basement walls (cut stone, etc.), and rim joist areas (sometimes, there are other ways to insulate rim joists too). For most "regular" places, there are other ways to insulate that will work just as well as spray foam, and sometimes better, and the "other ways to insulate" are nearly always cheaper too. Cellulose, for example, is probably the cheapest attic insulation to install, but it's also one of the best performing insulating materials for this application.

   Bill

4. Expert Member
   Dana Dorsett | Jun 25, 2021 01:59pm | #4

   There is no "payback" on closed cell foam except in the case of it saving the roof from uplift during a hurricane. Vented attics are at much higher risk than unvented in hurricane zones, but retrofitting to code with purpose made steel ties is probably cheaper than the upcharge for closed cell foam.

   At any given R value cellulose has favorable thermal diffusivity characteristics in a cooling dominated application than closed cell foam. This is a thermal mass effect. Lower diffusivity both lowers & delays the peak ceiling temp. See:

   https://gutex.de/en/product-range/product-properties/insulation-in-summer/

   ^^Closed cell foam has the same diffusivity as the "PU rigid foam" in that bar chart, which is about twice that of cellulose.

   https://gutex.de/fileadmin/uploads/Produkte/Produkteigenschaften/Content/phasenverschiebung-EN.svg

   You may have to zoom in to be able to read the scales on that image, which compares the differences in temperature and over time the peak ceiling to room temperature deltas between very low diffusivity fiberboard insulation and a high diffusivity insulation such as fiberglass, as well as the time lag. With the diffusivity of cellulose the delay and peak suppression will be something in-between, but much closer to the wood fiberboard insulation than the fiberglass.

   With as little as 1" of closed cell foam (R5 minimum) to air seal and protect the roof deck the rest of the R can be done with cellulose, usually at a much lower price point than closed cell foam, having perhaps the best of both worlds. But it takes something like 3" of closed cell foam to provide a significant hurricane resilience factor.

   Bottom line, the benefits of closed cell foam are pretty thin, and mostly about structure (at the full code-R) and moisture protection for the roof deck. All else being equal cellulose would provide more comfort and efficiency in an FL climate.

   Only a local real estate agent would be able to tell you the magnitude of improvement (if any) on resale value over time. (I suspect from a resale point of view the $8K would be better spent toward kitchen & bath updates, but I don't work in real estate, and I'm nowhere near your real estate market area.)

5. 
   walta100 | Jun 25, 2021 02:21pm | #5

   The only real way I know to understand the pay back is to build a computer model of you home with your weather your fuel costs and run the numbers.

   BEopt is the program
   https://www.nrel.gov/buildings/beopt.html

   With that said my gut says you are unlikely to live long enough to save 8k in fuel by adding insulation.

   Walta

6. 
   alexza | Jun 25, 2021 02:54pm | #6

   Thanks for all the replies. Let me add some information. When I say "health benefits", I mean a conditioned space, closed off from the outside, which can lead to less dust, better quality air, humidity. I could achieve that with cellulose and air sealing but I also have HVAC in the attic. My current cellulose was done in 1992 and varies from 2-10" on the floor. The existing attic has evidence of creatures (mice, etc) getting in at one time, we tented when we moved in (Feb '21) so little can survive that. The all ceiling penetrations are leaky (lights, fans, etc) when viewed with a Seek thermal camera.

   My roof will remain vented, leaving a 2" air gap between the roof deck and the rafter insulation. I posted a message a few days ago about using rigid foam in the rafter bays to establish the 2" gap and then going over it with closed cell. The feedback consensus was to do it all with rigid board, multiple layers, well sealed - which I decided to do.

   I trying to guesstimate something like:
   - moderate to poorly insulated - electric bills ~400/m avg over the year
   - tight and well insulated attic - savings of ~15% per month (I'm guessing) - which equates to ~10 yr payback - not great

   This doesn't include less stress on mechanicals, potentially better resale, etc.

1. 
   woobagoobaa | Jun 25, 2021 05:09pm | #7

   OK mechanicals are currently in non-conditioned attic space. I assume you intend to remove all the existing cellulose / clean up the attic.

   Why the need to leave the roof deck vented?

7. 
   alexza | Jun 25, 2021 05:55pm | #8

   Yes mechanical are there, also will vacuum out and clean. Cedar shake roof w skip sheathing in good condition. Shake needs to dry from both sides to prevent curling.