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Adding to existing foam insulation

minute_basset | Posted in Energy Efficiency and Durability on

While getting quotes to insulate our standalone, 2 story above grade garage (with studio on top), we thought we’d take the opportunity to also increase the insulation in the roof of our house.  It’s located in VT.

The house was a circa 1960 cape in mostly original condition with full shed dormer along part of the back. A new System 2000 had been installed, but the rest of the house needed new windows, siding and roofing.

As part of a 2011 gut/remodel all the exterior walls – basement, 1st and 2nd floor and roof were insulated with open cell spray foam. The house has 2×6 walls, so R-19+ in the walls, and 8″ rafters, so R-28+ in the roof where the foam also covers the rafters. A Fantech ERV was also installed at the same time with the unit in the attic. For the time period, this all seemed appropriate and fortunately we have not had any issues with excess moisture. 

We’d like to increase the R-value in the roof and it seems the only option is to add another coat of open cell foam. One recommendation is to add 4″ for an additional r-16, bringing the roof up to r-44. Adding closed cell seems out of the question as we’d be laying a vapor barrier on top of the open cell which at any point in time could have vapor passing through it.  We’d essentially be trapping moisture if I understand the mechanisms correctly and that could lead to potential sheathing damage.  I understand that building products and building science strategies and recommendations continue to evolve over time based on study and testing.  I’d like to determine what’s the best approach to move forward (or not) in our efforts to increase the roof insulation.

I had been brushing up on the latest foam conundrums, reading the experts, and foam manufacturer websites to prepare for discussions with installers about the garage. Surprisingly, in the past week I have been told many contradictory things about open/closed cell foam in addition to getting different recommendations about the amounts of foam to install.  I have also found that quite a few installers have abandoned working with open cell altogether.  

For our upcoming garage project, we had settled on:
– a combination of 2″ closed cell flashing under the roof  for vapor/air barrier followed by open cell  for r-49
– open cell in the exterior walls for r-22
– 2″ closed cell on the garage ceiling/studio floor for R-14 so we get the benefit of both an air barrier and vapor barrier preventing garage odors from migrating to the living space above.
– 2″ closed cell on top of 5′ of exposed foundation along the one wall that’s partially built into the hillside.

So 2 questions:
1.  What’s the best thing to do to improve the R-value in the house roof?
2.  Does the insulation approach to the garage look reasonable?   Do I need to go to an all closed cell approach?

From reading the building scientists and experts, there seems to be place for, and benefits to, both open and closed cell foam as long as it’s installed correctly and care is taken to provide a vapor barrier and ventilation (and it’s less expensive than all closed cell).
But in the field right now, it seems like open cell has become the boogie man, and installers just want to work with closed cell.   I’ve also been getting recommendations to forgo code in the roof due to the diminishing returns as you go above R- 35 to R-38 where the incremental cost to go to R-49 is not worth the additional investment.  It does seem to be a valid argument, but I’d like to stick to code especially where the upstairs will be living space and it should be comfortable.

We plan to keep the garage at 40+ during the winter and higher when working there. And eventually heat and finish the studio with a bath and kitchenette. We also plan on putting in spot HRVs in both the garage and studio.

Hopefully we can find one company that is still using both open and closed cell foam. Otherwise it’s just cost inefficient to bring out 2 different installers to apply the 2 different foam types.   

Thanks in advance for reading and providing any insights/recommendations.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi A Walker.

    I am not familiar with the practice of installing closed cell spray foam over open cell spray foam in a roof assembly. If your roof rafters are encapsulated in open cell spray foam insulation, you not only have decent R-value, but you are mitigating thermal bridging. I'm not sure what the returns on your investment will be by adding additional insulation. Perhaps a GBA user who has modeled a similar scenario will come along and offer some data.

    You mention not wanting to add a "vapor barrier" to the insulation. I think that you should have a vapor retarder covering open cell insulation in a roof assembly, particularly if the roof is unvented. Read this for more info and to make sure that you don't have a risky roof assembly already: Open Cell Spray Foam and Damp Roof Sheathing

    As far as the garage goes, there is nothing wrong with your approach. The spray foam will do a good job air sealing and insulating, though it is not a total air sealing strategy and other areas will need to be addressed. It is also an expensive insulation strategy that does nothing to address thermal bridging and thermal bridging diminishes the R-value of the walls. Is it possible to install exterior rigid foam on the garage or is the exterior already finished?

  2. minute_basset | | #2

    Thanks Brian,

    My not wanting to add a vapor barrier in the existing house roof was because at the time, it was typical to spray all open cell directly on the roof. So, for my install there isn't a technical vapor barrier against the sheathing, just the 8" of open cell covering the rafters. Even though this isn't always an issue (and it hasn't been for us), there have been enough failures resulting in the guidelines of installing a vapor barrier against the underside of the roof sheathing. I had also read the article you linked to so that's what made me concerned about applying closed cell over the existing open cell - would I be trapping any existing vapor passing through the open cell up against the sheathing and create a problem I don't have now, down the road?

    So for the garage, I was definitely planning on installing a vapor barrier to the roof sheathing before anything else got installed.

    The garage is an enclosed shell with a layer of advantech sheathing, air barrier and shiplap vertical siding - so no option for exterior foam. I understand the issues with the thermal bridging. I think I'm a bit less concerned about the garage space. The foam is pricey, but it will probably do a better job doing the air and vapor ceiling. One thought I had to potentially limit thermal bridging was to install strips of drywall to the face of the studs in the garage and then hang drywall on top of that. It might turn out to be too crumbly though. I was trying to think of a leaner approach than installing a strip of rigid foam to the stud and strapping on top of that before hanging drywall. Thoughts?

    Upstairs, in the living space, I can address the thermal bridging more readily - the knee wall area can be sprayed and studs covered as we'll be building another wall in front of it. We'll also have a flat ceiling on the second floor, so most of the roof rafters could get sprayed as they won't be in the way of installing drywall.

    I had been toying with dense packing the roof, but I just went through that while building a habitat house. I have a good hands-on appreciation of all the sealing details required to be successful plus building out a gusset for part of the roof assembly to achieve the depth/R value I'd want - it's a lot of work. I also can't see my other half taping the backs of outlets and taping smart membrane to the front of an outlet...

    I guess I'm being lazy. And I'm hoping to do it all this season. Now I just have to settle on a heating strategy.

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