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Board foam insulation under an unvented cathedral roof

TUGdo7DovU | Posted in Energy Efficiency and Durability on

Hi, We are building a passive solar house near Knoxville, TN (zone 4). The house is in the dry with a “cool color” asphalt shingle roof already on. We are getting ready to insulate.

The walls are from the outside: Hardie, 3/4″ rain screen, Tyvek, 1 1/2″ strapping with ESP insulation in between, OSB, and 2×6 walls 24″ oc. We are planning on using dense pack cellulose in the walls with air tight drywall. This seems OK to us.

The real question is about the cathedral ceiling insulation. The ceiling has 2×10 rafters and 1 1/2″ thick strapping under that. The roof has no ridge vent, and the bays between the rafters at the top plate and roof sheathing have been sealed up with 2x’s and canned spray foam.

To help with the air-tightness of the drywall at the ceiling plain, and for a firm place to attach the drywall gaskets, we put some blocking above each partition wall that intersects the cathedral ceiling. Between the partition wall top plates and the solid blocking, we added a layer of 12″ wide vinyl flashing (overkill?). We are really trying to do the air tight drywall well/right.

We were planning on just using dense pack cellulose in the ceiling. Then we heard about the flash and batt/fill technique and read Joe Lstiburek’s article “Don’t Be Dense”. So, we were going to go that route (flash and fill). I was researching the spray foam polyurethane (SFP) when I came across the thread started by Paul F. about the possible risks of SFP. Now we don’t want to take the risks of using closed cell SFP.

I only see 2 choices: 1. Use dense pack cellulose and hope we get the airtight part right. 2. Use a board insulation (polyiso or XPS -don’t know which or if it matters), cut it into strips and placed under the roof sheathing with canned spray foam. Then do dense pack cellulose. We can get a hold of some 1/2″ polyiso (Rmax) with foil facing, or 1″, 1 1/2″ or 2″ polyios (Firestone) with a fiberglass face.

Don’t know which to chose. Thank you in advance.

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Replies

  1. TUGdo7DovU | | #1

    Hi,

    First time poster and had problems with the formatting. Sorry about that. I have reformatted and condensed my question.

    We are building a passive solar house near Knoxville, TN (zone 4). The house is in the dry with a "cool color" asphalt shingle roof already on. We are getting ready to insulate the cathedral ceiling.

    The ceiling is 2x10 rafters with 2x3’s furred down under the rafters. The roof has no ridge vent, and the rafter bays between the top plate and roof sheathing have been sealed up with 2x's and canned spray foam.

    We have decided against using closed cell spray foam polyurethane insulation under the roof sheathing.

    This is what we are planning to do:

    Use either XPS or polyiso rigid foam insulation (R-15) cut into strips and foamed or caulked to the underside of the roof sheathing. Then put in dense pack cellulose and air-tight drywall.

    Any comments? Thank you.

  2. GBA Editor
    Martin Holladay | | #2

    Ken,
    In case you didn't see it, your question (and many other related questions) is addressed here: How to Build an Insulated Cathedral Ceiling.

    In your climate zone, your chosen method of insulation requires a minimum of R-15 rigid foam, as you correctly noted, and a total roof assembly R-value of at least R-38. That means that you will need 6.25 inches of dense-packed cellulose in addition to the rigid foam.

    Your plan will work, although you'll find the installation of all those rectangles of rigid foam to be fussy, labor-intensive work. Remember, every rectangle needs to be air sealed at the perimeter with caulk or canned foam.

  3. TUGdo7DovU | | #3

    Martin,

    Thanks for your reply.

    When we started this house (2005), we were going to insulate the roof with just dense pack cellulose and a correctly installed air-tight dry wall ceiling (with the approval from ORNL Building Science people). That is why the ceiling is blocked in and furred down.

    We are afraid of the health risks from spray foam polyurethane.

    If we use caulk, what kind would you suggest?

    Thanks again. This site is wonderful!

  4. GBA Editor
    Martin Holladay | | #4

    Ken,
    I've been satisfied with the performance of silicone and polyurethane caulks. You can also cut the rigid foam deliberately undersized and fill the gap with canned spray foam.

  5. Bill_Hul | | #5

    Hello Ken,

    We would recommend dense packing the roof cavity full with cellulose at an installed density of 4.0 lbs/cuft to prevent separation from the roof sheathing. Ideally this would be installed above Insulweb fabric so that you can QC (feel) the density of the cellulose above, which will feel very solid at these densities. The Insulweb will bulge between the rafters and will need to be rolled flat by the installer to allow drywall to be installed. You will need to find a local installer who is familiar with these techniques.

  6. TUGdo7DovU | | #6

    Once again: thanks to all of you who have been so generous to take the time to respond !!!

    In response now to Bill Hulstrunk (though others should feel free to comment):

    Your reply prompted a lot more reading. One article turned up the possibility of a warranty so I want to pursue this further as an alternative to the time-consuming requirement to install and seal 2.5" - 3" of rigid foam insulation between the rafters.

    I forgot to include some info in my earlier post that may or may not influence the way you answer the questions that follow. First: although the roof has conventional tarpaper with asphalt architectural shingles, there is an additional peel and stick membrane (Grace brand Ice and Water Shield) beneath the tarpaper on the south-facing 4/12 pitch roof ( but no membrane on the north 12/12 pitch.) This membrane would definitely prevent any drying to the outside for this part of the roof. The south-facing roof also supports two 4x7 foot water heating panels. Second: we have a programmable Trane Energy Recovery Ventilation system. I mention this because you made a statement earlier in another article that there needed to be active management of indoor humidity in the winter; someone else commented that this was a liability that a builder would not want to have.

    In your article titled "How to install cellulose Insulation "(BGA, 3-11-2001) you made the same recommendation (4 lb / cu ft) and then you stated on page 4: "We feel comfortable enough to warrant the material in that assembly for the life of the building. We will stand behind these installations. " The installations that have had issues have always been related to density problems. We tell our installers, 'If you aren't sure of the density of your applications, you shouldn't be using this technique.' The material has to be at the right density, and in full contact with the exterior sheathing. "

    Q #1: I assume therefore that your warranty is conditional on having one of your approved installers to the job. Correct? If not, can a DIY owner-builder do a warranted installation? Note: to be really green, I am aiming for a 200-year building :)

    Q #2: How many times have you had problems with a dense-pack unvented cathedral roof installed by one of your approved installers? Can you provide references to verify that you met your warranty obligations in case of a failed system?

    Q#3. Later in the same article you said: "The argument we make is that when you consider the combination of dense-packed cellulose and the drywall, the assembly is impermeable. The intent of the code is to prevent warm moist air from having contact with the sheathing, and all we need to do is convince the code official that that won't happen." What are the requirements of your warranty regarding drywall? What is your track record on convincing code officials?

  7. Bill_Hul | | #7

    Hello Ken,

    First and foremost, you will need to find an installer who is familiar with this technique, if you cannot, than I would recommend against it. I am not familiar with cellulose installers in that area since it is outside of our service area. Next, you will have to discuss this application with your local code official to see they are willing to accept the use of dense pack cellulose in this application. These two issues may unfortunately be the deal breakers in your geographic area.

    A #1: National Fiber warranties the performance and durability of our cellulose in this application when installed at the appropriate installation densities, a twelve inch rafter would require 4.0 lbs/cuft installed density. This warranty is not conditional on a approved installer, and could theoretically be done by a homeowner. Unfortunately, it is unlikely to find a rental machine capable installing the cellulose at these high densities, and without training, it would be more difficult to know if you were doing it correctly. National Fiber does offer an installer training class.

    A #2: I am personally not aware of any unvented roof failures, although on two jobs, the density was insufficient and caused separation from the roof deck and moisture accumulation in the cellulose. In both cases, water dripped from the bottom of the roof assembly indicating a problem above. No damage to the cellulose or surrounding materials occurred in either case, and the additional cellulose was injected to bring the roof up to the correct density. The original installer corrected one and National Fiber payed another installer to correct the second one. There are thousands of these in the Northeast and many more being insulated daily.

    A #3: Their are no requirements for drywall in our warranty, and we frequently blow cellulose behind Insulweb or Typar which has been strapped below. Our track record while good is mixed, in areas were this has been historically done since the 1980's, the local code officials understand and allow this practice. In areas where this is new, or in areas where the code officials are not comfortable with "alternate methods and materials" provision of the code, it may not be possible. I would consult your cellulose supplier in your area to see if they will assist you. We have information on our website, but most code officials will likely want this from your cellulose supplier.

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