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What’s the best way to retrofit interior with rigid foam?

goodnatured | Posted in Energy Efficiency and Durability on

I’m located in Portland, Maine. These questions concern a Cape Cod with a shed dormer running across the back side of the home — 2×10 rafters, 4/12 roof pitch, with 3 feet of original 10/12 pitch left on either side of the dormer roof. The opposing side of the roof has 2×6 rafters with 10/12 pitch in full.

My plan is to apply Dow-R 2″ rigid foam on the interior over the studs and rafters – 2×4 walls. I will be meticulous about air sealing seams etc.

I will run the rigid foam down the ceilings first, then up the walls. Dense-packed cellulose will be professionally blown in following this.

I have been doing a lot of reading! My question is the air/moisture permeability against the roof sheathing/rafters. I have read that this is a must re: rot and mold. If this is a must could I use an Ice and Water self-adherring membrane (like Grace) against the sheathing and rafters to create that barrier?

I also am planning to apply 5/8″ fire-code gypsum following this and right up against the foam board. Would I really gain a large advantage if strapping is used for an air space before the gypsum is attached? Would the answers apply to the exterior wall sheathing also?

Hope there is enough detail in my question. – Dean

Oh, nonvented, conditioned roof, full Ice & Water Shield on the exterior of the dormer roof, then architectural shingles. First 6 feet of opposing roof have Ice and Water, the ‘turkey runs’ have full Ice and Water Shield. Dormer roof sheathing 5/8″ Advantech tongue-and-groove, the remainder is 1′ pine space 1/4 inch apart.

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  1. GBA Editor
    Martin Holladay | | #1

    Before you install dense-packed cellulose, you need to install ventilation channels on the underside of your roof sheathing. That's a code requirement -- as well as a building science necessity.

    You'll also need soffit vents and ridge vents -- and if your dormer construction makes ventilation channels impossible, you may need to choose a different insulation strategy.

  2. goodnatured | | #2

    Great, I have followed advise of a insulating company, and the roofing company is working in conjunction with him. The roof is alreay done, no ridge, soffet or gable vents. This plan as described before was approved by the city before work started. There for the strange question of ice and water shield applied to the underside of the roof decking (including raftrs) before insulating is done, obviously I started reading about air sealing, moisture trouble after the process started, I do heat with forced hot air in the winter so it is very dry interiorly.. thoughts, suggestions? Is my idea of the ice water shield application ridiculous?

  3. GBA Editor
    Martin Holladay | | #3

    No, you can't solve the problem by adhering Ice & Water Shield to the underside of your roof sheathing and your rafters, for several reasons: the materials would be expensive, the work would be awkward, it's impossible to do a perfect job, and you would be creating a vapor-barrier sandwich with roof sheathing in the middle -- a disastrous approach if you ever get a roof leak.

    If you want to stop vapor from reaching your roof sheathing, you'll have to install closed-cell spray polyurethane foam. The other alternative is to retrofit some soffit and ridge vents.

    (By the way, after having worked in Armenia for a year and a half, I can't resist greeting you as a long-lost cousin. Barev -- vants es?)

  4. dankolbert | | #4

    Dean - we have done many dense pack unventilated roofs without incident (that we know of, at least). It is indeed a controversial topic these days. I recently had a long talk with Bill Hulstrunk, the technical rep for National Fiber. He is admittedly a biased source, working for a major cellulose supplier, but he's seen more cellulose insulation projects in NEw England than anyone else I know, and presumably has a vested interest in their success. The short version is he hasn't seen any problems with well-installed insulation. The full ice & water job on the roof is a bit of a drag, but I think you should be OK. And I too have had local code officers sign off on "hot" roofs.

    And speaking of cousins, I used to live in the same apartment building as Marcie.

  5. goodnatured | | #5

    Thanks for the info guys. Dan, I almost fell out of my chair- too funny, small world. How in the world did you make the connection of Marsce being my sister? Have we ever met?

  6. dankolbert | | #6

    How many Manoogians can there be? I don't remember - it was many years ago (the Porter's building on Pine St - late 80's).

  7. GBA Editor
    Martin Holladay | | #7

    Dan made a good point. When I was offering advice, I was thinking of other GBA readers tempted to build a roof like yours. But your roof is already built, so you have to go forward with a decision on how to proceed. If you can find a cellulose installer who really knows how to dense-pack -- not just a guy who blows the bays until they are full -- you could leave the bays unventilated.

    To me, it's not the best solution. But it may be your best option under the circumstances.

  8. stuccofirst | | #8

    Cellulose has a considerable settling ratio. I'm not sure how this will affect things in the future.

  9. Bill_Hul | | #9

    Hello Dean,

    The key to the durability of your roof assembly will be on its ability to dry to the interior of the home. There are several foam board type products that will allow this; glass faced polyiso board (Non Foil Faced!), expanded polystyrene (EPS), or extruded polystyrene (XPS), in the order from most to least moisture permeable. You will want to avoid using a poly vapor barrier, foil faced polyiso and Ice and Water Shield on the interior since these will inhibit your roof system from drying towards the inside and create the self composting double vapor barrier situation.

    The cellulose installed in enclosed building cavities including non-vented roof assemblies need to be dense packed at a density of 3.5 lbs/cuft for a 2 x 6 increasing to about 4.0 lbs/cuft for a 12 inch cavity to become self supporting and will not settle over time. Installers should consult our Expanded Bag Coverage Chart posted on our web site for the appropriate sqft per bag of cellulose. Cellulose loose blown in a flat attic will have some settling over time down to its settled density of about 1.4 lbs/cuft. These flat attics will need to be ventilated to the outside to prevent diffusional moisture from condensing on the underside of the roof.

    For building systems that can only dry towards the inside, it is important that we maintain the indoor relative humidity levels at no more than 50% during the cooler months, so that the building assembly will be able to dry towards the interior. This can be accomplished with an HRV, ERV or even a continuous duty bathroom fan such as a Panasonic or equivalent on a timer, and monitored with a digital temp/humidity gauge.

    Although we would prefer having your non-vented roof assembly dry towards the exterior or even better yet be able to dry in both directions, there are thousands of examples of durable roof assemblies filled with cellulose that dry only towards the interior. We have eliminated the need for Ice and Water Shield products by insulating roof assemblies with insulation products that actually work.

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