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Converting 3rd floor to unvented, conditioned living space — insulation dilemma

user-5553622 | Posted in Energy Efficiency and Durability on

My house was built in 1902 and includes a 3rd floor that has always been used as a conditioned living space. The furnace was located above collar ties/ceiling on this floor with all insulation being blown-in cellulose on top of collar tie flat ceiling framing and batts behind knee walls. Furnace didnt pass inspection and was removed along with ductwork. My bottom two floors are zoned, each with a new energy efficient central HVAC system.

There was significant damage to plaster on 3rd floor and I recently finished removal of all ceilings and walls. We plan on the 3rd floor being a master bedroom suite with a walk-in closet, bathroom and bedroom and were hoping to vault the ceilings as opposed to a lower flat ceiling and installing a mini-split system with 2 indoor units to condition this space (approx 800 sq ft).

The roof framing is very complex. It is a 12 pitch hip roof with 6 hipped dormers. From attic floor to ridge is 16′. Insulation was removed along with plaster.

I am located in the city of St. Louis, which I believe to be zone 4a.

Based on all the research I have done it seems that in a roof deck with such complex framing and so many dormers such as the one I have, ventilating it properly via ridge and soffit vents becomes nearly impossible.So… I decided an unvented attic would be my best option. I have gotten several quotes and simply cannot afford spray foam as an insulation option for underneath roof deck between rafters. The thing is… I would bite the bullet and go for it IF any of the many options these companies gave me presented an incredible r-value upon completion. My rafters are old true 2X6 boards. Even when filling it completely with closed cell insulation my r-value doesnt even meet the current standard according to 2015 code for attics.

After obsessing over it for too long I have decided to do with the cut and cobble method many have spoken about on this site.

I still have a few questions regarding specific recommendations for my application:

1. Should I leave an air gap (or baffle) on main parts of the roof where this is feasible (continuous air flow) prior to installing my rigid insulation? Or should i apply rigid insulation directly under roof deck?

2. I am still unsure if I will flash and batt or use the stacked rigid foam application, which is preferred?

3. There will be storage tucked into every knee wall and i therefore intend to insulate all the way to the eave for a proper seal in attic space, how do i address the eaves? The roof frame sits on the solid, 3-wythe brick exterior walls- where should i place my soffit dam in terms of the brick exterior walls? Inside edge where brick wall below meats the interior space, middle of the brick wall or above the outermost layer of brick?

4. With the cut and cobble method which rigid insulation is preferred and in what order should it be installed?

5. Prior to installing my first layer of rigid foam insulation where should I seal with can spray fiam?

I know this topic had been addressed and I apologize for being redundant but every house presents a unique situation and I am hoping someone will be familiar with a situation such as my own and offer advice for movnig forward

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, I feel duty-bound to warn you that it is unwise to insulate your roof with a method that isn't recommended. I do not recommend insulating unvented cathedral ceilings with the cut-and-cobble method. The risk includes sheathing rot.

    OK, you've heard the warnings. If you decide to go ahead with a method that isn't recommended, at least you have been warned.

    1. Leaving an air gap between to outermost piece of rigid foam and the underside of the roof deck will reduce your risk. If some rafter bays can be vented, I urge you to vent those bays.

    2. If you intend to limit your insulation to what fits in the 2x6 rafters, you aren't going to have much R-value. Ideally, you would add strapping or furring to provide a deeper cavity for insulation. If you like cut-and-cobble, that means including at least one layer of continuous rigid foam installed on the underside of the rafters.

    3. "Where should I place my soffit dam in terms of the brick exterior walls?" As far toward the exterior as possible.

    4. "With the cut-and-cobble method, which rigid insulation is preferred?" If you put one layer of EPS toward the exterior (as the uppermost layer of foam), followed by multiple layers of polyiso, you'll get slightly better performance than you would if you went the all-polyiso route. (Polyiso doesn't perform well at low temperatures; the EPS will keep the polyiso warmer.)

    5. "Prior to installing my first layer of rigid foam insulation where should I seal with canned spray foam?" I don't think you have to use any canned spray foam until after you have installed your first rectangle of rigid foam.

  2. user-5553622 | | #2

    Upon further inspection of the roof I realized that not only do I not have a ridge vent but I only have soffits on the front and rear of the house. What are my options for ventilating properly if I go with a ventilated roof and install baffles?

  3. GBA Editor
    Martin Holladay | | #3

    If this is a hipped roof with dormers, you have no "options for ventilating properly." You can jury-rig a system that is better than nothing if you want, but it won't be a properly vented roof assembly.

    It's possible to install vents in a fascia, although not everyone appreciates the look.

  4. Dana1 | | #4

    You're probably looking at fewer than 1000 board-feet to do a flash-foam of closed cell on the underside of the roof deck, which is more than sufficient for protecting the roof deck from interior moisture drives. Since it's full dimension lumber it may even be under 800 board feet. That would run about $900-1000 in my neighborhood, a bit more if doing it with DIY foam kits.

    You can then fill the remaining 5" with compressed R21 fiberglass or R23 rock wool (trimmed for width, since a standard batt width is designed for milled 2x framing and 16" or 24" spacing, not full-dimension which has narrower cavities.) The IRC prescriptives call for 30% of the total R to be the closed cell foam, but you can cheat that (by a lot) if you have at least 1" of closed cell foam, and use a smart vapor retarder on the interior side of the fiber layer, between the insulation and ceiling gypsum. (A 8' x 100' sheet of Certainteed MemBrain runs about $100 at Menards online, if not in stores, but HD & Lowes are also now starting to carry it- also online, if not in stores at different sized & prices. ) With 1" (R6) of foam and a compressed high density batt (~R18-R20) the foam-R won't quite be the prescriptive 30%, but it'll be well over 20%. The smart vapor retarder would be more than sufficient mold protection for the fiber layer at that ratio.

    This would likely come in cheaper, easier, and definitely more reliable than a full-depth cut'n'cobble solution, even if you had to buy a couple of 600 board foot DIY spray foam kits to pull it off. Though the kits are usually a bit more expensive than the pros, but a pro might not want a job that's under 1000 board feet if they're busy and quote some ridiculous amount of money. A DIY approach would be in the $1500- range. It's not rocket science, but do read and understand the instructions before embarking on that path.

    If you want higher performance, a layer of 2" UNfaced EPS below the rafter edges works. You'll need 3.5-4" screws to hang the gypsum, but if you tape the seams of the EPS and stagger seams of the gypsum to those of the EPS the vapor retardency of the EPS will be adequate protection from wintertime moisture drives, but for a couple hundred the broad-sheet vapor retarder is still good insurance.

    Even without interior side EPS thermally breaking the rafters it's unlikely that you'll need more than about 1- ton of minisplit to heat & cool that space, 1.25 tons tops. Most 2-head multi-splits are bigger than that, with 18-20KBTU/hr compressors as the smallest offering. If the 2-head idea is a matter of distributing the heat/coolth between two rooms partitioned off from one another, a 1-1.5 ton mini-duct cassette installed at the partition can easily serve both. Fujitsu's mini duct cassettes can even be mounted vertically, in a 9.5" cavity, so it could be in a fat spot in the wall, or in a closet space between the two rooms. But if you have a closet space between the rooms, a horizontally mounted mini-duct cassette below the closet ceiling would work with the lineups from most vendors. Oversizing the mini-split such that it rarely modulates takes a toll on both comfort and efficiency, so you want to get this right, if you can.

  5. user-1072251 | | #5

    I tried the "cut and cobble" method on a roof in my home, and removed a section a few years later to check and found that the roof sheathing was soaked! Then I had it spray foamed.

    Dana's method would work well; another method would be to suspend 2x4's under the rafters & fill them with additional insulation - then the roof would be thermally broken with sufficient insulation.

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