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Insulation questions on a gut rehab

tkaj | Posted in Energy Efficiency and Durability on

Hi all,

I’d like to comment on the usefulness of the website while conducting my research during our gut-rehab;we have been reading a ton of information about best practices during construction and what we should and should not do.

Some facts about the house:
Tri-level brick home in Chicago
2×4 walls
2×6 roof rafters
Roof pitch is 4:12
Forced air HVAC
Roof is 5 years old, one layer of asphalt shingles
Home built in 1965

Some facts about the project:
This will be a gut-rehab and a regular 8 foot ceiling will be converted to a cathedral/vaulted one by using a ridge beam to open up the kitchen living space
The plan is to install MDF tongue and groove on the inside of the ceiling
Wall and ceiling insulation will be updated to make the home more energy efficient
Roof will not be touched on the outside since its in great condition

We have some specific ideas/questions about how to insulate the home but need some guidance on the best approach that wont blow our budget completely. From reading all articles we came up with 4 potential solutions.

Closed cell spray foam for walls and vaulted ceiling to make it air tight. We were thinking to go with 4 inches closed cell foam in the ceiling between the rafters and 3 inches of closed foam in the walls. This is very expensive ; the most expensive option and there is some risk associated with closed cell foam do to off gassing and no conclusive studies that give any evidence that his might be a hazard or not. Also, its hard to figure out who is a quality installer since a company comes and bids but a worker comes by to install and I’m not sure how to gauge the quality of the installer. We do have an 8 month old baby so its a major concern.

Closed Cell Spray + Rmax Thermasheath. There will be 1 inch of Closed Cell Spray and the rest will be filled up with Rigid Insulation. We can get our hands on Polyisocyanurate-Rigid-Foam-Insulation-Board and use it to add another 2+ inches to reach appropriate R values. This would be time consuming but we have time so that is not an issue.
Can this be even done or would a better option be to fill the rest with fiberglass? It will be hard to reach the same R value this way with current 2×6 rafters.
Is there potential for some condensation at some spots?
Is one inch closed cell foam enough of a vapor barrier or do we need more?
Should rafter bays and stud bays be filled completely or is it ok to leave 1/2 inch before applying drywall?
Should we use 1/2 inch Rigid Insulation on top of everything and tape the seams and drywall on top of that?

Closed Cell Spray + Rmax Thermasheath. There will be 1 inch of Spray foam on the roof and rest will be rigid insulation. Walls will be completed only with about 3 inches of Rigid foam cut and cobble pancake technique plus 1/2 inch of Rigid foam on top of that with taped seams covering the 2×4 and extending the wall a little bit and drywall would go over that. If I’m correct this will help with thermal bridging but I might be wrong.
Is there potential for some condensation at some spots when it comes to the walls?
Is one inch closed cell foam enough of a vapor barrier or do we need more when it comes to the walls?
Should we install the Rigid foam up against the wall or should we leave 1/2 between the wall outside wall and first sheet of insulation?
Should we use 1/2 inch Rigid Insulation on top of everything and tape the seams and drywall on top of that?

Rmax Thermasheath in both walls and ceiling between rafters using the cut and cobble pancake technique that I have read about here with spray can foam to fill the sides. I agree there is a risk doing this on the ceiling between rafters since this might not be 100% air tight.
I guess same questions as above.

Sistering the roof rafters with 2×10 or more to create more space for fiberglass insulation and vent channel. This will require some additional work but and not sure on the overall cost since we would be changing the roof assembly.
We currently have a similar set up in our current come and the room gets very warm plus this will require some additional carpentry work which is not an issue if this is the least risky and proven way to do it.

Notes: I agree the spray foam might be best practice but I’m just looking for guidance on all options in order to make the most educated choice that has the least amount of risk in the long run to cause problems. There are many options and ways to solve this issue and every application is different and it all depends on the home but I would like to get some professional advice to make the decision easier to make and to get educated when getting bids for the work as well.

Thanks for all the help and please let me know if any clarification is needed.

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  1. brendanalbano | | #1

    For your walls, the standard approach to interior insulation retrofits is closed-cell spray foam, as you have identified. If you are wondering how thick of foam you need, the individual foam manufacturers should say at what thickness the foam serves as a "Class II Vapor Retarder". You want that much thickness minimum. For example, the water-blown Icynene ProSeal Eco (MD-R-210) is a class II vapor retarder at 2.4". The conventional Icynene Proseal (MD-C-200 v3) is a class II vapor retarder at 1.5", but is blown with a blowing agent that has a very very high global warming potential. Other manufacturers values will differ, but probably be in a similar ballpark.

    If you are interested in non-foam solutions for interior insulation of masonry walls, they exist, but require greater attention to detail. One example is discussed in "Figure 9: Smart Vapor Barrier Approach" at the very bottom of this article:

  2. Expert Member
    Dana Dorsett | | #2

    Spray foam isn't necessarily a best-practice, and 3" of ccSPF between wall studs is barely R1 better from a whole wall performance level as 3.5" of open cell foam. See:

    If you're committed to foam, use open cell foam. A full 3.5" of half pound open cell foam in the stud bays uses 1/4 of the polymer of a flash-inch of closed cell, and water instead of HFC245fa for the blowing agent, and it's less than half the cost of 3" of closed cell. Open cell foam is as air-tight as closed cell foam, and SO much greener!

    Using cut'n'cobbled foam board between rafters or studs is similarly low bang-per-buck, and a lot of work.

    If the roof deck & wall sheathing are plywood, caulking the framing to the sheathing with polyurethane foam and taping any seams in the plywood is as air-tight as foam cavity fill. Damp spraying the cavities with cellulose or 1.8lb density JM Spider fiberglass, scrubbed flush with stud edges provides comparable or better thermal performance as open cell polyurethane. If you feel you need more wall performance, an inch of rigid polyiso on the interior side of the studs would bring the wall performance up to current code minimums. Tape the seams with a high quality aluminum tape (eg Nashua 324a, and caulk the top & bottom edges to the framing with polyurethane caulk.

    If there is an inch of vented air space between the brick and the wall sheathing it qualifies as a vented cladding, and you don't need an interior side vapor retarder in US climate zone 5 (Chicago), though using a membrane type smart vapor retarder behind the wallboard isn't a bad idea for fiber or open cell foam insulated walls, but not necessary if you added the inch of interior side polyiso.

    Either vent the roof, or spray 4" of 2lb polyurethane on the underside of the roof deck.

    If spray foam, use water blown Icynene MD-R-200 (or MD-R210) would be sufficiently vapor retardent to protect the roof deck, but still sufficiently vapor open that it's a reasonable drying path. All HFC or HFO blown 2lb polyurethane would be far more vapor tight but also somewhat higher-R (not in a meaningful way, though), but if you must, see if you can't find a vendor of HFO blown foam, which is far more benign blowing agent. Most HFO blown 2lb foam can also be installed in lifts of 4" per pass, whereas HFC blown goods need to be blown 2" at a time, with a cooling/curing period between lifts. This is both a quality and a fire-hazard-while curing issue. (Lapolla is one vendor who has already made the transition to HFOs, but others ar cropping up.) Run away from any closed cell foam installer who claims they do lifts of 3" or 4" in a single pass with HFC blown stuff.

    Rather than sistering on 2x10s (which are heavy, awkward, and thermally bridging), use 11.5" x " 11.5 (or 12" x 12") half-inch OSB gussets spaced 3' on center (or some spacing that works glued &screwed onto a 2x3, so that when you glue & screw them on onto sides the existing rafter it makes a webbed truss, with a 2x6 and a 2x3. Then damp spray it full of cellulose or Spider, scrubbed flush with the 2x3. With 4"/ R20+ 2lb foam on the roof deck you have sufficient dew point control for ~R30 or slightly more of fiber insulation. That would be 8" of cellulose, or 7.5" of 1.8lb density fiberglass. It would add up to the code-min R49-R50, but due to the lower thermal bridging of OSB webbing instead of milled timber it will outperform a code-min R49 between joists or in a vented 2x raftered roof.

    If vented you need more depth to hit code min, but it can be an all-fiber insulation show. The exterior side air barrier/chute has many options, but if dense-packing fiber, half-inch asphalted fiberboard works pretty well, is cheap, moisture resilient, fairly vapor permeable, and it's easier & greener than using cut up foam board. (Asphalted one-side fiberboard is available at Menards, if you can't find it elsewhere.The asphalted side faces the ventilation gap. Home Depot sells fully asphalted fiberboard. You may have to order online and have it shipped to your local store.). With a vented roof and a fiberboard exterior side air barrier for the fluff you don't need an interior side vapor retarder other than standard latex paint. It takes 13" of cellulose to hit R49 (the fiberboard gives you another R1), or 12" with Spider/Optima/L77 etc (only Spider can be damp-sprayed, the others need to be blown in mesh or a closed cavity. As with the unvented example, building it up as a truss is lighter weight and higher performance than milled lumber.

  3. brendanalbano | | #3

    Good catch Dana: I was jumping immediately to interior retrofits on old Chicago buildings with multi-wythe masonry bearing walls (which is what we encounter a lot on the commercial side of things), not brick veneer over stud walls, which it sounds like is the actual situation in question here!

  4. tkaj | | #4

    Thanks these are great ideas. Some clarification on the wall assembly that is currently in place starting from the outside Brick -> Framing -> Fiberglass -> Drywall. There are no vapor or air barriers installed. I think my largest concern is with the roof. Its doe not have sheeting just boards and felt and shingles no OSB. Just like in attached picture.

  5. brendanalbano | | #5

    Quick clarification on your clarification on the wall framing:

    When you say "Brick -> Framing -> Fiberglass -> Drywall"

    Do you mean that there is no sheathing? When you remove the drywall on the inside you can see the bricks? Or is there some sheathing of some sort between the bricks and the framing?

  6. Expert Member
    Dana Dorsett | | #6

    Is there any air gap tor wood sheathing between the framing and the brick?

    Or is the framing in contact (or nearly in contact) with the brick, and braced with let-in bracing (rather than sheathing)?

  7. tkaj | | #7

    No sheathing between the brick and framing. Minimal gap between framing and brick. Typical 1960 to 1970 ranch/split level construction in Chicago area from what I have seen so far. I guess I'm lucky that it has any wall insulation.

  8. GBA Editor
    Martin Holladay | | #8

    First, let's talk about your roof assembly.

    Some basic facts:
    1. You have chosen to create a cathedral ceiling in a space that didn't have one. The work requires the retrofit of a structural ridge. This is a very expensive remodeling project, so I'm going to guess that your budget is adequate. Otherwise you would just live with a horizontal ceiling.

    2. You are leery of spray foam because of IAQ and health concerns.

    3. Your roof has 2x6 rafters.

    4. You don't want to disturb the roofing or the exterior. You want all insulation work to occur on the interior.

    OK, those are the basic facts.

    Here's my advice:

    1. Don't use spray foam, since its use makes your nervous.

    2. Don't use the cut-and-cobble technique for your roof unless you are willing to implement a vented approach. (The cut-and-cobble method has been associated with condensation, mold, and rot when used for unvented cathedral ceilings.)

    3. A vented approach makes the most sense for you, but a vented approach will only work if your roof has no skylights, dormers, hips, or valleys. If any of these obstructions exist, you'll need an unvented assembly. And if you want an unvented assembly without the use of spray foam, you'll need to install rigid foam on the exterior side of your existing roof sheathing.

    4. Your rafters aren't very deep. Step one is to talk to a structural engineer to find out whether the rafters need to be beefed up for structural reasons. If you are beefing up your rafters anyway, you may end up with rafters that are deep enough for insulation.

    5. If you want to insulate your roof assembly from the interior, be realistic. Don't expect your 5.5 inch deep rafters to provide enough room. They won't. You'll need to deepen your rafters somehow. Dana's suggested approach is one good way to do it (again, assuming that your engineer OKs the 2x6s).

    6. When you retrofit a structural ridge beam in an older house, you often end up with air leaks near the ridge. You need to come up with a good detail the ensure that your ridge is (a) airtight, and (b) adequately insulated. An exposed ridge beam probably won't work.

    -- Martin Holladay

  9. GBA Editor
    Martin Holladay | | #9

    Next, let's talk about your walls. The most striking thing about your walls is that they lack sheathing.

    Step one for your walls is similar to step one for your roof assembly: talk to an engineer. You need to find out whether your walls are adequately braced to prevent racking. Your engineer may require interior plywood or OSB (at least in a few locations) as bracing.

    Step two is to read this relevant article: Insulating Walls in an Old House With No Sheathing.

    -- Martin Holladay

  10. brendanalbano | | #10

    Martin: That wall insulation article seems focused on building with no sheathing and wood siding.

    In this case, it sounds like the situation is a brick exterior and no sheathing.

    Would that affect the advice in that article?

  11. tkaj | | #11

    Thanks Martin.
    Lets talk about my walls. After reading the article you reference I have an idea which way I will go.
    1. Like the article states I will install vertical sticks to serve as spacers to create air gap.
    2. Insert Rmax Thermasheath-3 2 in. x 4 ft. x 8 ft. R-13.1 Polyisocyanurate Rigid Foam Insulation Board on top of those sticks and tape all the corners. Will this be enough to provide a good air barrier?
    3. On top of the first Insulation Board I will install Rmax Thermasheath-3 1-1/2 in. x 4 ft. x 8 ft. R-9.6 Polyisocyanurate Rigid Foam Insulation Board using the pancake method and tape. Will add probably need to add some spacers to add depth to wall cavity.
    4. Should I install something like MemBrain on top of everything just in case.
    This should give me about R-22 between the studs in my walls. I know I can go with mooney wall but not sure if I want to.

    Also the house is 52 years old and only had fiberglass as insulation and there is no sheeting or anything and everything is dry no mold all the studs look perfectly fine. So the other option which is less work intensive could be just add 2x2 to extend the wall cavity and put R-21 fiberglass insulation add MemBrain on top of that and be done. Would this approach be a lot worse then using rigid boards and taping them?

  12. GBA Editor
    Martin Holladay | | #12

    Basically, the principles are the same.

    1. You need to establish some type of air gap (using a plastic mesh product or "spacer sticks" and rigid foam).

    2. You need to establish an exterior air barrier (ideally with a product that is moisture-resistant -- again, rigid foam is usually a good choice).

    3. You need to insulate the stud cavities on the interior side of this air barrier.

    -- Martin Holladay

  13. GBA Editor
    Martin Holladay | | #13

    N/A N/A,
    Q. "I will insert Rmax Thermasheath polyisocyanurate rigid foam insulation board on top of those sticks and tape all the corners. Will this be enough to provide a good air barrier?"

    A. I'm not sure what you mean by "tape all the corners." You have to tape the entire perimeter of each sheet of rigid foam, not just the corners. This approach will work, as long as you have chosen a high quality tape that sticks to lumber and that sticks to the type of rigid foam you are using.

    Q. "On top of the first insulation board I will install Rmax Thermasheath using the pancake method and tape. Should I install something like MemBrain on top of everything just in case?"

    A. No, you don't need an interior vapor retarder with the cut-and-cobble approach. The rigid foam is already a vapor retarder.

    Any time that you decide to install insulation between the studs, you'll still end up with thermal bridging through the studs. One layer of continuous rigid insulation (installed on the interior side of the studs) would go a long ways toward improving this wall's R-value, no matter what material you decide to put between the studs.

    The best way to use rigid foam is as a continuous layer, not cut up into narrow rectangles and inserted between studs.

    -- Martin Holladay

  14. Expert Member
    Dana Dorsett | | #14

    Adding 1" air gap from the brick and using 1" foil faced polyiso for the exterior air barrier in the stud bays leaves you about 1.5" of stud bay depth for fiber.

    Rather than adding 2x2s onto the stud edges (which would only bring it up to 3" of depth), glue a sheet of 1.5" polyiso onto a sheet of half-inch OSB, and rip 1.5" wide strips out of your laminated stack, and bugle-head screw those onto the framing. (You'll want to make some 3" wide strips for doubled-up top plates, jack studs, etc.) That would leave a 3.5" cavity depth suitable for R15 batts, and you can hang the wallboard on the OSB.

    The 1" polyiso on the exterior side provides sufficient dew point control for the 3.5" deep batts, so you can skip the MemBrain, etc, and The foil facers make good vapor barrier against the intensesummertime moisture drives of the brick, to avoiding condensation inside the assembly during severe air-conditioning days. The 1.5" of polyiso + 0.5" OSB edge strips puts about an R9.5 thermal break on all of the framing. The net performance will be better than an IRC 2015 code minimum R13 +R5c.i. wall.

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