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Community and Q&A

Rigid polyiso in side attics?

Ryan Griffin | Posted in Green Building Techniques on

As a spray foam alternative, I’m considering using continuous rigid foam along the 32ft long x 7ft slope in my side attic and just spray foaming in the lower joist bays and places where taping is difficult. The attics are about as simple as can be with no dormers or other obstructions. I will need 8″ of polyiso to meet my disired R-value (R-48), while keeping the 2×4 rafter bays wide open for ventilation. There are also HVAC ducts and plumbing in the attic, which is why I’d like to convert it conditioned space. I would need to cut the 4×8 sheets down to 2×8 sheets to get them in the access panel.

On top of that, Rmax polyiso is only sold in 1″ 4×8 sheets in my area ($15ea) and the price jumps to $60 for a 2″ thickness via special order. That has lead me to believe that it would most economical to use 8 layers of 1″ Rmax (roughly $700-800 material for the side attic). While still spray-foaming at the top and bottom (possibly with low expansion foam).

My 3) questions are:

1) how hard will it be to hang 8″ of rigid from the attic ceiling, and are there any tricks, like maybe screwing the first few layers down and then glueing the rest to that? I can’t imagine any fastener long enough to go through 8″ to grab the rafter

2) can I glue foam board together ahead of time to create 2″ or 4″ layers and

3) would I be better off just spray foaming to R-48 (also almost 8″)?

Ryan Griffin
1450SF 1.5 story
Minneapolis MN
climate zone 6

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  1. GBA Editor
    Martin Holladay | | #1

    Gluing foil-faced polyiso to foil-faced polyiso is tricky. The glue dries very slowly because the foil is impermeable. Perhaps some types of glue dry faster than others, but most foam-compatible glues that come in a cartridge dry slowly when used to glue foil facing to foil facing.

    Cap nails can be purchased in lengths up to 8 inches long -- not quite long enough for your plan, but good enough for the first 6 inches of foam.

    Timberlock sells screws up to 10 inches long; you can add a fender washer if you want when using them for rigid foam.

    Your plan is doable but fussy. If you are patient, though, the plan will work. Make sure that you pay attention to airtightness -- seal the rigid foam at the seams and the perimeter.

  2. Ryan Griffin | | #2

    Thanks as always, Martin. Given the glue drying situation, I'm a little reluctant to try this. I looked in the cavity, and I would be performing most of this work on my back laying across floor joists, so messing around with hundreds of 10" screws doesn't exactly sound fun. A contractor recommended that I could spray foam the floor with 1-1/2" foam and side wall with 3" foam instead of the slope and fill the cavity with cellulose. I'm thinking I might just install air cutes in the rafters and fill the whole side attic top to bottom with cellulose.

    That brings me to my next question... In the top attic there is barely enough clearance to get in (15" at the highest point. In a perfect world, I would do the same treatment up there, spray foam 1-1/2" across the ceiling joists and then blow cellulose to fill up the rest of the space, leaving air chutes. The issue is< i don't think a man could safely get in there to foam. even if he did, there is no way he could reach the far end of the low 3/12 pitched roof. where rafters meet ceiling joists (think salt box). So, now that I'm back to thinking spray foam, will I run into any issues where spray foamed slpes meet cellulose top attic? See diagram. I would need to hang a sealed air barier from the existing drywall ceiling. Please let me know if there is a better way that doesn't involve tearing off the roof. Thanks,

  3. GBA Editor
    Martin Holladay | | #3

    The best solution with this type of roof is always to install new rigid foam above the existing roof sheathing, followed by a second layer of roof sheathing and new roofing. But you don't want to do that.

    Your plan will work, however, especially if you can gain access (if necessary, by demolishing interior finishes) to do the work properly.

    Two things puzzle me: You mention, "I would need to hang a sealed air barier from the existing drywall ceiling" -- but why isn't the existing drywall already an air barrier?

    Also, your diagram says, "Add 3/4" tongue and groove vapor barrier to ceiling" -- to which I can only ask, "Huhn? What's that?"

  4. Ryan Griffin | | #4

    Martin, if we were to do the exterior roof insulation, is it possible to do just the back (low pitched portion? The main thing that has stopped me from doing this is that the city requires a minimium 1" air gap and no insulation may touch roofing materials. This adds a second layer and additional cost, though may be possible. I also thought that it may be challenging to transition between the convention insulation method on the steep slope to 'outsulation' on the flat portion. I could be wrong, so I appreciate your input. if you were to replace the roof, would you still suggest having no insulation between the ceiling joists?

    My goal here was to find the most cost effective way to achieve R50 with a full vapor and air barrier. The 3/4" ceiling material was tongue and groove foil covered polyiso, taped at all seams. The idea was to apply the minimum effective air/vapor barrier to the underside of the ceiling since it would not be possible to install anythng on the attic side due to space constraints.

    If there is a better way to acomplish airtight R50 here, that is the approach I want to take. Anything can be demo'd since it's a major renovation, but I wnat to keep what we can.


  5. Richard Beyer | | #5


    If you insist on using spray foam learn as much as you can about it to avoid the pitfalls of hiring a novice installer. Note that all existing insulation should be removed prior to installation of SPFI and the room MUST be fully ventilated to the outside during installation and up to 72 hours after. NO ONE other than the installer(s) should be in the home during installation and up to 72 hours after installation. Be sure to choose your installer very carefully and be sure to verify with the manufacturer of the "chemicals" their training is up to date.

    This product should not be used by anyone who has not fulfilled the proper training.

    NOTE: "The existing insulation has the ability to leach the odor created during the install of the SPF Insulation. This leaching action can cause the SPF install odor to linger indefinitely."

    Training criteria for low-pressure.. aka..Froth Paks...

    High-Pressure Criteria.... Foam installed utilizing specialized equipment.... The BEST training is through the actual chemical provider and third party trainers like SPFA..

  6. GBA Editor
    Martin Holladay | | #6

    Yes, it would be possible to install exterior rigid foam on just one side of the roof (the side with the lower slope). However, I would only take this approach if I were committed to eventually doing the same work on the other side of the roof.

    Yes, the problem with this approach is maintaining continuity of your insulation barrier. That would be tricky, but doable.

    Your suggested approach (the approach that doesn't require any exterior work) is probably less expensive, so I certainly understand why you would prefer it.,

  7. Aj Builder, Upstate NY Zone 6a | | #7

    I actually agree with Richard, spraying closed cell in an existing home is too risky for me. I have a good company and I would let them spray open cell Icynene, but for a few years now I am doing everything in my power to avoid spray foam even in new construction which is where it only should be approved for use. If the foam stinks, your home will never be the same. And I would abandon it instead of remediate it.

    Save a dollar. lose a house.

    Change your plan to internal rigid foam lose some head room and then use vents and cellulose.

    Get as much R as you can. Then deal with air leaks and other parts of your home to save on energy bills.

    The best way to save on energy...

    Heat less to no space
    Smaller home, less rooms heated
    pellet stoves
    wood stoves
    mini splits
    move to mild climates
    live on a boat in a mild climate... no tax no heat bill no AC bill... swimming year round.. fish for dinner.

    Any way.... rigid foam on the inside.... best cost and safe install and easy peasy.

    Richard, see we do agree sometimes... how do yaa like them apples? When's the last time you heard that phrase?

  8. Ryan Griffin | | #8

    AJ, I love the idea of rigid on the inside. It's just hard to get anyone willing to to quote it for me. Unlike the NY, where I'm from, nobody in Minneapolis seems to touch the stuff. Of course, I was also complaining about the thought of hanging it myself and boxing in all window and door trim. Please shoot me a message if you have any recommendations for contractors in the twin cities area who do this kind of work.


  9. Expert Member
    Dana Dorsett | | #9

    In Minneapolis you'll get more performance out of the rigid foam if the outer ( "cold side in winter") 25-35% of the foam thickness is EPS rather than polyiso, due to the funky derating curve of polyiso.

    When the average temp through the foam drops below the 75F ( the median foam temp at which it is tested for R-labeling) the performance both polyiso and EPS begins to rise. But with polyiso there is an inflection point at which it's performance begins to fall rapidly, whereas with EPS the performance keeps climbing with falling temp.

    If you're going for a max thickness of 8" you'll get better performance out of 3" EPS / 5" polyiso or 2" / 6" stackup than an all-polyiso solution, since the outer couple of inches of colder polyiso would be performing at less than half it's tested & labeled R during the coldest hours. That outer EPS would be performing somewhere between R4.5-R5/inch during the coldest hours, but if it were polyiso it would be R2.5-3, even though the inner layers of polyiso would be performing near (or slightly higher than) the labeled R-value.

  10. Ryan Griffin | | #10

    Dana, that's a very interesting and helpful tip. I knew performance of both degraded compared to XPS, but I hadn't heard that EPS is better than Polyiso. Can you point me to a chart for this by any chance?

  11. GBA Editor
    Martin Holladay | | #11

    For more information on the topic that Dana raised, see In Cold Climates, R-5 Foam Beats R-6.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Ryan, i really don't have a feel for this project. Much more info and pictures needed or a airline ticket. Like shark tank, I'm out. Disregard my earlier post.

    If I was there, I start with talking needs and budget. Then options, and a really good look at all. You need someone there who has the talent to do this job. It's not a hard job but very very few contractors are doing this type of work, almost none.

    Post more details.... for now... shark tank style, I'm confused,.... and out. A turkey is vying for my attention.

  13. Expert Member
    Dana Dorsett | | #13

    EPS does not degrade relative to XPS, but rather gains on it slightly with ever lower temperatures. See:

    Note that the separation between the red line (XPS) and black line (EPS) is slightly narrower at the cold end of the scale than at the warmer end. This is partly a function of the blowing agent of the XPS. (That blowing agent dissipates over time- after 5 decades XPS is performance is only ever so slightly better than EPS of the same density.)

    But note that the crossover between the EPS line and the brown polyiso line occurs at a median foam temp of about 8C / 46F. So, when it's 35F on on side of the polyiso layer and 55F on the other they're at about the same performance level, and when the warm side of a polyiso layer is only 40F, it will under-perform EPS of equal thickness. But when the cold side of the polyiso is 35F with an interior temp of 70F, the polyiso will likely outperform the EPS, since the average temp through the polyiso would be above the +8C crossover point, per Karagiozis' chart.

    This varies a bit by manufacturer & process. The folks at Building Science Corp are allegedly working on teasing more of the derating details out of the woodwork with more lab testing. The very preliminary and known-incomplete discussion of the topic begins here:

    Note that in all of these testing the temperature is an average temp through the layer, not the cold-side temp. The delta-Ts across the foam will likely have an effect, as well, but I await further discussion based on BSC's testing.

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