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Rigid foam attached to underside of rafters: Good idea?

noahniceview | Posted in General Questions on

I live in zone 5a Connecticut , lucky me. Have mechanicals in attic and would like to semi condition space while maintaining a vented roof. Thinking 4″ board, foil on both sides with a value of r28ish. Would like to use attic for storage as well. This method I believe if air sealed and installed well may be a good solution. Am I nuts or on the correct path. Thankyou, Noah.

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  1. wjrobinson | | #1

    My vote, nuts.

    But It's your nuts not mine you're squirreling away. Have at it.

    Option 2: ebay

  2. noahniceview | | #2

    Alright, why nuts? Looking for a solution I can do myself with a low cost attached.

  3. GBA Editor
    Martin Holladay | | #3

    You're not nuts. A few observations:

    1. In your climate zone (climate zone 5), the building code calls for a minimum of R-38 ceiling insulation, not R-26 or R-28.

    2. If your attic has gable walls, don't forget that they need insulation too.

    3. You'll also need to air seal the tricky area at the perimeter of the attic, where the rafters bear on the exterior walls.

  4. noahniceview | | #4

    So, what I should then do is install say 4" between rafters and then screw 4" under rafters. This would give me a r56, overkill? I have about a r19 of fiberglass on attic floor, will most likely deck it for storage.

  5. GBA Editor
    Martin Holladay | | #5

    I wouldn't bother cutting up strips of rigid foam to insert between the rafters. That's pretty much a waste of perfectly good foam -- too many seams.

    Either make the continuous layer of rigid foam under the rafters a little thicker, or be satisfied with R-26.

  6. noahniceview | | #6

    Can I attach more rigid at a later date on top of the foiled rigid or would that create a issue? Tight budget at this time. Can do the 4" myself for $1600 and come back later and double up. If done in stages should I order only single sided foilfaced and point that to the roof deck, then when I double up get single sided again and put foil side down. Or does it not matter. Thankyou for your help, I don't want to cut up foam either. Also should I go a different route? My brain hurts thinking of all the ways to go. This application makes good sense to me, but I'm no expert.

  7. wjrobinson | | #7

    Waste of money. No ROI. I stick with my vote. Nuts.

  8. noahniceview | | #8

    What would you do AJ? I am looking to stop massive ice dams, create a decent environment for hvac, save on fuel and have a comfortable home. For $1600.00 comfort trumps roi. This little project is going to be implemented stages. Stage 1 fix leaky attic and hvac, make it happy up there. Stage 2 insulate basement and stop stack effect. Stage 3 windows. Would like to go from burning 8 cords in a 2200 square foot cape to 3-4 cords, so when I'm old it will be manageable.

  9. Expert Member
    Michael Maines | | #9

    Noah, there is no problem with adding extra layers of rigid foam in the future. I don't know what the ROI would be but with mechanicals already up there, only R19 in the attic floor now, and ice dam issues, I'd say you are on exactly the right track.

    If headroom is an issue I would go ahead and put strips of foam between rafters, as long as they are sealed to the rafters with spray foam. If headroom is not an issue than an additional continuous layer is better and easier.

  10. Expert Member
    Dana Dorsett | | #10

    To stop ice dams you have to block the thermal bridging of the rafters too, not just the center cavity. You can use high-density cathedral-ceiling batts leaving 2" of clearance to the vented roof deck and at least 2" of interior iso (interior must be side fire-rated Thermax) glued to the rafters, seams taped, and held in place with 1x furring long-screwed to the rafters 24" o.c. to minimize the thermal bridging via the fasteners. Assuming 2x6 rafters and R15 batts, that would give you a center-cavity R of ~R27, but it would outperform a code-min R38 low-density batt solution since even at the rafters you'd be at ~R18. Be sure to stack foam (or use closed cell spray foam) at the top of the studwalls at the bottom of the rafter bays or you may still end up with ice dams.

    For coastal CT that would probably be enough to prevent ice dams but for somewhat higher altitude interior locations (eg Waterbury) you'd likely need at least ~R40 center cavity, ~R35 at the rafters.

    In southern New England there are multiple sources for reclaimed roofing foam from commercial sources (try craigslist first, or if you have a truck and are willing to drive to Framingham MA).

    With a vented cavity above the fiber don't even think about cheaping out with low-density batts, and make sure you fit them PERFECTLY, since every gap or compression leaves a tiny thermal bypass to eat up performance.

  11. noahniceview | | #11

    Thankyou for your input. Not worried about headroom, storage is for holiday crap and seasonal clothes etc. Only issue is I need to frame in a real attic access. I think starting with the 4" foam will make a significant improvement. Thanks again.

  12. jhrockwell | | #12

    I'll echo the importance of taping the seams on the interior surface and sealing the junction of roof to floor. After you apply rigid foam to the inside face of rafters, the cavities and roof deck will be relatively colder in heating season, and any escaping moisture vapor is more likely to condense on those colder surfaces, with no chance of drying to the interior.

  13. wjrobinson | | #13

    Noah, your home needs air sealing. Installing the rigid foam is difficult and dicey. Insulate your rim joist and air seal the basement. Air seal windows doors by pulling molding. Air seal attic floor completely. Seal opening to attic. Methods to do all available here and all over the web. Your utility may have lots to offer you.

    Very unnoticed gaps in your rigid plan will leak out your 8 chords just like it doesn't exist. That's what many don't understand. Possible but not probable that you will get the results you are hoping for IMO.

  14. noahniceview | | #14

    Will be cutting rigid and foaming in place at wall plates as well as foaming gable wall top plates. Then foam sealing rigid seams. Staggered and taped as well. I have miles of foil tape. Don't like the idea of any type of batt insulation, unless their is something I don't know about. Seems foam in its various forms would be superior.

  15. noahniceview | | #15

    I understand what AJ is saying. The way I see this is (and please correct me if I'm misleading myself) snap some lines, install straight and tight, seal seams very well. Were there are obstructions, improvise don't comprise. Ive had quotes of $4500.00, one was for open cell sprayed to underside of roof deck 5.5" giving me r20ish. The other was for 4" silver glo rigid foam also r20. Both to expensive, I'm sure they work well but I'm a hands on type of person and the product I would like to use has a better r value and is also foil faced in both sides for much less money. I am in the trades a fully understand how important the install is. Understanding I will need more insulation up there when I can afford it. Baby steps due to buget and stages for managing. All the heat goes up and out at my house, so why not stop it there first.

  16. noahniceview | | #16

    What does Imo mean?

  17. noahniceview | | #17

    I guess what I need to know is this. What is the best way I can create a semi conditioned attic in zone 5a with keeping a ventilated roof and not calling the spraytruck. Thanks guys you've been very helpful.

  18. Expert Member
    Dana Dorsett | | #18

    As much as I detest batts in general, the additional benefit of cut'n'cobbled foam over high-density batts for the rafter bay fill just isn't there. The rafters rob ANY cavity fill of performance, and even foaming the cut'n'cobble in place would have dubious air-sealing integrity whereas you can do pretty well with mounting foil-faced foam on the rafters themselves. With the bridging of the rafters factored in, with a 15% framing fraction the difference between 4"/ R15 h.d. batts and 4" of poly iso is about R2, a difference that can made up with another half-inch of foam thickness on the interior-side rigid foam. The performance of h.d. batts is FAR superior over temperature than the low-density cheapies that everyone (rightly) detests.

    The scrap rate on a cut'n'cobble is also quite high, which is another reason to go with reclaimed goods if you're hell-bent on the approach. The cost per unit R for reclaimed goods is less than virgin stock mid density batts, and about 1/4-1/3 the cost of virgin-stock rigid iso (even less if you take the beat up goods with the dented & broken corners, which probably works for this app.)

    Installation tip: If you're going to cut'n'cobble, get some 2" thick goods and glue-in a 2x2" foam tight to the roof deck & rafter as spacer-rails for the vent cavity, then cut the center-goods at least an inch narrower than the cavity, and foam it in with FrothPak or other 2 part foam kit. It'll go quicker and be tighter. If you go with the 12 board-foot FrothPak kits from box stores at ~$40/pop you'll be into it for some real money, but not as much as you'd be spending on 1-part foam cans. There are bigger foam kits out there (FomoFoam, TigerFoam, etc) that you might consider- you do the math on the total volume needed and $/volume. At 600 board feet you can get the spray foam cost down to about $1.20/board-foot, compared to $3.35/bf-ft in the 12 b.f. FrothPaks, or $5+ for 1-part foam cans.

    Spraying o.c. foam on the underside of the roof deck would provide a more reliable air seal, but has long term issues related to roof rot since the unvented roof deck would get a lot of moisture cycling through high-permeance foam in a CT climate. (Plank roofing would tolerate this much better than plywood, and OSB would have a crazy-short lifespan.)

    AJ has it right, unless you're going to blower-door test and rectify the leakage through your cut'n'cobble before adding anything to the interior it's bound to fail miserably as an air barrier-it's just too tough to make perfect even with edge-foaming. It's worth verifying & fixing an interior-side taped-seam iso approach too.

    AJ also has it right that you need to treat both the bottom and the top of the stack to make a serious dent in stack-effect infiltration losses, and the biggest untreated unseen air leak in most houses is at the foundation sill & band joist, which is at least as important as dealing with the attic.

    Also, unless you have already finished the basement, insulating the foundation and band joists/sills with 2-4" of reclaimed roofing iso is also a HUGE benefit in a CT climate, and easier to implement than a cut'n'cobble between rafters. (Keep the bottom edge well off the slab to avoid wicking, use 1x furring through-screwed to the foundation with TapCons 24" o.c. on which to mount the code-required half-inch gypsum as an ignition barrier.

  19. noahniceview | | #19

    Not going to cut n Cobble anything. Just want to attach product to underside of rafters. Well aware of stack effect and how it works, can't Do it all at once. Need to make upgrades as money is available(unless you need to get rid if some). Need to start somewhere or I will get nowhere. Basement is next. We will have a major rehab of that craphole next year and will address all those issues at that point. The 2 part foamkits are a part of the plan. No half assing around.

  20. GBA Editor
    Martin Holladay | | #20

    Q. "What does 'Imo' mean?"

    A. Thank you, Noah! I'm glad to welcome you to join my group -- the "Let's Stamp Out Confusing Internet Acronyms Association." (By the way, no one is allowed to refer to the group as the LSOCIAA.)

    I'm just guessing here, but IMO may mean "in my opinion."

  21. jnarchitects | | #21

    I have the same set-up in my own attic with the exception of a couple key details. I have a hot roof (not vented) w/ dense packed cellulose. The ridge and perimeter of the foam was sealed with great stuff (apparently not as great as the name suggests).
    This summer I noticed some staining streaks on the interior face of the foam. I cut the tape along one of the seams. Guess what I found...water.
    Turns out the cellulose was not packed to an adequate density and the great stuff probably isn't sealing as well as necessary. Last winter some warm moist air made it into the rafter plane, then the nice warm summer sun drove it back to the interior condensing on the sticky side of the tape (not on the backside of the foam).
    We repacked the cellulose and will be sealing perimeter and ridge with 2 part foam. There are also a couple of places where ducts dive down along the perimeter into the ceiling plane that need to be better sealed. I'm considering adding strips of rigid along the taped seams, so any gaps b/w the rigid boards (which will happen) don't allow for a condensation space. The only problem with this approach is if anything happens in the future it will be harder to detect as the taped seams will be covered.

    Just wanted to pass along recent experience....

  22. GBA Editor
    Martin Holladay | | #22

    As you probably realize by now, your method of insulation is not recommended -- in fact it's a code violation.

    If you are dense-packing your rafter bays with cellulose, you MUST include a ventilation gap between the top of the cellulose and the underside of the roof sheathing UNLESS to have an adequate thickness of rigid foam ON TOP of roof sheathing, or an adequate thickness of closed-cell spray foam sprayed against the underside of the roof sheathing (flash-and-batt or flash-and-fill).

    More information here:

    How to Build an Insulated Cathedral Ceiling

    Creating a Conditioned Attic

  23. jnarchitects | | #23

    It is a code violation now, but was a configuration allowed locally for years successfully and supported by several local building science professionals. Oddly enough, it is an install that is still warrantied by the cellulose producer despite the new code. But yes, if I had to do it all over again, I would absolutely be putting nail base on the roof. At the time, it was a budget issue, I could install the rigid in the attic myself, but had to pay someone to do it on the roof.

    Live and learn...that's the best part and worst part of working in the construction field. It has been a great process to workout what was happening and how to address it.

  24. wjrobinson | | #24

    As this thread grows... so will you see that "your plan" to any of us, sounds great, as we dream up ideas and solutions to what can go wrong... what we are doing is trying to guarantee that "all space shuttles will always fly safely." Over time, I see lots of unexpected failures in the use of rigid foam no matter how it is used, Inside, outside, on OSB, continuous over rafters or floor joists.

    The good thing about this case is that failure leads to loss of warm air but not rot (Chris's and others unvented disasters). What is getting lost is the savings that one is dreaming of. Spending a few thousand to save a few dollars is.... OK I won't repeat myself.

    This is my opinion (IMO), many will differ. So did the engineers of the space shuttle.

  25. noahniceview | | #25

    AJ and Dana, I'm with you guys and I get it. I know I have to do the basement as well. Attic is were I choose to start my energy saving upgrades. I will address those other areas when the cash flow is available. What I don't get is what AJ is saying about spending a few thou to save a few dollars. When one has poor insulation in attic and one would like to add more one would expect to spend some money. AJ said to air seal the floor and blow in some insulation and I agree. However, I need to create a semi conditioned attic for my hvac and storage. So.. by adding the insulation under the rafters to maintain a ventilated roof at a cost of say1600 for a r28 airsealed radiant barrier product with the ability to add more if needed at a later date, how is that a bad idea if an air sealed attic floor w/blown in will cost roughly the same and I still won't have a good environment for hvac etc. That's what I'm having a hard time with.

  26. wjrobinson | | #26

    Noah, do your plan, if I was there I could help critique any mistakes. good luck my man.

  27. noahniceview | | #27

    Thankyou Aj, I only want to do this correctly.

  28. Expert Member
    Dana Dorsett | | #28

    OK, no-cut'n'cobble (I misunderstood!)- you'll get there. Double-layer the foam, staggered seams, minimize the number of fasteners to minimize thermal bridging. Use reclaimed goods for 3-4x the R/$.

    It's still worth putting batts in the rafter bays as long as you leave 2" of air gap to the roof deck.

  29. toddimt | | #29

    I posted a similar scenario although maybe not stated as eloquently. I live in Essex County NJ, which I believe is climate zone 4a. I have a similar circumstance in that my A/C air-handler and duct work is in the attic. I have a split level home.

    I like to preface that my house WAS very leaky with old windows etc. This also made it that the humidity couldn't be increased by to much in the winter, since you would get condensation on the windows. Well, the windows have all been replaced and the lower level was dense packed with cellulose and the lower attic was blown with cellulose. Now you can increase the humidity level in the winter. Its Auto set using automatic aprilaire humidistat with outdoor temp sensor. On a whim, I happened to go into the upper attic on a cold day. I had just installed a replacement type of pleated filter for my furnace in the basement that I figured I would do the same on the A?C airhandler. When I opened the filter door, I was hit with hot humid air. I saw a good amount of condensation inside the filter area and mold starting to grow on the edges of the filter door. This all looked like a new issue, since the mold amount was small. With the house very leaky before, the indoor humidity was lower and this was all probably a non issue. I also never noticed the convection currents before of the air going up into the return vent, in the upstairs hall, only to fall out cold on the lower level. House was so drafty previously. This is an issue not discussed but an issue that can now be introduced by making improvements elsewhere. Area's like the external pleated filter compartment on a a/c system, doesn't have that much insulation internally and can't really be wrapped, due to the need for access.

    Thus, I was exploring a similar idea to what Noah had proposed. However, I wasn't looking to convert the entire attic to conditioned space. I was looking to just make like the middle half - 2/3 of the attic as interior space. This would encompass the airhandler, return duct, main trunkline, and most of the supply lines and attic steps. There would probably be a couple that would have to run out of the conditioned room. I would think it would be easier to build knee walls and shrink the space down then try to go from ridge to soffit. especially since you couldn't run rigid foam all the way down to the top plates anyway. Thus you would have Thermax foam walls. could cut pieces of thermax or plywood to seal between the joists under the knee walls. Would then have the thermax ceiling.

    Noah's plan calls for the Rigid foam to be installed under the rafters. Possibly 4" worth and then maybe add additional. I understand it may not be the easiest to cut rigid insulation and attach between the rafters, but it seems I read about this practice all the time as a way to create baffle vents. What's the difference between rigid foam or plywood? I don't get the use of fiberglass hi-density batts being above the foam and vent channel idea. Wouldn't the fiberglass loose almost half its R-value hi-density or not sitting in the rood air ventilation path? I also, didn't get Dana's post about cutting foam down and gluing it to the roof sheathing for the blocking for the foam. I can't see glue keeping this in place all on its own. I would think fasteners would need to be used but no way I see doing that foam to foam. Why would 2x material ripped down be bad? Its up against the rafter anyway and above the conditioned space.

    I understand the thermal bridging argument of the rafters but how is this any different then a wall, in an older house that has framing that extends from plywood sheathing to drywall? The R-value here is almost zero. Is using ore screws going to really increase thermal bridging? Think of the same example of a wall and attaching drywall with the tons of screws. The 4" of rigid foam under the rafters at least give you R26. But a 2x6 rafter with a 1.5" vent space , would give you the ability to put 4" of rigid insulation (R-26) in that space which was not filled. 2" would be to create the baffle in the 1st place. With the 4" below the rafters, you would then be R-26 below the rafters and R-52 between No? Staggering the seams, taping and adding mastic "should" cut off the airflow from within the conditioned space from flowing outward. Single ply's of foam to me would be harder I would think then multiple staggard ply's.

    If I was looking to go like 6-8' in either direction from the ridge towards the soffit. I "could" run the air baffle all the way to the soffit but then I would think that would only need to be the 1st layer of foam.

    While this might be a "cobbled" approach, I would think it is one of a few options that would correct a condensation issue. While closed cell foam, applied to the underside of the roof deck, I have read too many horror stories when applied in retro attic situation. Thus, I was looking for an alternative.

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