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Community and Q&A

Spray foaming the attic with a twist

mculik5 | Posted in Energy Efficiency and Durability on

I will be having my attic sealed and insulated with closed-cell spray foam. However, the design of my house makes this a bit more complicated because my roof cuts into the second-story ceilings at the front and rear edges of the house (will make more sense if you look at the attached pic).

Above this part of the ceiling, there is only some very old batt insulation (less than R-8) and the roof deck. The house doesn’t have eaves/soffits, so there is no venting in this area.

As shown in the attached pic, the spray foam contractor suggested building up the spray foam to form a sort of mini-knee wall at the edge of the attic, separating the attic from the space above the sloped ceiling.

This approach makes me very nervous because the sloped space will essentially be uninsulated and unvented. I’m not so worried about energy loss as I am about ice dams.

This is in climate zone 5A.

Do I have a valid reason to be concerned? Can I just blow cellulose in this area and seal it up?


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  1. mculik5 | | #1

    Not sure why my text got cut off, but here's the whole thing:

    I will be having my attic sealed and insulated with closed-cell spray foam. However, the design of my house makes this a bit more complicated because my roof cuts into the second-story ceilings at the front and rear edges of the house (will make more sense if you look at the attached pic).

    Above this part of the ceiling, there is only some very old batt insulation (<R-8) and the roof deck. The house doesn't have eaves/soffits, so there is no venting in this area.

    As shown in the attached pic, the spray foam contractor suggested building up the spray foam to form a sort of mini-knee wall at the edge of the attic, separating the attic from the space above the sloped ceiling.

    This approach makes me very nervous because the sloped space will essentially be uninsulated and unvented. I'm not so worried about energy loss as I am about ice dams.

    This is in climate zone 5A.

    Do I have a valid reason to be concerned? Can I just blow cellulose in this area and seal it up?


  2. mculik5 | | #2

    Weird...there is a glitch with GBA where if you use a less-than symbol in your post, all the text after it will be cut off. I tried to use the symbol for "less than R-8," and the remainder of my post was cut off.

    Apologies for the first answer. I attempted to re-post, and, as you can see, experienced the same issue.

  3. Expert Member
    Dana Dorsett | | #3

    The symbols are read as HTML code, which is why some symbols screw it up.

    Sealing an attic with some amount of closed cell foam makes sense, but more than the minimum amount required for moisture control and air sealing isn't so green, since nearly all ccSPF is blown with HFC245fa, which has about 1000x the global warming potential of CO2. How thick were you going with the stuff, and what was the stackup? Is the attic above still a vented attic?

    What is the full rafter depth in the sloped ceiling section?

  4. mculik5 | | #4

    Thanks, Dana.

    You had replied to another of my posts and informed me about HFC245a. I'm investigating water-blown closed-cell as an alternative.

    I realized that my first post didn't make it clear that the spray foam contractor can't get his guns into the area where the ceiling is sloped, hence his suggestion for a foam mini-knee wall.

    Hopefully, the attached picture will give you a better idea what's going on. The white area with the red question marks is the area I'm asking about.

    Again, the contractor can't fill this area with foam, and because the house has no eaves/soffits, it's not "officially" vented (though I highly doubt it's air tight).

    To answer your questions directly:

    - Rafters are 5.5" deep, including sloped ceiling section
    - Foam will be 4" thick to comply with code requirements for R-20 and class-II vapor retarder
    - The attic will be sealed, as it contains HVAC equipment
    - There is no insulation stackup - just foam (so, from the top - asphalt shingles, felt, wood roof deck, 4" foam, with 2x6 rafters 16" OC)

    Is it a good/bad idea to blow cellulose into the question area? Leave it as is? Other ideas?

    Thank you!

  5. mculik5 | | #5


    Does anyone have any ideas?

    I had another contractor out today who said he would just shove some rock wool in there.

    Weather I put insulation in there or not, my concern in the summer is that warm, most air will enter the space and condense behind the ceiling because it will be cooler from the air conditioning.

    My concern in the winter only applies if I don't put insulation (I think). In that case, the warmth going through the ceiling and open cavity will heat the roof deck and cause ice dams when it snows.

    Any other comments?


  6. GBA Editor
    Martin Holladay | | #6

    It looks like you double-posted this question. I posted an answer on the other thread where you posted the same question; I'll copy and paste my answer here to make sure you see it.

    The solution is to remove the drywall on the sloped ceiling so that the spray foam contractor has easy access to the roof sheathing.

    Once the insulation work is complete, you can install new drywall and paint. Remember, drywall is cheap.

    One point worth mentioning: you are planning to insulate your roof to about R-26. That is well below the minimum code requirements for roof R-value in your climate zone. (Although you inform us that northern New Jersey is in Climate Zone 6, it is, in fact, in Climate Zone 5.)

    According to the 2009 International Residential Code, the minimum R-value for roofs in Climate Zone 5 is R-38. If you wanted to provide that much R-value with closed-cell spray foam, you would need to install 6 inches of foam. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

  7. mculik5 | | #7

    Thanks, Martin.

    While I understand it is the best approach, pulling the sloped ceiling down is a much bigger project than we're interested in doing right now. We have plaster walls over what appears to be a precursor to drywall (fortunately, no lath), so it's not as easy as take some drywall down, put some drywall up.

    Are there alternatives to what you're suggesting? Blowing cellulose into the area? Sliding pieces of rock wool in there? Honestly, I'd leave it as is if there isn't a risk of condensation and/or ice damming.

    Also, while I fully concur that R-XX is R-XX whether it's spray foam or any other form of insulation, spraying more than 4" puts this project out of our price range. My plan is to add more insulation when we re-roof the house in a few years, and I believe that spray foam at R-26, turning the attic into conditioned space, will be more beneficial than cellulose at R-38+ and keeping the attic vented.

    For context:

    Our overall goal is to increase comfort upstairs. We're not trying to save money, or specifically meet new energy efficiency/house tightness standards. Right now, we have about R-8 worth of fiberglass batt on our attic floor, and it leaks like a sieve. Any insulation work we do will be a considerable improvement. The reason I decided that spray foam is the way to go is because:

    - Our HVAC equipment is in the attic
    - The house has no eaves, so venting is tough as it is
    - Spray foam in the rafter bays has generally the same effect as cellulose on the attic floor
    - Keeping insulation off the floor makes accessing electrical and HVAC equipment much easier
    - Spray foam can be a consistent thickness, whereas there will be more cellulose in areas not used for storage/HVAC equipment, and less where we have to cover the joists with decking


  8. GBA Editor
    Martin Holladay | | #8

    If the existing ceiling is now insulated with only an R-8 batt, then this roof assembly could certainly cause ice damming.

    It's hard to know what to suggest if your budget isn't big enough to address the problem, other than to visit a local bank and to take out a home improvement loan.

  9. mculik5 | | #9

    So my "science" is right regarding the ice damming.

    Any concerns with simply filling the area in question with cellulose or rock wool and sealing it up with the foam mini-knee wall? ~5" of either insulation would provide an R-value in the neighborhood of R-19.

  10. GBA Editor
    Martin Holladay | | #10

    You can't fill rafter bays with cellulose insulation (or any similar air-permeable insulation) unless you also include a ventilation channel between the top of the insulation and the underside of the roof sheathing. This ventilation channel needs to communicate with soffit vents that allow air to enter the bottom of the ventilation channel, and with ridge vents at the top (or with a well-ventilated attic at the top). This ventilation channel is required by most building codes for all types of air-permeable insulation.

    For more information on the correct way to insulate a sloping ceiling, see How to Build an Insulated Cathedral Ceiling.

  11. mculik5 | | #11

    I don't mean to be a pest with the back-and-forth, and I'm learning something new each time you post, but I'm still not fully understanding.

    The thing that seems to be confusing the entire issue is that the house has no eaves/soffits. The roof does NOT overhang the exterior walls. The only ways I could "officially" vent the areas in question are:

    - Drill vent holes through the sides of my house, and/or...
    - Drill vent holes in the trim boards on the gable ends

    The only ventilation the area in question gets presently is natural convection. In the winter, warm air radiates through the walls into the cavity, the warm air in the cavity rises towards the ridge, and cold air FROM THE ATTIC (not through soffit vents) replaces the warm air.

    Does that change anything?

    The way I see it, the only purpose of the insulation in the area in question is to prevent heat transfer to the roof in winter. Whether there are baffles doesn't matter because I'm not expecting air to flow from bottom to top in this space.

    Again, thanks for the help. Just trying to do the best thing I can within the budget that I have, and without tearing the ceiling down. The insulation contractors have been very little help because all they want to do is sell insulation and forget about the details. This forum is fantastic.

  12. GBA Editor
    Martin Holladay | | #12

    If you install air-permeable insulation in your rafter bays without a ventilation channel, moisture can accumulate in your roof sheathing. In some cases, the result is rot.

    The construction of your roof is not conducive to the installation of a vented assembly. That's not unusual. In a house like yours, you need to create an unvented roof assembly by installing spray foam against the underside of the roof sheathing, or by installing a thick layer of rigid foam above your existing roof sheathing (followed by a second layer of plywood and new roofing). Your options are explained in the article I linked to.

    To install spray foam insulation, you need to demolish your ceiling. If you can't afford to do the work, you are in a pickle.

  13. mculik5 | | #13

    So, I'm in a pickle. That's what I was afraid of.

    You're probably going to hate this, but here's my list-ditch idea:

    Spray foam the attic as planned, BUT leave the gable vents/roof vents OPEN, and DON'T build up the mini-knee wall.

    My thought is that this approach won't change the current ventilation setup of the attic, but the insulation in the rafters will reduce the amount the attic heats up because of the sun.

    Reducing solar heat gain is my primary objective. The HVAC equipment in the attic is used only for air conditioning in the summer, so there's no issue with the attic being vented in the winter from that perspective. If I can get the summer attic temp down, that will significantly improve the efficiency of the HVAC equipment, as well as comfort (though not as much as truly sealing the attic). I understand this solution won't improve winter comfort, but that's not a major issue.

    I used a data logging thermometer to create the attached chart, which shows ambient and attic temps/humidity levels/dew points. Again, my main goal is to reduce the attic temp, as this will reduce radiant heating through the ceiling upstairs, and help the HVAC equipment in the attic.

    Then, when we put a new roof on in a few years, we can pull the decking off, spray the back of the ceiling, add rigid foam, and fully seal the attic.

    I understand this isn't an ideal solution, but it would be better than nothing, right? Any issues with this approach?


  14. GBA Editor
    Martin Holladay | | #14

    I can't recommend your approach.

    If you want to install spray foam on the underside of your roof sheathing to create a conditioned attic, it's nuts to leave your gable vents open.

    If you are worried about ice dams -- and you write that you are -- then it's nuts to leave your sloped ceiling the way it is (either uninsulated or insulated with an R-8 batt -- I can't tell which).

  15. Expert Member
    Dana Dorsett | | #15

    When you re-roof don't pull the decking off unless it's all punk and really in need of replacement. If it's in good shape you can just drill from the top to dense-pack the cavity, then install the requisite amount of foam-R above the roof deck for dew-point control.

    It's possible to use slow-rise 2lb closed cell polyurethane directly over the existing R8 batts in the sloped section right now with no demolition, and be done with it. But it's expensive, comes with some amount of blowout or outgassing risk, and at 5-5.5" would dramatically reduce the drying capacity of the assembly. From a total cost point of view I suspect you'd still be better off taking the sloped ceiling down and doing the unvented slope "right".

    If your HVAC equipment includes the heating equipment, insulating the roof deck & gables but leaving the gable vents open would be a large thermal bypass. and increases the stack effect draw pulling conditioned air into the attic all winter (compared to a fully sealed & conditioned attic.)

  16. mculik5 | | #16

    Based on the data I've gathered, it seems my attic temp typically runs about 35F - 40F above the ambient outdoor temp on a sunny summer day. To me, this indicates two things:

    1. The asphalt shingles, lack of insulation on the underside of the roof deck, and sun are the cause of significant solar heat gain in the attic
    2. The current ventilation system isn't exhausting hot attic air and bringing cooler ambient air in

    And because there is very little insulation on the floor of the attic, all of the heat in the attic radiates through the ceiling into the second floor of the house. It also heats the HVAC equipment in the attic that I'm paying to pump ~60F air through. This means the HVAC equipment, which is used only for cooling, has to work harder than it should, and also results in seesaw temperatures on the second floor.

    At the end of the day, my goals, in priority order are:

    1. Increased comfort during summer (winter comfort is not a goal at this point)
    2. Increased efficiency

    Based on everything I've read related to attics containing HVAC equipment, the key to achieving these two goals is to reduce attic temperature - ideally, by sealing the attic and having it be the same temperature as the rest of the house.

    In my case, fully sealing the attic isn't an option because I'm not in a position to tear down a considerable part of my second floor ceiling. I could seal part of the attic, but that's not a good idea because it will cause problems in the unsealed part. So, until I'm ready to tear down my ceiling or lift up my roof deck, I'll have to have a vented attic.

    Thus, the question becomes - how do I reduce attic temperature in my vented attic? I can think of three options:

    1. Increase ventilation - Because the house has no soffits, this project is as major as tearing down the ceiling. No go, unfortunately.
    2. Supplement the existing ventilation with a powered fan
    3. Add insulation to the underside of the roof deck.

    The reason I believe #3 is the right answer is because everyone agrees that the best solution in my situation, aside from removing the HVAC equipment from the attic, is to fully seal it. If I went with #2, I'd be removing the fan when I decide to fully seal the attic. Why not get a jump start on the best solution by applying as much spray foam as possible now, and finishing the job a few years from now when I put a new roof on?

    I understand this is far from the best option. I'm trying to do the best thing I can right now, and at the same time, avoid doing something that is going to prematurely rot my roof or cause other problems. The roof doesn't leak today, and ice dams aren't a problem. I'm not trying to improve resistance to leaks or ice dams; I just don't want to make the house more susceptible.

    The reason I keep posting is because it seems like you are suggesting I go with option #4 - don't do anything until I can do everything. Am I interpreting your responses correctly? If it were your house, you'd leave R-8 on a leaky (air, not water) ceiling until you were ready to tear part of it down? You wouldn't consider options #2 or #3?

    As always, I do appreciate the responses. Thanks.

  17. Expert Member
    Dana Dorsett | | #17

    Attics of course are subject to the passive gain of the roof. It would take a LOT of attic ventilation to make an appreciable dent in that. Increasing the attic ventilation via mechanical means during the day has nearly universally been shown to increase the cooling energy use of the house, and simple soffit ventilation wouldn't buy you more than a couple of degrees relief at the peak.

    Attic ventilation has far more to do with purging moisture, and very little to do with attic or roof deck cooling, so scratch # 1 ad #2- they really aren't even close to being a cooling solution.

    If you insulate the roof (or stapled radiant barrier to the rafters) it would measurably lower the peak & average attic temperatures while raising the roof deck & shingle temperatures a handful of degrees. Insulating the roof deck would provide a lot better wintertime performance however- radiant barrier is a highly over-rated band-aid.

    So, yeah, I guess #4 is really the cheapest way forward if you're going for low risk. If the pitched roof is facing south you would probably get away with dense-packing them with cellulose, but on the north side it would be more risky. Since most of your roof gains would be on the south side, it might be worth risking dense-packing there(?), along with insulating the roof deck in the open attic, but leaving the north side sloped ceilings until later. The average wintertime roof temperature on the south side is quite a bit warmer than the north side due to more favorable solar gains. In unvented insulated roof assemblies the north side is always the highest risk.

    If the roof deck is sheathed with planking rather than plywood or OSB it's less susceptible to rot issues, and you'd probably get away with dense packing it even on the north side for a decade. Plywood is considerably more susceptible to mold & rot than plank, and OSB more susceptible still. Some people refer to OSB ad "mold food", and won't use it anywhere, but if you treat it right it isn't really that bad.

  18. mculik5 | | #18

    Thanks, Dana.

    The good news is that the semi-conditioned attic approach I'm considering does have some value, and probably won't make things worse. However, it's certainly not the best solution, and the ROI might not be there.

    At this point, my options are clear:

    - Do nothing until I'm ready to pull the ceiling down
    - Address the entire insulation problem when I redo the roof
    - Do the semi-conditioned attic thing now, and fully seal it when I redo the roof

    I really appreciate all the replies and insight, and your patience with me. I learned a ton. Thanks.

  19. GBA Editor
    Martin Holladay | | #19

    This is a test of the "less than" symbol bug. <x displayed?

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