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Did an add-a-level, insulation options are of concern with solar panels

GBA Editor | Posted in Energy Efficiency and Durability on

I have just completed an add-a-level on a New Jersey cape cod home and am at the point where I need to decide how to treat the insulation issue up there. I might add that there is a furnace up there in the attic with lots of ductwork which the installer wrapped. The south facing roof will be completely covered with solar panels. The existing insulation on the first floor is admittedly not the best, but short of ripping out all the walls there wasn’t much I could do that I was aware of. The 2nd floor living space and attic are ready to go, with one room having a trey ceiling (10 ft., not cathedral). I am considering closed cell for the walls, but unsure of what to do beyond that. Is spraying the roof deck with foam going to increase the possibility of a major problem down the road if a leak should develop? There will be 54 holes drilled for the solar mounting system, and even though the installer assures me they won’t leak….well….of course they will say that! Will foam on the walls make a difference upstairs even if the lower level is not great? Will making the upper level and attic air tight come back to bite me in the butt because there will be air infiltration on the lower level? Soo many things to consider, I’m a bit overwhelmed and undereducated on this. Any experience with a similar situation? Or maybe point me to a link that might address solar and attic insulation? Thanks in advance.

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  1. Doug McEvers | | #1

    I would not spend a dime on solar panels until the entire building envelope is considered, the scenario you present is showing a lack of planning. Please give more detailed information on the original part of the house and also the new portion, wall thickness, rafter design, etc.

  2. Monica | | #2

    Doug, the architect worked with the Solar Company from the start to insure that the new roof was pitched properly and the rafter design was correct to support the number of 8.1 kW system. Adjustments were made according to the Solar Company's engineers. I'm pretty sure I'm OK with that part and the state has already approved the rebate. The lower level is a classic war home cape with a walk-out basement, block foundation, footprint about 24x33. The add-a-level cantilevers out 2 feet in the front. Nothing special about the construction in that respect. The old insulation downstairs appears to be a combination of fiberglass bats and old blown in from what I can tell. There was a fire about 25 years ago and most of it is fiberglass where the walls were replaced. The house is tyvek wrapped and sided with Certainteed CedarBoard insulated vinyl. Roof is 35 yr GAF shingles over plywood sheathing. There are 2 furnaces, one in the basement (92% ECM) and one in the attic (80% ECM). Central air will be installed in the spring. Hope this info helps. There seem to be a zillion different opinions and approaches!

  3. Riversong | | #3


    Given that there is HVAC equipment in the attic, however well (apparently) sealed, it makes sense to move the thermal envelope and air barrier to the roof plane. But you're quite right to be concerned about the effect of possible (likely?) roof leakage.

    A study was published in the ASTM Journal on just this issue. The computer modeling study examined 24 different roof assemblies for each of the two climates: Boston MA and Miami FL. The study was run for a 10-year period, using hourly local weather data, and examining only the last 8 years - after the construction moisture content reached equilibrium. They also introduced a single event leak (to simulate a wind-driven rain storm) of an average 0.20 lb water/ ft²hr for a total duration of eight hours in the third summer of each simulation to examine the effects of incidental leakage.

    Each roof assembly included: asphalt shingles; either felt or self-adhering membrane (SRAM); either OSB or plywood; 9¼" of either fiberglass, open-cell spray foam or closed-cell spray foam; either poly VB (with fiberglass), or variable VB (with open-cell foam), or no VB (with closed-cell foam) in the ceiling; and gypsum wallboard.

    The following conclusions contradict the conventional wisdom that closed-cell spray foam does not require venting and protects from water damage and rot.

     Vented roof systems with permeable insulation in cold climates are durable because they include redundancies that can tolerate incidental moisture and provide visual indicators of roof leakage.

     The least tolerant roof assembly in either climate is the unvented closed-cell polyurethane insulation roof assembly with SRAM applied over the sheathing. This roof assembly creates a vapor trap and is slow to dry although the SRAM is supposed to prevent leakage from wetting the sheathing. Additionally, the closed-cell polyurethane foam will not allow leakage water to filter through and can promote deterioration of the wood roof structure with no visible indication of a roof leak.

     In cold heating climates, vented roof assemblies clearly outperform unvented assemblies with respect to drying potential. Multiple types of vented assemblies in this climate are considered durable, including permeable insulations such as glass-fiber batt and open-cell polyurethane insulation.

     In hot humid climates, the most durable roof assembly is the vented open-cell polyurethane with either felt or SRAM applied over the sheathing due to decreased drying time of the interior gypsum wallboard when compared to the unvented roof assembly.

    In your climate and with the solar panels on the roof and HVAC in the attic, I think the best approach would be to spray open-cell foam sufficient to meet IECC prescriptive values (R-38 for zones 4 & 5), after solar panels are installed, and keep an eye out for leaks.

    Air sealing the upper storey and roof will dramatically reduce infiltration in the lower levels, since air cannot enter unless it has an exit path. But, while you're spraying foam, you might also seal the basement sills and band joists if they're exposed.

  4. Doug McEvers | | #4

    Robert has given a great synopsis for your project, by all means air-seal where you can. My question is why does a house of this size need 2 furnaces? I have always been able to find space for a chaseway for trunk lines when adding a 2nd story. Open web trusses keep the ductwork inside the envelope. To add some encouragement, my last project, similar in scope to the one you describe appears to have lowered the heating demand by about 35%, no improvements have yet been made to the lower levels. I must add however, the new addition was very airtight and very highly insulated. As Robert said, you must seal very well the new contruction on the 2nd floor, this keeps infiltration at bay for the rest of the house (lower levels).

  5. GBA Editor
    Martin Holladay | | #5

    The sad part of the story is that it makes no sense to install an 8.1-kW PV system — one that probably cost about $60,000 before rebates and incentives — without first implementing much more cost-effective energy-efficiency strategies. If a careful analysis was actually performed to see how to implement ALL strategies cheaper than PV — something that was clearly not done — you would have an air sealing and insulation plan in place by now. And the house would CERTAINLY not have two furnaces.

    I don't think anyone has mentioned your cantilevered floor yet. Be sure to get the details right there. It is very difficult to insulate a cantilevered floor properly with anything other than spray polyurethane foam. I hope the joist bays are still accessible, and I hope they haven't been filled with fiberglass batts.

    There are plenty of ways to install a PV system without 54 roof penetrations, but it may to too late to tell you that. To give just one example, it's possible to install a standing-seam metal roof, and to support the PV modules with S-5 clamps ( Anyone who can afford a $60,000 PV system should be able to afford a standing-seam roof.

    Installing a PV array with 54 roof penetrations in an asphalt shingle roof? That makes me nervous — especially because the array was specified before you had an insulation plan.

  6. Monica | | #6

    Thanks so much for the info on the study. For some reason here, no one uses open cell on the roof deck, I spoke with foam insulation contractors and they all said closed cell but thinner if foam is the chosen method. One said that open cell will hold moisture like a sponge and it will never dry out. Is that true or does it in fact "drain"?
    Are you saying open cell, and keep the soffit and ridge venting? What is SRAM? I doubt you are talking about Short Range Attack Missiles (although they could come in handy these days)...LOL!

    Doug and Martin,
    I should have clarified the furnaces. They are natural gas, not electric. No one, and I mean no one, heats with electric here because the rates are so high. The PV system size is based on the previous 12 months electric usage, the state requires proof of that before issuing rebates. Payback period here is 5-7 years which ain't too bad in my book...especially since rates go up, never down. Plus SREC's continue for 15 years. I know a few people who installed similar systems a couple of years ago and they said it looks like they will break even in even less than 5 years.

    As for 2 furnaces, yes, I could have done it with one, albeit difficult. After talking with 3 HVAC contractors, several homeowners who went zone, and considering my lifestyle, I decided to do it with 2. Chase space was marginal at best because the old flu was gone and converted into a closet... the plumbing barely fit (every inch counts in a small vent furnace now in basement) I don't think the cost was a huge difference in the scheme of things.

    The floor is already in place and insulated, so no the joist bays are not accessible on the cantilevered section.

    The 54 mounting points will be using Quickmount flashing ( attached to the L bracket and Unirac rails, which is better than what they normally use..just an L bracket right on the shingle.

    And Martin, don't assume because I am installing a PV system that there is a money tree growing in my back yard. I am a single income household, selling residential real estate in a miserable market with no paid benefits or pension. Going solar is the right thing to do in the long run...from both an ethical and business perspective.

  7. homedesign | | #7

    54 Roof penetrations is worrisome....
    Is the shingle roof already in place?
    Have you asked the PV contractor what they would charge to come back in 15 or 20 years to remove and replace the PV?
    How much more will your replacement roof cost if the contractor has to replace and or re-weave the brackets into the replacement roof?
    If the shingles have not been installed ......then a metal roof (at least on the south side..may be more practical?

  8. GBA Editor
    Martin Holladay | | #8

    I'm sorry if my answer sounded as if I thought there was a money tree in the back yard. I understand that almost every project has budget limitations.

    Here's my main point: the details you provided raise enough questions that it seems highly likely that the $60,000 spent for the PV system could have been used for building envelope upgrades like better air sealing, a thoughtful insulation system, and even, perhaps, better windows. (I'm speculating on that last item). Moreover, had the envelope received $40,000 or $60,000 in upgrades — or even upgrades costing much less — it would have been possible to heat the house with only one furnace.

    In the long run, the money spent on PV would probably have been better spent elsewhere. That way, insulation details wouldn't be an afterthought.

    I'm sorry if this reaction is unhelpful; I know this advice comes too late for your project. But the advice may prove useful to other readers.

  9. user-723121 | | #9

    To add to Martin's line of thought,
    Our 1978 rambler was upgraded with a 95% natural gas furnace, a 16 SEER 2 ton AC, air-sealed attic and 24" added blown insulation, basement walls insulated on the interior with R-10, all for $10,000.00. The annual heating bill is under $600.00.

    Upgrading existing housing need not be expensive and should have a positive return on investment, address the building envelope first.

  10. homedesign | | #10

    Monica ...
    I am curious what your PV cost is before incentives and after incentives?

  11. Monica | | #11

    Shingles are in place, and disrupting the integrity of the roof at this point is not an option...long story there. Window are Anderson SmartSun. I foresee using only 1 furnace at a time for the most part, no need to run the lower level very much at night and vice versa. the PV decision was not all about cost savings. I also feel it is the right thing to do. Plus it increases the value of my house significantly without increasing the tax assessment. The money saved from electric will be primarily from routine consumption which doesn't have much to do with insulating the envelope.

    The going rate for PV systems in NJ is about 7.49 per watt, minus state rebate being 1.75 per watt. Then 30% tax credit, plus about 4,000-5,000 per year for SREC's for 15 years, assuming the price remains somewhat stable. There are other ways of financing by selling the SREC's all at once, but you get a much lower price.
    As for the number of anchors, it does seem like a lot, but then again, there are 36 panels up there. Perhaps I'll call unirac in the morning....

  12. Riversong | | #12


    Unfortunately, there is no ideal solution to the design problems already integrated into your addition. HVAC should not be installed in an unconditioned space. Since it's already in place, the only reasonable option is to move the thermal boundary to the roof deck (but how do you get the required R-38?) . As you suggest, closed-cell foam can hide roof leaks until they do structural damage (and possibly loosen PV mounts). Open-cell foam can retain moisture if there is not good drying potential, such as venting between foam and roof deck (which also serves as a drainage plane to move bulk water to the soffits).

    With any foam applied directly to the roof deck, it's imperative to allow the roof to breathe upward so that there is some drying potential. SRAM is self-repairing adhesive membrane (bituthene or Ice & Water Shield), and it prevents any drying to the outside. This is more of a problem with OSB sheathing than with plywood. Composition shingles on traditional felt underlayment can breathe to the outside.

    If you can install rigid vent baffles from soffit to ridge vents, spray foam against them and tight to the wall plates for air sealing the attic, and then install sufficient blown cellulose or fiberglass on the attic floor to bring the total R-value to 38 or better, you'll have a reasonably durable and energy efficient "hat" which will significantly reduce infiltration.

  13. GBA Editor
    Martin Holladay | | #13

    If any of your windows face south, then Andersen SmartSun glazing, with a SHGC of only 0.28, was a poor choice for that orientation. However, it might have been a good choice for east or west windows.

  14. Monica | | #14

    well Martin, I think this might be one of those "6 of one, half dozen of another" type deals. There are south facing windows, which in the summer could use that .28 (I don't like blinds or curtains very much). I also like the 95% UV blocking.

  15. Monica | | #15

    Robert, could you post a link as to the type of rigid vent baffles you are referring to? Thanks!

    Doug, can I use your contractor? You must not live in NJ to get prices like Labor costs alone kill us here. I used to live in Texas and the cost of everything was a LOT less!

  16. user-723121 | | #16


    I did the air-sealing and basement insulation, the rest was by others, labor is not cheap in Minneapolis either.

  17. Riversong | | #17


    There are many vent baffle products, made from foam or cardboard. I make my own on site with wooden spacer/nailer strips and ¼" hardboard (Masonite) for rigidity.

    Here is one option for a vented spray foam roof system: Another, which attaches to the wall plate to block the soffit, is

  18. Monica | | #18

    Thanks for the info Robert. I see that the AccuVent system has one product designed for spray foam and cathedral ceilings which blocks the soffit and continues to the ridge.
    With these products, you don't need SRAM, right?

  19. Riversong | | #19


    The AccuVent should provide adequate roof venting, as long as it is coupled with continuous soffit vents and a continuous ridge vent with external wind baffles. Without the baffles, wind-driven rain and snow will enter the roof and the required negative pressure at the ridge will be reversed. The only two shingle-over ridge vents with baffles that I know of are Air Vent Shingle Vent II and Lomanco OR-4.

    With proper roof venting, SRAM and other bandaid "solutions" to ice damning are unnecessary - "solutions" which, themselves, create potential problems.

  20. Monica | | #20

    Robert, am I missing something? Does the Accuvent have pieces that continue up to the ridge in each bay? I don't see that on their website.

  21. Riversong | | #21


    The Original and Energy Accuvents are designed for batt or blown-in ceiling insulation, and are intended to be installed only in the eaves, down to the wall plate. The Cathedral version, has both starter strips (for either 16" or 24" oc framing) and extension strips for continuing up to the ridge.

  22. tpbPhMwNHx | | #22

    I would get multiple opinions on combining solar panels with spray foam insulation. Spray Foam Direct sells a green spray foam product and they should be able to help you with your questions. Ask a few contractors at least before you have the work done. That can provide you with more information and save you a lot of money in the long run.

  23. Robet | | #23

    I am curious: given the same basic set of circumstances minus ridge or soffit vents (no overhang), would you then apply closed or open cell directly to a roof deck? Let's assume there is only builders paper and asphalt shingles with Ice and Water-shield for the first 3" of roof at eaves.


  24. GBA Editor
    Martin Holladay | | #24

    If I were installing spray foam between rafters, I would first ventilate the rafter bays with rigid site-built channels using plywood, Masonite, or rigid foam to create a 1" or 1 1/2" air space between the top of the rigid foam and the underside of the roof sheathing.

  25. Robert | | #25

    Thanks Martin. Would you do this even if there were no opportunity for eave/ridge ventilation? house in question has no overhangs.

  26. GBA Editor
    Martin Holladay | | #26

    Robert, who used to be Robet,
    Probably. But you can get special types of dripedge that allow for some ventilation of roofs with limited overhangs. There are many ways to ventilate -- it's hard to specify a product without knowing more or looking at your roof.

  27. nuFH78yDd4 | | #27

    I would add a few additional resources in addition to what victor gave so that you can get unbiased reviews of spray foam products. One website is and the other is
    Both of these sites have a wealth of information about the topic. I personally think utilizing both is a great option. Solar has really come down in price and will continue to do so as the cost to make becomes cheaper. The foam insulation is great to be more efficient.

    Just my two cents.

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