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Community and Q&A

How to insulate with furnace in attic

Kevin Schramm | Posted in Energy Efficiency and Durability on

I am currently looking to put forced air heating and air conditioning into my house. I am located in Long Island, NY in zone 4a, right on the border of 5a. We recently gutted the entire house down to the studs and we are replacing the whole heating system. We will be using gas heat. The contractor that I think we are going to go with is going to install a furnace in the basement for the basement and ground floor, and a furnace in the attic for the second floor. Well also have a blower in the attic for the air conditioning. The contractor told us that we should insulate the underside of the roof, instead of insulating the attic floor. I’ve heard a lot of conflicting arguments and I want to make sure that is the right thing to do. From what I gather, my three options would be to insulate the attic floor, insulate between the rafters, or build a small room to go around the furnace and insulate only that (in this case, would I also need to insulate the air handler for the ac unit). Any advice, recommendations, explanation, etc. would be highly appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    The right way to do it is to insulate the sloped roof above your attic. For more information, see Creating a Conditioned Attic.

  2. Kevin Schramm | | #2

    Thanks. That article was very informative. One more question though. I was planning on insulating with Roxul mineral wool. Is this material considered air impermeable, and would I therefore not need to leave any space between it and the roof sheathing? I was thinking of using 5.5" of roxul and 2 layers of 1" overlapping foam boards underneath. Would this be a good combination? Additionally, we're going to need a new roof very soon, which will give us the opportunity to insulate the top side of the roof sheathing. I wasn't sure after reading the article if we could put foam above the rafters, and below the rafters, with roxul in between. Thanks again for the response and the link.

  3. Jesse Smith | | #3

    Why do you require two systems?

  4. Kevin Schramm | | #4

    I'm not sure why two systems are required. We have had 5 different contractors over to give estimates though and every one of them have suggested a system in the attic feeding the upper floor and a system in the basement feeding the basement and lower floor. I was wondering if this was the only way to do it or if there are other options. The house was built around 1930 and is basically a cape that has an attic. Excluding the basement, its around 1500sqft, if that is any help. Thanks again.

  5. Jesse Smith | | #5

    Without seeing it, my guess is that it's way over the top for 1500 sq ft. Especially in the context of a total gut. Were the people making the recommendations the same people selling you the two systems? What did they feel necessitated the 2nd system?

    Make a chase for duct-work in the basement or use ductless minisplits. If you use ductwork, run Manuals J & D. It's really worth studying manual D yourself - it's available on ACCA's website - because your HVAC contractors won't.

    Roxul isn't impermeable. You'll probably want to use the space behind the kneewalls for duct-work. With the money you save on not installing an entire 2nd system, you can either 1) use vented, dense-packed cellulose in the rafter cavity with 2" Thermax inside the rafters, or 2) add foam (search 'nailbase') to the outside of an unvented dense-packed cellulose rafter cavity.

    Animation of an interior retrofit which doesn't show the venting:

  6. GBA Editor
    Martin Holladay | | #6

    As Jesse pointed out, Roxul mineral wool insulation is air-permeable.

    You wrote, "I wasn't sure after reading the article if we could put foam above the rafters, and below the rafters, with roxul in between." This type of foam sandwich can work, but it is somewhat risky. If moisture ever accumulates in the roof assembly, it's going to have a hard time drying out. It's best to install rigid foam on just one side of the assembly -- not both.

  7. Kevin Schramm | | #7

    Thanks again for the responses. I've got some more reading to do and some more questions to ask the contractors. Jesse - Yes, the contractors were the ones making the recommendations and selling the two systems. Should I be contacting someone to come see the house and design or recommend a system and then have a separate contractor install that system? One thing I forgot to factor in when I said the house is 1500sqft is that we're planning a 400sq ft extension on the ground floor. I did make sure to tell the contractors when they priced out their systems, and they all took this information into account. Would that extra 400 sq ft make the two systems necessary? Thanks again.

  8. Expert Member
    Dana Dorsett | | #8

    If you limit the interior side roof deck foam to an inch of closed cell spray foam (~R6) there is no issue of trapping moisture in the roof-deck, but at 2" or more the drying rates would be super-slow. An inch of closed cell is about 1perm, give or take, at most a bare minimalist class-II vapor retarder.

    Alternatively, with Icynene's high-density open cell foam (MD-R-200) you wouldn't get down to 1 perm until it's 4" thick (R20, the IRC prescriptive minimum for zone 5) and still a very acceptable 1.3 perms @ 3" (R15, the IRC prescriptive minimum for zone 4.) See:

    But if it's rigid foam boards, putting it on the exterior of the roof deck is by far the better solution.

    But IRC prescriptive R aside, 2" of closed cell spray foam on the underside of the roof deck would be sufficient vapor-retarder + non-wicking condensing surface to be able to use Roxul on the interior, and would still allow the roof deck to dry. See table 3 in this document (and at least skim-read the whole thing for context.)

  9. Expert Member
    Dana Dorsett | | #9

    On the mechanicals front: Almost all available gas furnaces are already oversized for the heat load of a code-min 1500-2500 house in climate zones 4 & 5, and TWO of them would be ridiculous!

    Even though L.I. electricity prices are some of the highest in the lower-48, it's probably better/cheaper overall to do the whole thing with a single right-sized high-efficiency variable speed air source heat pump like the Bryant Evolution Extreme series (heat pump only, not the hybrid, at your likely heat loads) than two furnaces PLUS central AC.

    Though not as efficient as ductless mini-splits (which might be an even higher efficiency solution if it's an open floor plan, and worth pursuing if it is) it'll still be much cheaper to heat with one of those than with propane or oil, but more expensive than natural gas. But instead of three systems you'll have one, and without flue issues to deal with it's probably a cleaner installation all the way around, and even though it's on the expensive side for an air conditioner, it's got to be cheaper than two furnaces AND and air conditioner, and quieter than any of the proposed solutions too.

    The smallest in the line is a 2 ton which can probably handle your heating load at +15F (the 99% outside design temp for most of L.I.) with margin, but if not the 3 ton almost surely would. With continuously variable speed on both the air handler and compressor it modulates with load, and has a much higher efficiency at part-load than when running full-out (much like ductless systems.) If you want to wade into the weeds a bit on it, see:

  10. Jesse Smith | | #10

    You could consider hiring a separate person to do the design. I think David Butler would be excellent if he's available.

    2 systems for 1900 sq ft is ridiculous especially in the context of a gut. It should be really easy to install and conceal new ducts.

    IMO, one of the central ideas of energy efficient design is to do progressive things with the building shell - high R, airtight, good water management, etc. In part the higher enclosure expense is offset by lowered heating and cooling installation and operation costs. The addition of a second system not only violates this principle, it tends to do so in a way that permanently compromises the integrity of the building enclosure.

    Dana - I suspect that all the HVAC contractors proposed both furnace and ac for both systems, one of which would be in the attic. I don't know pricing for the Bryant model you recommend. Is it possible that it's a repackaged/knock-off of the Carrier GreenSpeed? Carrier owns Bryant so it seems likely that two models are related. Locally the GreenSpeed would be significantly more expensive than a 98% AFUE furnace w >16 SEER ac.

  11. Kevin Schramm | | #11

    Great information. Thanks. Since we need to redo the roof anyway, does anyone know the additional cost of insulating the deck with something like nailbase? Thanks again.

  12. Jesse Smith | | #12

    Polyiso is an expensive insulating material relative to cellulose. But in my area nailbase is cheaper to buy than separately buying the same thickness of polyiso and osb separately. Compact unvented roofs are a tricky retrofit. Make certain that you really understand everything before tackling the project. BSC has some good stuff on their website.

    For a newbie I would not recommend using the chainsaw retrofit approach. Cut the lower level sheathing at the eaves and block and spray foam vertically above the exterior walls. Connect that to the lower sheathing. Then cover the entire lower roof deck in ice and water shield, then add the nailbase. Use dense-pack cellulose in the rafters, net and strap the unenclosed rafter cavities.

  13. Expert Member
    Dana Dorsett | | #13

    Jesse- as I understand it the Bryant & Carrier units have some of the same compressor & refrigerant volume valve components under the hood, but I'm not sure about the coils and controls. I'm sure a pretty-good oversized hybrid HVAC unit would be cheaper than GreenSpeed but would TWO oversized (which seems to be what had been proposed) exceed that cost? (Some friends are currently looking at the Evolution Extreme as a retrofit- they havent received the quote yet, but I was asked to vet the capacity of the proposed unit for the actual heating load at their 99% design temp of +5F.)

    I found this slick interactive graphic page for the GreenSpeed that shows blocks of temperature bins for different locations and output graphs for the unit which may be useful once you've calculated your actual loads:

    Clicking on the "Heating Capacities" tab and selecting the model in the lower left corner it looks like the 2-ton is good for ~24-25,000BTU/hr @ +15F (depending on the which interior unit is selected) the 3-tonner is good for ~28-31,000 BTU/hr @ +15F, any of which are reasonably easy design targets for a tight 1900' house. (Many code-min homes that size would already be there.)

    A 2-stage heat pump is probably easier to right-size for the heating end than any gas-burner hybrid version on even a modestly higher-R house, but won't be as cushy-comfortable or quiet as the Greenspeed/Evolution-Extreme ilk.

    New roofing iso f.o.b. the distributor's warehouse runs about 9-10cents/R per square foot (eg R20 would be $2/foot or $58-64 for a 4' x 8' sheet, R15 would run ~$1.5/foot, etc.) I can get reclaimed roofing iso from commercial demolition & re-roofing for about 1/4-1/3 that from multiple vendors locally, making it an attractive base layer for chainsaw-retrofit type foam-overs etc. Some of these vendors advertise primarily on craigslist. The closest such operation I found to L.I. is pretty far up the river in Newburgh:

    There are similar outfits operating in central MA/CT/RI, eg:

    (have truck, will travel...) will deliver (for a price), but unless you're buying a whole truckload the sweetness of the deal isn't nearly as good.

  14. Jesse Smith | | #14

    Dana - that's pretty cool. I'm sort of ambivalent about the whole ducted heat pump thing. Generally think that ductless minisplits are probably better. But I suppose if you can use the same duct system for ventilation, then the ducted systems might make more sense. Dunno. Would love to see more research. Have you seen the 2nd section of this report?

    Modulating furnaces often start in the 20-30kbtu/hr range, so it's not like the furnace thing is that bad. I also wonder to what extent many historic problems have been driven by improper installations, or at the very least a failure to think creatively about how to solve fairly minor problems.

  15. Bob Irving | | #15

    have you had an energy audit? I would urge you to get one now. With the stud bays open it's an excellent time to do a blower door test - you'l be able to easily find the problem areas and seal the house, leading to a lower heat loss requiring less btus and a smaller heating system. All of this will save you installation costs and ongoing fuel costs. The auditor should be able to do an analysis of the insulation and recommend upgrades or strategies to increase the insulation value of your house. If he can't, there are excellent resources that people on this forum can recommend.

  16. Kevin Schramm | | #16

    Thanks again for all the responses. Does anyone have a diagram of a properly laid out system in a two story house showing all the duct work, furnace and ac equipment? Everyone is telling me that this is the way forced air systems with ac is done and I'd like to be able to see in a diagram showing a more appropriate way to lay out the system. Thanks.

  17. GBA Editor
    Martin Holladay | | #17

    Duct systems are usually designed by HVAC contractors or (sometimes) mechanical engineers. There is no "one way" to do it. A furnace can be in a garage or an attic -- usually bad ideas -- or a basement or mechanical room (better). Ductwork can be installed in an attic (bad) or a basement (better) or between the first-floor ceiling and the second-floor subfloor (good).

    Here is more information on ducting options: Keeping Ducts Indoors.

  18. Expert Member
    Dana Dorsett | | #18

    Jesse- I share your ambivalence about the ducted heat pump, and no, I hadn't seen the skimpy bit o' data in that document you linked to. According to the chart on p24 the variable speed unit tested in the CC2 unoccupied house tested at an HSPF of 9.5 @ 37F outdoor temp, which is pretty close to the 3-month winter binned average temp at locations in L.I. A measured HSPF of 9.5 indicates 9500 BTU-out for every 1kwh in (or a COP of about 2.8) which is somewhat AHEAD of I would have expected of the variable speed ducted system to come in for a heating season average in a L.I. climate. The HSPF on the Green (occupied) test house averaged an HSPF of 11.2 @ 45F, which is also very good, but too warm a temp to be a useful benchmark of the seasonal average in a Long Island location.

    By comparison best in class mini-splits would deliver a COP of about 3-3.5 in that climate, but if resistance heating is used for temperature balance in the doored off rooms it could be either better or worse than the ducted variable-speed system for overall heating efficiency.

  19. Kevin Schramm | | #19

    I am going to meet with the contractor tomorrow and see if I can clarify a few things. Thanks again for the information. One more question if I do decide to condition the attic. From the first linked article in the first response -

    If you want to use air-permeable insulation in this location [between the rafters], you have two choices: include a ventilation channel between the top of the insulation and the underside of the roof sheathing, or install rigid foam insulation on top of the roof sheathing to keep the sheathing warm. (Don't do both, since these two solutions are incompatible.)

    Does that ventilation channel have to go all the way up the roof, or can you just use one of those baffles that just goes down by the eaves. I was thinking of maybe taking a trip up to connecticut or newburg to pick up some second-hand rigid foam and just putting roxul in the bays. Thanks.

  20. GBA Editor
    Martin Holladay | | #20

    Q. "Does that ventilation channel have to go all the way up the roof, or can you just use one of those baffles that just goes down by the eaves?"

    A. I'm not sure I understand your question, but I'll take a stab at answering. The ventilation channels need air inlets at the soffit; these are the soffit vents. The channels need air outlets at the ridge; this is the ridge vent.

    If there is a ventilated attic under the ridge, the ventilation channels need to extend from the soffit area to the ventilated attic, so that air can flow unimpeded from the soffit vents, through the ventilation channels, to the attic, and then out the ridge vent.

  21. Kevin Schramm | | #21

    Sorry, I may be misunderstanding. Right now my house has no soffit vents. I have two vents a few feet below the ridge line and a gable vent with a fan. The hvac contractor was suggesting that I insulate the rafters but leave open all of the vents. However, from what I read, if I choose to insulate the attic, I need to close up those vents. After closing up the vents, I was planning on using Roxul in the rafter bays with rigid foam over the top of the rafters. But, from what I understood from reading, an air permeable insulation like Roxul should not be in contact with the underside of the roof sheathing if rigid foam is going to be used below it. Is my understanding correct? I'm guessing it may not be. Thanks again.

  22. Jesse Smith | | #22

    Was it the same contractor that recommended two systems? If so, it makes perfect sense. The only way to increase a 1900 sq ft building load to the point where 2 systems are required would be by massively increasing infiltration. And what better way to increase infiltration than by leaving several large openings and a fan punched through the thermal boundary of the house? This is why we can't have nice things - HVAC contractors just wreck stuff.

    Dana - skimpy is right! Would be nice to see more studies. Where did you get your numbers (COP 3+) for minisplits? What units do you use?

  23. Kevin Schramm | | #23

    Well, as a layman, it didn't make sense to me either. Open vents inside the conditioned space of the house seem like a terrible idea, but I don't have any real knowledge in this area. I forgot to mention that we need to have our roof redone, and we can easily close up the vents above if it is advisable.

  24. Expert Member
    Dana Dorsett | | #24

    Jesse: InRe mini-split efficiencies in different climate zones, the NEEA has site-monitored field data on 69 real-world systems in occupied houses, over a substantial period of time, some of which has been condensed to a tidy 156 page report. See:

    See the executives summary, including table ES-1 on page 10 (.pdf pagination) for the ultra-condensed summary. The Eastern Idaho region is the warm edge of US climate zone 6, but had some of the newest latest-greatest units whereas Inland Empire is zone 5, but had a preponderance of older lower HPSF rated units. (That info is buried in the text much deeper in the report, but it explains the low average for the warmer climate, and what's possible elsewhere.) The Puget Sound and Willamette clusters are both US marine zone 4, and suffer more defrost efficiency/capacity losses than would normally be seen in a Long Island (cold edge of zone 4A) winter. The Boise/Twin cluster is somewhat drier in winter than L.I., but significantly colder, at the cold edge of zone 5, and the cluster average COP was 2.96.

    At L.I. winter temps & humidity I'd expect a better-class mini-split to average a COP better than 3, but probably not more than 3.5, more in line with the Willamette and Puget Sound clusters.

    In the NEEA study they didn't break it down by manufacturer, but in other NEEA papers about half of all units installed under the Ductless program were Mitsubishi, with Fujitsu coming in second and Daikin a distant third in raw numbers, with the rest widely distributed among other manufacturers. But this is over several thousands of installations, not the 69 installations continuously monitor in the field study report. See table 4 of this intermediate marketing report:

    In the field report they imply that the 1 & 2 ton Mitsubishis and 1-ton Fujitsus were the most represented in the monitored field units, which led them to bench-test and develop models for two of the most popular 1-ton units, using those to project and verify the anticipated performance at the various cluster sites. The bench-test report lives here:

  25. Expert Member
    Dana Dorsett | | #25

    Kevin- since you're re-doing the roof, this is the opportunity moment to put the requisite R15 rigid foam on the exterior of the roof deck to allow you to go un-vented with fiber on the underside of the roof deck. Given the crazy air leakage common on vented attics on houses like that it's worth the up-charge IMHO, whether you site the mechanicals in the attic or not.

  26. Kevin Schramm | | #26

    Dana- I'm talking to a roof guy tomorrow and I'm hoping that they can lay down some foam like you recommended at a manageable price. It's really frustrating that it's so hard to find someone to design and install an air heating and cooling system the right way. I'm starting to think maybe we should just install a boiler with radiators and be done with it. Just go without ac for a while and use the money we save to do other work on the house. It doesn't seem like the contractors are really taking advantage of the open walls anyway because they are just running ducts up from an unfinished basement and down from the attic. I can't imagine that the cost would be much different if we waited a couple of years and installed ac after the house is finished. The whole idea of forced air and ac was to get it done while the house was gutted because it would be a good opportunity to get it done well.

  27. GBA Editor
    Martin Holladay | | #27

    Q. "The HVAC contractor was suggesting that I insulate the rafters but leave open all of the vents. However, from what I read, if I choose to insulate the attic, I need to close up those vents."

    A. You definitely don't want to have a powered attic ventilator (a fan). To learn why, read this: Fans in the Attic: Do They Help or Do They Hurt?

    If you decide to insulate between the rafters, there are two ways to do it: you can build a vented roof assembly or an unvented roof assembly. If you choose to have a vented roof assembly, you need both soffit vents and ridge vents. To learn more on this topic, read How to Build an Insulated Cathedral Ceiling.

    Q. "I was planning on using Roxul in the rafter bays with rigid foam over the top of the rafters. But, from what I understood from reading, an air permeable insulation like Roxul should not be in contact with the underside of the roof sheathing if rigid foam is going to be used below it. Is my understanding correct?"

    A. If you install Roxul between the rafters with no ventilation chutes -- in other words, if you are building an unvented roof assembly with air-permeable insulation -- then you need to install rigid foam above the roof sheathing. This method will require new roofing.

    If you decide to install a layer of rigid foam on the interior side of the rafters, with Roxul in the rafter bays, you'll need to have a ventilation gap between the top of the Roxul and the underside of the roof sheathing. This approach requires both soffit vents and a ridge vent.

  28. Expert Member
    Dana Dorsett | | #28

    Going with a hydronic boiler now and AC (ducted or otherwise) later will be almost certainly be dramatically more expensive than doing a heat pump-only solution (ducted or otherwise) now. Popping in a couple of mini-splits later won't be dramatically different than installing them now, but it they can handle the heating load at a comparable operaing cost as the hydronic solution it means you didn't need to buy that system.

    But I can certainly appreciate how frustrating it is to deal with the dearth of competent HVAC contractors, particularly when addressing the low loads of high efficiency building envelopes, or even current code min! IRC 2009 homes are quite a bit more efficient than typical homes of he 1970s, but most of those 1970s rules of thumb (that reliably oversized systems even then) are still commonly used today.

  29. Jesse Smith | | #29

    Dana - thanks so much for posting that info! Really helpful!

  30. Kevin Schramm | | #30

    I noticed a geothermal truck the other day. I didn't know that was available here. Could that be another option?

  31. Jesse Smith | | #31

    If you feel like you want to push your experiment with HVAC incompetence just a little further, then yes. Otherwise probably not. The beauty of ductless minisplit heat pumps is that there isn't a tremendous amount that can go wrong with the installation. The installation of duct systems introduces a moderate amount of complexity. GSHP's take that complexity into another realm. Furthermore, installation quality is very difficult to verify. I'm aware of several GSHP installations in my area where the units are nearly permanently stuck in electric resistance. I would bet that an average GSHP installation would perform at a significantly lower COP than the minisplits Dana referenced.

  32. Kevin Schramm | | #32

    Yeah, I think I'd rather keep it as simple as I can. I'm going to ask the guy coming today about heat pumps.There is another contractor that I'm going to contact who I think mainly works with the Carrier Greenspeed. I like the idea of ductless in the bedrooms, but not so much on the first floor. Would there be any reason that we should avoid ducting the lower level while doing ductless above? Thanks again to all.

  33. Expert Member
    Dana Dorsett | | #33

    If you spend the difference in cost between mini-splits and geothermal on insulation and even rooftop PV you'll end up with a lower heat load, and lower electricity use (and higher comfort) for the money.

    Geothermal is not cheap- runs $9K/ton installed (before subsidy) for 4-5 ton systems in nearby CT & MA, but is cheaper, ~6-7Kton for trenched in slinkys in other places. Ductless is less than $4K/ton, and while not as efficient as the best-in-class geothermal installations (COP of 4-4.5) it's right in there with the typical 3rd-party verified all-in geo efficiency (again, very few data points, but some), which tend to run between an annualized heating COP of 3 and 4. Mini-split efficiencies in your climate are ahead of hack-implemented geo of which there are all too many real-world examples, if website help boards are any indication.

    With geo every system is a custom design, with the efficiency & capacity of the system in the hands of the designer. Just the designer's time on a best-in-class geo system can cost as much as a 1-ton mini-split, installed, so in a small house small load situation the economics are never going to add up.

    It's perfectly fine to put a mini-split upstairs and a GreenSpeed or other type of system downstairs, with the caveat that any doored off rooms upstairs will end up with temperature differences at the outdoor temperature (or solar gain) extremes. If it's fairly open, it's not much of a problem. With doored off rooms upstairs is might be worth springing for higher performance windows though. To rationalize a ductless head in an individual room would need a heating load greater than 7KBTU/hr or a cooling load of 5KBTU/hr. The smallest ductless multi-split heads run about 7K cooling (higher in heating mode), and if you oversize them for the peak loads by more than ~50% they spend a lot of time in standby-idle, and cycle rather than modulate, cutting into their efficiency. It's sometimes possible to split the output of a mini-duct cassette between two adjacent rooms though, which might ultimately be the right thing to do for bedrooms if the individual room loads are too small.

    Otherwise, putting a single head in a common area and keeping the doors open most of the day works, only closing them when privacy desired. I have a relative in WA who heats her whole house with a 1.5 ton Mitsubishi, using a small amount of resistance heat in the bedroom with the door closed on only the coldest nights, leaving the door to the bedroom and other rooms open to the common area most of the time, letting convection balance the temps. Even with her sub-code R-values and U0.6ish windows that's been working pretty well, and now that she sprung for U0.28 windows throughout it'll work even better. (The majority of old windows were leaking water during high winds, having been mis-installed prior to her owning the place- the decision to replace them this was not on energy-use grounds.)

  34. Kevin Schramm | | #34

    Thanks again for all the information. I had a roof guy come yesterday and he recommended against putting foam under the roof on my house. He said he doesn't like putting foam on a steep roof and he said its harder to nail things down without cracking the wood below when driving through foam. Its tough trying to figure out how I am going to get things done the right way. I'm sort of in a hurry to get the heat and insulation done because we'd like to move in the the house asap. Hopefully I'll be able to make use of all the information that has been given here by finding someone who can actually do the work.

  35. Kevin Schramm | | #35

    I just spoke with a contractor about the Carrier Greenspeed. He had already come to give me a quote on another system so I just asked him over the phone about doing the heat pump instead. Coincidentally he said that he has one in his own house. Anyway, for some reason his quote was much higher for the heat pump. It went from around $17k with a gas furnace and ac to $25k for the heat pump. Hopefully I'll get a better quote later today from another contractor that is coming to the house.

  36. Expert Member
    Dana Dorsett | | #36

    Your roofing guy doesn't get it- you may have to show him some pretty pictures. You never NAIL through 2.5" of foam! On a roof foam-over you either put a nailer deck directly over the foam that is through-screwed to the rafters using pan-cake head timber screws (eg. FastenMaster HeadLok ) on a 24" o.c. spacing, and fasten the roofing to the nailer deck.

    An alternative is to put 2x furring over the foam through-screwed to the rafters 2x" o.c., and nail the nailer deck to the furring, and vent the 1.5" cavity with Cor-A-Vents or similar below, and a standard ridge vent above, which lends some resiliency to the nailer deck.

    It takes a bit of practice/skill to hit the rafter consistently with screws longer than 5-6" but that's as long as you're going to need. To do R15 iso vented with 2x furring with 12.5" foam takes at least a 5-1/2" timber screw, but unvented you're fine with 4". This hardware is available (including the spider-drive bits) even at box stores:

  37. Kevin Schramm | | #37

    The frustration continues. I can't even get someone to give me an estimate on a ducted heat pump or over the roof sheathing insulation. I called another roofer and he didn't know anything about insulating with foam above the roof. Both of them said that foam insulation is only used above flat top roofs. I know that's not true from what I've learned here and I've seen pictures of them myself elsewhere.

    The heat pump has been the same thing. The heat guys say that they don't recommend the heat pump if you have gas in the house. I insisted that the last two guys give me an estimate for a heat pump along with their other suggestions. So far one sent me 3 quotes, none of which involved a heat pump, and the other has yet to get back to me. The first guy did include a quote for a minisplit system though, but it was $5k more expensive that doing the dual furnaces and ac.

  38. Expert Member
    Dana Dorsett | | #38

    You may well be stuck with a zoned 2-stage or modulating gas furnace as your heat source and a standard 2-3 ton AC package as your best-case scenario on the HVAC end. A dearth of local talent make breaking the mold difficult.

    NYSERDA maintains lists of contractors who at least have some sort of a clue- you may be able to dig up the right contractor(s) there:

  39. Torsten Hansen | | #39

    I've been absent from the boards for a while but I want to add my two cents worth about your insulation questions. We have worked on a ton of houses like yours and it sounds like you find yourself in a snake pit of confusion.
    Exterior foam installed in connection with a re-roof is great. If you can get it, you can insulate your rafter bays with any material you like. I'm guessing that your rafters are 2x6s, which will limit your cavity insulation to about R-19 with traditional materials. Exterior foam should be as thick as you can get it and no less than R-10.
    If you decide against exterior foam for whatever reason, your absolute best choice is to create an unvented roof system with air-impermeable spray foam insulation. Foam densities between one and two pounds will give you r-values between 25 and 35 in the rafter bays and whatever you want in the attic areas. Do not use 0.5 pound (open cell) foam in your roof. Your existing gable vents will be sealed off and no, you should NOT install an attic fan. Leaving a big hole in your house doesn't make a lot of sense.
    Foam is incredibly good at keeping out the summer heat and your conditioning needs for the second floor will be minimal. We find that even attics that are not actively conditioned stay very close to the temperature of the rest of the house once the roof is foamed.

  40. Kevin Schramm | | #40

    Thanks for the replies. I'm actually leaning towards the mitsubishi ductless hyper heat systems. I got a quote for a 7 headed system that wasn't too far off what I got from forced hot air and ac after factoring in all the costs of having to hook up the gas line and other incidentals. I like the idea of not having to sacrifice space for the blower and ductwork, and I like that there won't be conditioned air running through the attic. My only concerns are the bathrooms and the real world efficiency of the units. Can anyone tell me the best way to keep bathrooms warm. The contractor I spoke to didn't really explain how bathrooms are kept warm. Is it possible to use a room to room ventilation fan? I have an 2nd floor bathroom adjacent to a bedroom, and a 1st floor bathroom adjacent to a den. Would it be advisable to use a ventilation fan to make sure that the bathroom stays warm, or are there better ways keeping the bathroom warm? Thanks again.

  41. Expert Member
    Dana Dorsett | | #41

    Calculate the heat loss of the bathrooms- odds are it's a pretty tiny load. (Bathroom windows tend to be small). If the doors are normally left open to the common space, convection will even things up reasonably, no supplemental heat or active circulation necessary. If you WANT some supplemental heat in there, a tiny resistance cove heater operated under both thermostat & occupancy-sensor control keeps it comfortable while still being pretty stingy on power use.

    I'm a bit dubious about any implementation that would put 7 ductless heads in a 1500' house. The very smallest heads are good for about 7000BTU/hr cooling, and 9000-10,000 BTU/hr heating, which means with 7 heads would be ridiculously oversized. Were you going for a single head per room or something? On rooms with tiny loads you can split the output of mini-duct cassettes.

    Do an I=B=R heat load calc on a room-by-room basis. Only rooms that have a design heat load of greater than 7000BTU/hr would be candidate for an individual head.

    Rarely would using a ventilation fan to share air with the adjacent room be a necessary or right solution. The temperature difference between the rooms would be very small most of the time, and at a low delta-T the efficiency of the fan running continuously could be even less efficient than a thermostatically controlled electric cove heater or panel radiator.

    I will often heat my sub-code 1920s house solely with a wood stove at temperatures near or below your outside design temp. The bathroom off the living room with the wood stove has a ~8 square foot U-0.5 window and about 40 square feet of 2x4 exterior wall area, only of which 30-35' is currently insulated with cellulose. Simply keeping the door to the bathroom open keeps it within 3F of the living room.

    Two bedrooms also have doors opening into the common space. The one with 12' of U0.5 window and ~100' of exterior wall (all but 5' of which is fully insulated) tracks well within 5F with the door open when it's +15F outside. The other has ~35' of window and over 200' of exterior wall, with ~30' uninsulated, and with the door open it stays well within 10F of the common space. Overnight with the doors closed the deltas increase when it's +15F out, but the less lossy bedroom never drops below 65F if we keep the living room at 72-73F on those nights.

    This is alljust to illustrate the point than in a smaller house with low individual room loads it's sometimes better to slightly overheat a common space with a ductless head, and let convection keep the adjacent spaces warm, especially when individual heads would be more than 1.5x oversized for the doored-off rooms. Managed well, most current code-min homes can be point-source heated with way fewer than 7 heat sources and stay comfortable. The only space that I can't heat my sub-code house with the just wood stove comfortably at +15F outdoor temps is the family room on the far end of the house that has a huge glazing fraction to take in the views. (It's on it's own radiant floor zone to keep it comfortable.)

    So while not having a ductless head in the bathrooms or other low-load may be a cause for concern, it shouldn't be. Installing a small amount of resistance heating that is use only when necessary works for most people. Radiant cove heaters are more comfortable than baseboards, since they heat the humans, not the air. Suppress the urge to oversize them to be able to bring temps up quickly, since you want it to feel like the warm glow of a sun-beam, not a broiler.

  42. Kevin Schramm | | #42

    Thanks Dana. I gathered from earlier posts that 7 heads might be too much. He was planning one for each bedroom (3 - all of which are pretty small), one for the dining room, living room and den, and one for the basement. I'll definitely talk with him to try to bring down the number of heads as you described.

    I've been trying to estimate my cost per month for different alternatives. I found a link to a calculator from a link on this site -
    Assuming that the alternative to mini-splits would be an 95% gas furnace with r-8 ducts in the attic and gas is $1.25 per btu, it looks to me like I would need at least an 11 hspf heat pump if electricity rates are going to be $.20 per kwh. Ideally I would want higher because I know that hspf usually drops as temperature drops. From what I've seen, It looks like the hyper heat mitsubishi units get up to around 10 hspf, which would get me reasonably close, and from what I understand, I wouldn't have to worry about the unit not working in case it gets extremely cold outside. Do these figures sound right as far as a cost estimate goes? I don't want to end up going with the ductless and realize that I calculated wrong and it is going to cost way more than a high efficiency gas furnace. Thanks again.

  43. Expert Member
    Dana Dorsett | | #43

    The seasonally adjusted average HSPF of better-class ductless for a L.I. climate is between 10 and 12, but only if it's sized reasonably for the loads. Oversizing for the heating load by ~25% yields a modest uptick in as-used HSPF, but upsizing by more than 50% results in a lower HSPF due to low duty cycle and excessive standby.

    In L.I. you don't really have to worry about extreme cold. It almost literally never gets much below 0F, and brief periods of operation at those temps are not a problem for most mini-splits. The cold weather mini-splits such as the HyperHeat or Halcyon XLT-H series are needed in places where the binned hourly average temps in January are +15F or lower, but that's you're 99% outside design temp, not your average. Anything with a rated output at +5F/-15C or lower (which is most of them) would handle your climate.

    I don't have the all-in delivered retail cost of natural gas in your neighborhood, but when running the apples-to-apples don't forget to include the pumping &/or air handler power of the fossil burner in the analysis too.

    An HSPF of 10 means it delivers 10,000 BTU/ kwh, for which you're paying 20 cents (all delivery & energy charges included?). At 20 cents/kwh all-in cost that is $20.00/MMBTU of heat delivered to the house. An HSPF of 11 would be $18.18/MMBTU.

    Buck-fifty/therm gas in a 95% burner is $15.80/MMBTU plus pumping/air handler cost. Call it $17/MMBTU.with power use included (just a WAG, no a real analysis.)

    The $64,000 question is where gas prices are going vs. electricity prices over the lifecycle of the systems. My gut feel is that electricity will continue to track with the wholesale price of natural gas in the near term, but as more wind & solar resources in the northeast over the next 15 years gets built out the price of electricity will decouple, and track lower than natural gas, which is currently being produced at or below-cost from shale formations at $4/MMBTU well head price. At a 25% capacity-factor (the average of existing wind developments in the northeast) wind power is on par with just the FUEL costs of legacy-nukes, and only slightly more expensive than combined cycle gas power at the $4/MMBTU contract price. Rooftop PV is still in the 14cent/kwh lifecycle cost (and falling), below your retail price and below the peak wholesale price but it too will have a moderating effect on electricity pricing, since the marginal cost per kwh, like wind is near $0. Right now we're on the thin edge of the wedge for those in the north east, but it's definitely coming- it's already in-progress, and it will transform the power utility business dramatically over then next decade or two.

    Predicting future energy prices has always been a dangerous pastime, but I don't foresee anything but flat to slightly falling power pricing regionally, but natural gas has almost no where to go but up from here, since the cheap & easy gas has already been had, and tight-gas is only barely economic to produce at the current wholesale price. As gas prices rise electricity will slightly, but it will eventually decouple. Wind is currently only ~3% of the NYISO grid source ( )and nearly non-existent on L.I., but the economics of wind are becoming compelling. The pie charts can change pretty fast- if someone had told me 10 years ago that in 2013 more than a quarter of all kwh moving on the grid in Iowa would come from wind power I would have dismissed them as green-pipe-dreamers. This train won't be stopped, even if the production subsidy does.

  44. Kevin Schramm | | #44

    Thanks again Dana. Very helpful. I think I may have miscalculated the cost of electricity. It looks like after 400kwh the rate drops by more than $.03. If I'm calculating right, the real cost per kwh of the heat pump is probably around $.16 per kwh. If that's right then the heat pump is probably going to be cheaper, but either way it looks basically around the same. The Mitsubishi website doesn't really give that information in the product showcase area. Is there a better place that anyone knows of to find the efficiency and capacity of the different Mitsubishi units that are on the market currently. Thanks.

  45. Kevin Schramm | | #45

    Sorry, one more question. I had a tech also suggest using a cassette to share one head between two rooms. This seems like a good idea, but if we do that for the bedrooms, won't I be back to running ducts through the attic?

  46. Dan Kolbert | | #46

    There's this thing called a general contractor....

  47. GBA Editor
    Martin Holladay | | #47

    The cassette can go in the ceiling assembly or in a soffit. In either case, it's possible to install an air barrier and insulation above the cassette.

  48. Kevin Schramm | | #48

    So far I've gotten two proposals for mitsubishi mini splits - most of the technicians that have come to the house haven't wanted to give a proposal for mini splits to be used as the only heat source. Both of the proposals that I have gotten include 7 heads and 2-36k btu compressors. From what I have read, this seems like a lot of total btu and a large number of heads. My question is, if these are the proposals I'm getting, is it going to be any better than the original proposal with two furnaces? I'm worried that if I'm not getting a properly sized mini split system, it will wind up short cycling all the time and the efficiency is going to be terrible. Thanks for all the helpful responses.

  49. GBA Editor
    Martin Holladay | | #49

    This is the dilemma faced by all builders and owners of energy-efficient homes. In most areas of the country, HVAC contractors are stuck in the 1950s. It's a distressing problem with no easy solution.

  50. Aj Builder, Upstate NY Zone 6a | | #50

    If we could place bets. .. I would bet against this home project. Too late in the game to go into design options and never to me anyway would I think homeowners should study assembly choices and then somehow get a home built.

    What this site is good for is knowledge and then to me having a home built by those who already have experience.

    Lots of luck otherwise IMSHO.

  51. Expert Member
    Dana Dorsett | | #51

    "Terrible" efficiency with oversized mini-splits would be a COP of 2.5 in stead of 3.0 in your neighborhood. If you have 2 compressors, you have two complete systems- and if weather conditions allowed you could just turn one off.

    That's pretty common for 2-minisplit homes with one head per floor in open floor plan homes. Most of the time the upper floor unit can cool the whole house, and most of the heating season the lower floor unit can pretty much heat the whole house, the magic of convection. Not every home layout is optimal for doing that but even my 1920s antique can be point-source heated with the wood stove until it's below 30F outside.

    You might do what I do when dealing with HVAC folks: Run the heat load calculations on your own, figure out how many heads and where they would go, and where you might need supplemental heaters, find the ductless solutions & resistance heaters that fill the bill, draft up a spec and TELL THEM what you want, and that you're putting it out to bid with multiple installers. (When I did this for the hydronic heating system in my house the system designer for the contractor told me flat out that it wouldn't work. I told him I didn't care- it's what I want, and that I'd sign a waiver if they liked. Five winters later I'm still waiting for it to not-work- I thinkin' it probably really DOES work, but what do I know, I'm not a hydronic heating designer or boiler tech. ;-) )

    There were both good HVAC designers and real hacks in the 1950s too. Can't say it was at any better or worse ratio than today. But there's a bit less excuse for it today. Most customers don't really care as long as it heats & cools the place adequately, and any installer who has undersized a unit even once gets gun-shy about making the undersizing error again. If they don't show you a heat load calc, they're not really doing much other than hooking stuff up.

  52. Kevin Schramm | | #52

    We decided to do the ductless system. Thanks for all the input. I'll post my experiences with it once we have it installed for a while. One last question. I've read on this site and even in this thread that it is detrimental to put too large of a head in a room. One of our bedrooms is only about 100 sq ft. (none of the contractors wanted to split heads or share heads through open doors) I thought that if we are going to put a head in that room it should be a 6000 because that is the smallest Mitsubishi offers, but the hvac tech advised that we go with a 9000 because it will only work as hard as it needs to and both sizes cost the same amount. Should I insist on the 6000 or is he correct? I'm worried that it might overheat/cool the room. Thanks.

  53. Kevin Schramm | | #53

    Now that I have the mini-splits in, I was wondering if anyone could give me some tips on how to get the best efficiency. I wound up with 12000btu hyper heat unit for heating and cooling the living room. I also have a 48000 btu outdoor unit that is capable of handling 8 blowers, although I only have 5 blowers attached. I have a 9000btu unit in the basement, a 12000 btu unit in the first floor dining room, and in the three bedrooms upstairs I have one 9000btu unit and two 6000 btu units.

    The way I've been working it is to use the 12000 btu hyper heat unit the most because it is the most efficient and it is on the first floor, near the stairs. I usually have it set around 71 degrees when we are home, with the other unit in the dining room set the 68. The basement unit is almost always off because the basement is not finished right now. Upstairs, two of the three bedrooms are unoccupied, so those units are always off. My bedroom is often off because the first floor heat is almost always enough to heat the whole second floor. When my bedroom unit is on, we usually set it for 65 - it still almost never drops below 70 in this room. At night I usually keep only one of the first floor units on and set to 64 and I sometimes turn on the bedroom unit and set it to 65.

    My questions - Should I be using the 8:1 unit more (at most I am usually using only 21000btu of the 48000 btu capacity)? Is there a certain percentage of capacity that gets the best efficiency? Also, is it better to keep the temperature constant, instead of setting it back at night? Sorry for the the overly detailed question. I ask because I got a pretty high electric bill and I wanted to make sure that I'm doing things correctly. I can't say how much of the charges are from the mini splits as part of the bill cycle I was running space heaters before my mini splits were installed, and I'm sure that contributed to the high bill. Thanks for any help.

  54. Expert Member
    Dana Dorsett | | #54

    In general the efficiency of ductless systems peak when modulating at or near it's lowest compressor & interior-head speeds. Just because you are using heads capable of delivering 21,000BTU/hr doesn't mean that they are actually delivering that much heat when they're on.

    When using setback strategies during cold weather it means the interior head will run at it's highest speed (= lowest efficiency) during the recovery ramp to bring it up to temp quickly, and the compressor will have to run at a high-er (= lower efficiency) speed during that period. With 8 head multi-splits it's hard to say for sure if there is a crossover where you'd actually be slightly higher net efficiency with setbacks, but in general unless the zone is going to be off for 24 hours or more, deep setbacks are likely to be less efficient, since the lower efficiency during the recovery ramp can eat up more power than was saved by letting the house cool to the lower temperature and not running it (at a higher efficiency) just to maintain temp for 8-10 hours. Recovery ramps also typically start at or near the coldest part of the day, when the efficiency at any given compressor speed is lower due to the lower outdoor air temp.

    The exception would be an a house that leaks like a sieve and has no insulation, and falls to the setback temp quickly.

    During the shoulder seasons heating adjacent zones by keeping the larger zones warm and turning off the less used heads is probably going to be more efficient than having 8 heads all cycling on & off all the time. Having 2-3 zones running almost constantly at or near min-modulation keep the compressor cruising long near min-mod as well, whereas 8 zones calling for heat intermittently would introduce a higher number of cycles with little or no modulation.

    In general, with single head mini-splits or multi-splits, "set and forget" is going to be more efficient in a house that's occupied at least 12 hours out of the day. For multi-splits cutting back on the number of active zones during low-load weather should help a bit too. The turn-down ratio of the compressors isn't infinite, but most are capable of a 4:1 or better turn-down, so running it with long cycles at 1/4 of the output capacity at a given outdoor temp is pretty efficient.

  55. Kevin Schramm | | #55

    Thanks for the reply. So it sounds like a should be setting back much less at night. Also, by keeping the bedroom set so low and the dining room set 3 degrees lower than the living room, I probably am short cycling those units since the hyper heat unit in the living room running at 71 is probably enough to keep the living room higher than 68 and definitely keeps the bedrooms upstairs much higher than 65. I'm thinking it probably makes the most sense to run only the single hyper heat unit when it is not too. When it's colder, maybe it makes sense to turn on the larger unit, but to run the zones at an equal temperature to the hyper heat, that way the blowers won't be going on an off so much.

    One more question about the watt usage - In the Mitsubishi brochure it lists the 12000btu hyper heat's total input as 930W and my 8:1 unit's total input as 5270W. Does that mean if everything is running all out that I am consuming 6.2kwh?

  56. Expert Member
    Dana Dorsett | | #56

    Yes, if you can keep everything comfortable with just the mini-split, throw the breaker on the x8 multi.

    With everything running at it's maximum blower & compressor speed you'll be pulling 6200 watts, and 6.2kwh every hour. But the output would be pretty HUGE if it were all running balls-to-the-walls. When modulating at lower speed it's output is much lower, but at a higher BTU/watt ratio, thus the power savings is in greater proportion than the reduction in output.

    At 40F outdoor temps better class mini-splits will deliver roughly 1.5x as much heat per watt at minimum speed (or minimum output) than at max speed (maximum output.) If the thing can deliver say, 15,000 BTU/hr @ 40F at max speed while drawing 930 watts, that's 15 BTU/watt- a COP of 4.4 which is pretty good. But if at minimum modulation it's only drawing 150 watts, but delivering1.5x the efficiency that means it's delivering (1.5x 18BTU/watt=) 22 BTU/watt x 150W= 3300 BTU/hr, 22% of it's max-capacity for only 16% of the power and a COP of 6.4, which is more than pretty good, it's OUTSTANDING. (And why the efficiency hit of a steadier high temperature can still use less power than a 1-hour warm up ramp at max speed.)

    The actual power it's drawing at low-but-modulating load will vary a bit, and the manufacturers keep re-tuning their blower-speed to compressor speed algorithms to run as efficiently as possible at part load. It won't always be max-efficiency at absolute minimum on speed on everything, but that's the gist of it. And the above example is probably pretty close to reality for the latest & greatest new generation mini-splits rated with HSPFs of 13 or so.

  57. Kevin Schramm | | #57

    Thanks again Dana. Great info. I saw one of your posts where you talk about standby losses for individual units. Any idea what the standby power draw is on a big 8 zone unit? Thanks.

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