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Community and Q&A

Spray foam roofline in stone house

jackzylkin | Posted in Green Building Techniques on

I have a house made of dry-stacked stone (with a continuous layer of concrete parging on the outside, and mostly plaster on the inside of the walls, with some exposed dry stone in places the plasterers couldn’t reach. I regard the concrete on the outside as a pretty good air barrier, and I am debating whether to complete the envelope by spray-foaming the underside of the roof-deck to R-32. Currently the roof is insulated haphazardly with R-13 batts behind a finished drywall dormer ceiling.

I worry about a few things before proceeding with the plan:
1. My house has no central air conditioning (because it is basically a cave and it is already very cool in the summer, except in the attic.) It has hydronic heating, too, so there is no air handling in the house at all. Will moisture levels in the house become a problem if the roof is sealed up? For example, will this worsen the problem we already have sometimes have with condensation on aluminum windows in the winter? Will water vapor, no longer to escape through the roof, instead migrate into the stone walls and cause mold? We often get some water in our basement after spring thaw, so that has me concerned about humidity problems too.

2. Will all the old-house smells all of a sudden become more noticeable because they cannot escape? For example, smells from within the stone walls?

3. The attic is a finished room with original wide pine plank flooring. Under the floor is lots of loose-fill fiberglass. I worry this fiberglass will produce dust that has nowhere to escape to after tightening up the attic space.

4. More than anything I worry this is an unreasonable expense that will not pay itself back soon. We pay $2800 a year to heat the house with oil and wood heat, and the project will cost $5800. (The expense is because we plan to remove existing shoddy drywall on the ceiling before spraying, then replacing it after.) We could instead spend the money on some other project, like upgrading our oil furnace to propane or insulating the foundation or something.

Any advice would be greatly appreciated, as this is starting to cause me some anxiety. I hate wasting so much oil on heat each year, but an antique stone house is such a challenge for green upgrades!

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  1. jackzylkin | | #1

    Forgot to mention: Part of the reason for the added expense is also that I am springing for a low-GWP foam. So, the project is not being weighed entirely in terms of dollars and cents.

  2. Jon_R | | #2

    I'd pay someone to do an energy audit and report on the most effective things to do (measured in $ and in GW).

  3. jackzylkin | | #3

    Thanks. I will plan to do this but I would still like to know if closed cell foam is even an option for a house like mine.

  4. Expert Member
    Dana Dorsett | | #4

    Standing water in the basement is a problem, but not surprising if its a dry stacked foundation. This house needs a thorough assessment for drainage and water proofing on the bottom of the house (including a basement floor/slab vapor barrier) to be included as part of any proposed changes.

    A concrete parge over stone may or may not be a good air barrier, but without water proofing it's not a great moisture barrier. It may be able to wick & store quite a lot of rain-wetting moisture, which can contribute to high indoor humidity too.

    It's certainly possible to use closed cell foam on the under side of the roof deck, and making the house more air tight is still the right thing to do. Active ventilation will purge moisture in a more controlled fashion than random air leaks and stack effect drives.

    Where are you located?

    How deep are your rafters?

    R32 isn't a particularly high performance level, and closed cell foam is one of the most expensive ways to get there. That's only 4.5-5" of foam, and the thermal bridging of rafter depth that shallow really undercuts the performance of closed cell foam. If you have deeper rafters than that it's often cheaper to do just the bare minimum depth needed for dew point control with the rest of the full rafter depth filled with fiber insulation, which reduces the amount of heat transfer through the (now longer) rafter path. The ratio of foam-R to total- R needed for dew point control at the foam/fiber boundary varies with local climate.

  5. GBA Editor
    Martin Holladay | | #5

    Lots of issues here.

    Q. "Will moisture levels in the house become a problem if the roof is sealed up?"

    No one should expect the roof assembly to be an escape valve for moisture. If you have enough standing water in your basement, or other reasons for high indoor humidity problems, to make you worry, then: (a) you need to develop a plan to keep water out of your house, and (b) you need to develop a plan to control your indoor humidity level. (That usually means a ventilation system for winter and either air conditioning or a dehumidifier, if needed, in summer.)

    But your roof assembly has nothing to do with it.

    For more information, see this article: Preventing Water Entry Into a Home.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "The attic is a finished room with original wide pine plank flooring. Under the floor is lots of loose-fill fiberglass. I worry this fiberglass will produce dust that has nowhere to escape to after tightening up the attic space."

    A. You are ascribing magical properties to your roof assembly. Not only (according to your fantasy) is the roof assembly now allowing indoor moisture to escape, it is acting as an air purification system that removes dust from your house.

    This is magical thinking. Your roof assembly isn't removing dust. If you have a dust problem, you need to describe the problem in greater detail so that we can suggest remedies.

    I'll make two comments:

    1. If you plan to insulate along your sloped roofline, you don't need any insulation under your attic floor.

    2. It may be reasonable to leave the existing insulation under your attic floor in some circumstances. A new plywood subfloor should keep dust particles where they are. Otherwise, you can hire a contractor to remove the existing insulation.

  7. jackzylkin | | #7

    I am located in PA (Climate zone 5). I don't have deep rafters (only 6 inches) -- that is why I am opting for foam. The rafters are at 2ft centers, and there are 6 skylights, which are more of a concern as far as R-value goes. I'm more concerned about air leakage, because I am stumped on improving my stone house's R values.

  8. Expert Member
    Dana Dorsett | | #8

    "4: More than anything I worry this is an unreasonable expense that will not pay itself back soon. We pay $2800 a year to heat the house with oil and wood heat, and the project will cost $5800. (The expense is because we plan to remove existing shoddy drywall on the ceiling before spraying, then replacing it after.) We could instead spend the money on some other project, like upgrading our oil furnace to propane or insulating the foundation or something."

    Is this really about return on investment (ROI)?

    Changing over from oil to propane heating in most markets results in an INCREASE in annual heating cost, not a reduction. Propane is sometimes cheaper per gallon than #2 oil, but has ~33% less energy per gallon. In my area 95% efficiency condensing propane is more expensive heat than 85% efficiency oil. This week in my are both propane & oil are running about $3/gallon- propane would have to be under 2 bucks to beat oil on heating costs.

    In most markets changing from oil to ductless heat pumps (or displacing oil with ductless heat pumps) usually delivers a fairly dramatic reduction in heat loss.

    From a simple return on investment point of view air sealing is likely going to be your first most-obvious and best bet, not putting 5" of 2lb foam on the underside of the roof deck, followed by ductless (or ducted) mini-splits. With a $5800 budget, couple grand spent on blower door & IR imaging directed air sealing would leave you enough left over to buy a 1 ton cold-climate ductless mini-split.

    So, how many gallons of oil per year, at what price, how much wood (and what species / price) and what is your all-in fully delivered cost of electricity per kwh?

    What is your location? (A ZIP code is good enough.)

    In my area (zone 5A, 99% outside design temp of +5F/-15C, mean January temp about 23F/-5C) a cold climate mini-split reasonably sized for the load will deliver 10-12,000 BTU per kwh, averaged over the season. Let's lowball it at 10,000 BTU/kwh to make the math easy. That means it takes 100kwh to deliver a million BTU (MMBTU) into the house. If your power cost 15 cents/kwh, that's $15/MMBTU. If your power costs 20 cents/kwh, it's $20/MMBTU.

    In my area #2 heating oil costs about $3/gallon averaged over this past season, but it's volatile- it's been as high as $4 and as low as $2 at various points over the past dozen years. The most recent 10 years average is about $3. (It's $3.05 this past month, but was $2.54 the same month last year.) A gallon of heating oil has about 138,000 BTU/gallon source-fuel energy. Burned at 85% (a decently tuned up new-ish boiler), that delivers about 117,000 BTU/gallon into the heating system, but also uses some electricity. So it takes 1M/117K= 8.5 gallons to deliver MMBTU into the heating system. At $3/gallon that costs $25.50/MMBTU, and a mini-split using 15 cent electricity would save about 40% on the heating bill, but only 20% if your electricity is 20 cents. At $2/gallon it's $17/MMBTU, still somewhat more expensive than a mini-split using 15 cent electricity, and cheaper than a mini-split using 20 cent electricity.

    At $4/gallon it's $34/MMTU, more nearly 3x the cost of heating with a mini-split on 15 cent electricity. So if all hell breaks out, Iran's oil comes off the market and world oil prices spike for a few years the ROI of a mini-split is pretty fast.

    In my area a 1-ton cold climate mini-split is less than $4K, all-in, in competitive bidding. At $3/gallon oil and 15 cent electricity it'll be saving over a grand per year, with a simple payback in under 5 years. Air sealing could knock as much as 20-25% off the heating bill (just a WAG, based on the house description- it could be more, but it could also be less), and would save something like $500/year, also a sub-5 year payback. Insulating at the roof deck would tighten the house some, would reduce the ice-damming potential and last a lot longer than a mini-split, but probably won't save as much in the near term as a mini-split. A 1 ton mini-split isn't going to fully heat your house on it's own but it can displace a big chunk of your oil use, maybe all of it during the shoulder seasons. When it's in the mid 40s F outside it's efficiency will be a bit higher (more like 13-14,000 BTU/kwh) and it's capacity will be higher too.

    Insulating at the roof deck is a longer term investment than heating equipment. The conventional lifecycle projections for heating equipment is 15-25 years, and it has ongoing annual maintenance & repairs over that lifecycle. Insulation is more like a 25-50 years or more investment, and has no annual costs. So to break it down as to which is going to work better for you takes net-present-value type calculations, but at the marginal reduction in heating costs you'll see with just the roof insulation it's hard to see how at any discount rate it can end up with a higher NPV than air sealing &/or a mini-split heat pump, even if you have to replace the heat pump every 15 years.

  9. Expert Member
    Dana Dorsett | | #9

    Didn't see response #7 until I'd posted #8.

    In zone 5A you need a minimum of 40% of the total R to be on the exterior of fiber insulation to have reasonble dew point control at the foam/fiber boundary. With full dimension 6" rafters 2" of HFO blown foam would deliver R13-R14, and the remaining 4" could be filled with compressed R23 rock wool batts, which at 4" would deliver about R17. So that would be (worst-case) about 43% foam/total ratio, which would need no interior side vapor retarder (cheap latex on wallboard will do), and would be comparable in thermal performance to 5" of HFO blown foam.

    You could then apply the remaining high R/inch foam budget to insulating the foundation, but only AFTER dealing with the drainage issues.

    When it's time to re-roof you might consider adding 3" of reclaimed roofing polyiso (it's dirt cheap, performs at about R15 or better) and a nailer deck above the existing structural roof deck. That would bring the center-cavity R up to about R45 or so, but since there's now a thermal break over the rafters it would meet IRC code-max performance on a U-factor basis, despite being less than R49 (code minimum when compliance is based on R rather than U). That would involve re-mounting or replacing (or getting rid of some of) the skylights etc, so it's not a simple cheap re-roofing, but it would reduce the ice damming potential.

    It's possible to insulate the walls on the exterior and still meet IRC performance with a continuous 2" of HFO blown foam or 2.5-3" of reclaimed roofing foam (3.5" if EPS) on the exterior, but that would require adding siding or EIFS type finishing materials. The thermal mass of the stone means it only needs ~R13 of continuous insulation to meet code if it's on the exterior. A cementicious EIFS on blown 2lb foam might not look much different than your current concrete parge in the end. Reclaimed roofing EPS is a lot cheaper, but it would also require a lot more detailing than 2lb sprayed foam, and would usually work better with actual siding rather than a troweled-on coating.

  10. Expert Member
    Michael Maines | | #10

    Zach, others have the fine points and practical details covered, but for some simple math to answer your question, you're talking about reducing heat loss through your ceiling by 60%. It's hard to know what proportion of your heat loss is through the ceiling, and it will change as the insulation levels change, but a reasonably safe estimate might be 30%. 30% of 60% is 18%. So in theory, along with some miscellaneous air-sealing, you could cut your heating bill by 20%, saving $560/year, roughly a 10% return on investment using simple payback, more if you use compounding interest. That's better than most of us do in the stock market.

  11. dan7210329 | | #11

    "Concrete parging", which I assume is Portland-cement based, is probably fairly semi-permeable trapping a lot of moisture in the house. ( includes in its list a 3 coat stucco as .1-1 perms). Concrete parging is less permeable than original lime stucco and mortars found in many Pennsylvania stone buildings built before 1870. These stone structures were meant to breath and leak through the roof, windows, chimneys and even the stonework - all to allow moisture to escape - from weather, open fires, cooking and human bodies. With a concrete parge and a closed cell foam roof, you'll need to figure out a way to manage the humidity inside the house so you don't slowly rot the ends of floor joists sitting in stone pockets that are more moist and warm than before. Great Britain has millions of old homes with owners wondering how to make them more efficient. Here's a Guardian article that might help you understand how to deal with your old building. Hope this helps.

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