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Community and Q&A

Spray foam strategy for 1930 house (vapor barrier, ignition barrier, thermal bridging)

Rich Cowen | Posted in Energy Efficiency and Durability on

Greetings. I have been studying this forum over the past month in the hopes of understanding how to fix the building my wife and I moved into in July.

The house, built in 1930 as a bungalow, was renovated in 2004 with a new second floor and large attic, and then renovated once again in 2009 after it was purchased out of foreclosure. The renovations included replacement of all the windows, but not much else from an insulation perspective. We are located in northern Mass (climate zone 5) and the total square footage is 900 per floor.

The walls and windows in the living space appear well sealed. The problem with the building is that it has a fieldstone wall in the basement, 18″ thick, uninsulated on the slab, the walls, and the ceiling. Meanwhile, the heat produced by our gas furnace is quickly moving through the ceilings to a vented attic, and then is lost forever.

Basement walls, as of december 10, are already at 32 degrees, while it is still 68 degrees in our vented attic!

Because the air handler is in the attic, we want to start by moving the attic into the conditioned space (hot roof approach). Sealing the attic will reduce the “stack effect” that draws the cold throughout the entire building. This could be done with cellulose (we have generous 10″ rafters in the attic), but we are hoping to use foam, for superior air ceiling. Both approaches qualify for federal and utility company incentives.

One installer recommended 5″ of closed cell on the roof deck and 3″ on the gable ends. The other installer planned to completely fill the ceiling and wall cavities with open cell, giving us 9″ on the ceiling and 3 1/2″ on the attic end walls. So the alternatives from an R value perspective are:

Closed Cell: R33 on ceiling, R19 on gables (Bayer Baysystems)
Open Cell: R36 on ceiling, R14 on gables (Demilac Agribalance)

Both solutions require an ignition barrier to meet code — intumescent paint — if we don’t plan on finishing the space with drywall this year.


1. THERMAL BRIDGING. I am concerned that by partially filling the rafters, thermal bridging would be a lot worse on the roof with the closed cell option. The exposed surface of each roof rafter would be a full 9 1/2″ as opposed to 1 1/2″ so they would act as “heat sink fans” with closed cell.

2. FLAMMABILITY. With open cell the foam will be inches from the gas furnace, with close cell about 12 inches. The furnace is a 2004 model. I read here that the furnace should have “sealed combustion” for safety… how can I tell? I am thinking that a sheet of drywall right above the furnace would be an important investment in any case.

3. VAPOR BARRIER. I have read here that open cell might need a vapor barrier. Considering that all fans vent to the outside in the building, and that we will have no A/C, do we really have to worry about condensation with either solution?

My proposed solution is:

A. go with open cell

B. add staggered studs to the gable ends, coming out 2 inches to make a 2×6 wall. This would reduce the thermal bridging and permit an R value of 22.5, which seems better matched with the R36 on the ceiling. The lumber to do this would be only about $75… my carpenter could do this in a few hours.

C. spray on the intumescent paint myself to save on cost. There is is this one available locally:

Is this as good as the DC315 paint that is marketed for use with foam? link:

Both paints cost around $750 for 10 gallons, and I am assuming we would vacate the house for a few days to let the foam breathe before applying the ignition barrier. Spraying the paint ourselves, we save at least $1000 — but how can I ventilate the space for safe application once the foam is in place?

D. My hunch is that the paint could also reduce the ability of moisture to penetrate the open cell foam. Is this correct?

Well, that is enough for starters. I will describe the basement situation in more detail later on. And what ever happens, I can provide data to this forum on our natural gas usage both before and after the application of the foam.

Thanks in advance for your help!!

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  1. GBA Editor
    Martin Holladay | | #1

    Rich Cowan,
    1. Intumescent paints are somewhat controversial. There has been a lot of discussion for years about whether the tests used to prove that the paints reduce ignition time are really meaningful. If it were my own house, I would want to cover the spray foam with drywall.

    2. Open-cell foam definitely needs a vapor retarder in Massachusetts. The problem has nothing to do with air conditioning. During the winter, warm, moist interior air diffuses through the open-cell foam and condenses on the cold roof sheathing. Building scientists say that you can solve this problem with vapor-retarder paint applied to the cured foam. (Now you are up to two coats of paint.)

    3. Your plan for staggered studs on the gable wall makes sense. If you build a double-stud wall, you can choose from a wide variety of insulation types. You don't have to use spray foam on your gable walls.

  2. Rich Cowen | | #2

    thanks Martin --
    We have considered drywall, but framing the whole space would add $4-5K to this project now. Given that we are only using the attic for storage, my inclination is to leave it open for now (at least until we need to replace the roof in 5-10 years) so we can see how the foam is holding up.

    I know that any paint provides a vapor barrier but how much of one do I need? I would not mind putting "primer" on before intumescent paint, but would that reduce the effectiveness of the fire barrier? There is a web page at the EPA:

    which provides some guidelines on how to measure the vapor permeability of paint. However it also says that the best vapor barriers are oil paints, which would probably not contribute to fire resistance...

    A "double stud wall" is an interesting mean just add a second layer of 2 x 4s and fill the cavity with cellulose?? This would help with fire resistance because there is a bit of wiring running up the gable walls (all new wiring), connected to 3 lights at ceiling level. If we don't do this, I wonder if we need to insulate the spray foam from the wiring and fill the cavity with fireblock foam?

  3. ottobounds | | #3

    You mentioned you would describe the basement situation later, but I feel like I would start there before spending a ton of money on spray foam in the attic. Can't you stop the cold air from getting into the walls from the bottom up? Locate the points of entry at the foundation and I'm assuming ballon framed walls. Insulate the basement walls, which are 32 degrees already. Slowing down the stack effect in the attic makes sense, but I think I'd be looking at stopping the cold from getting in first. How does the air handler get heat to the basement, is it ductwork in an exterior wall?

  4. Riversong | | #4

    Sorry, OTTOBOUNDS, but you're out of bounds on this one. The attic is exactly where the improvements need to begin because of the location of the air handler.

    Rich, an air handler should never be located outside the conditioned space, but since you may be stuck with that, encapsulating the attic is essential. However, if that's a combustion appliance (not just a water-to-air coil), then it must be sealed combustion, direct vent. If not, then it will either have to be replaced or you may have an opportunity to move it and leave the attic as is, perhaps air-sealing and improving the insulation in the ceiling.

    If you do encapsulate the attic, then I would agree with Martin on the value of hanging drywall under the rafters (that should require little or no additional framing), and deepening the gable walls for better R-value. The drywall will cost a lot less than the intumescent paint.

    If you use paint as a vapor retarder, there are several latex VR primers on the market (such as Benjamin Moore Super Spec) which meet the 1 perm requirement (few other paints will).

  5. Michael Chandler | | #5

    We usually don't drywall the whole attic, just the area that is accessible from the living space for storage. Generally a 12x12 floor area just inside the attic access with that ceiling area framed down to avoid shaving the foam. And this need not include the area with the furnace in it. It's okay for the furnace to be in an area with exposed foam that is only accessed for service, just not the storage area that is immediately accessible from the living space. Close off that other area with a secure fire rated access door and you are fine as we interpret the code here. Still worth confirming with your local inspector.

    You can tell if the furnace is sealed combustion by looking for a combustion air intake pipe in addition to an exhaust pipe. It sounds likely that yours either is already or has that potential by what I read into the label info

  6. Rich Cowen | | #6

    Actually it is 3 or 4 degrees warmer than I reported earlier in my basement... I used a thermometer which was bad.

    Getting back to the attic: I think the furnace is sealed, how can I be sure? It is a Payne and the manual reads: "The model PG9M 4-way, multipurpose Gas-Fired, Category IV, direct-vent and non-direct vent, condensing furnace..." The size is 120000, the efficiency is over 90%. There are issues with the choice and placement of the furnace, but no matter what we do we will need a better insulated attic AND basement for the house to perform well.

    As for drywall, it is hard to draw the line on where to put it... do we drywall the eaves and above the collar ties? There is an additional expense because the open cell installer will charge $$ to trim off the excess foam. The attic has about 10 feet of height; the collar ties are at a height of 7 feet.

    Thanks for the feedback, and please keep the ideas flowing!

  7. Michael Chandler | | #7

    I wouldn't bother with the ignition barrier or the paint on the foam, use drywall as your barrier and paint the drywall for moisture, the less drywall the better, plan out your storage needs and even build some shelving as necessary to keep it efficient. Spend the money you would have spent on the ignition barrier on better duct sealing and thicker foam.

  8. Rich Cowen | | #8

    There is an intake pipe for combustion air from the outside.

    As for the issue of the vapor and fire retardants, the Demilec web site has some good info:

    1) For Agribalance, the water vapor permeability is 4.95 per 5 inches.

    2) They recommend the Benjamin Moore Super Spec primer in their Fall 2008 newsletter:,%20vol%201,%20issue%201.pdf

    3) They recommend a specific fire resistant product called FireShell F10-E and claim that Demilec is "approved" to be left exposed if sprayed with this product:,%20vol%201,%20issue%202,%20NFPA%20286%20test.pdf


    a) Does use of a primer in conjunction with an intumescent coating affect the fire rating? Has this ever been tested in a lab??

    Based on this discussion so far, I am not not sure whether to use foam at all. If we ever had a roof leak and had vapor retarders in place, it sounds like the open cell foam could never dry out!

  9. Rich Cowen | | #9

    Michael - our contractor gave us an estimate on drywall and framing the large attic, including a plywood floor, and thicker gable end walls, of around $6000. This was for 5/8" drywall, the kind that is marked as fire resistant.

    In our situation, is this overkill? From what I understand, 5/8" is thicker than building codes require. for example see:

    Even if we do drywall the main open space of the attic (it measures about 22' x 12'), it leaves the low areas under the slanted roof and behind the drywall unprotected from vapor penetration, so we'd still have to paint 40 percent of the roof with primer. And the area above a drywall ceiling... the triangle that would be above the drywall roof -- would this also need a vapor barrier on the foam?

  10. Riversong | | #10


    If you're not using the attic for storage, then you can perhaps skip the plywood floor (though if there's no floor it will be difficult to do the insulation, framing and drywall work).

    Foam requires a 15 minute heat and ignition barrier, not a ½ hour fire barrier such as type X drywall. Ordinary ½" DW is fine.

    Because of the difficulty in providing a proper vapor barrier, it may be best to use a closed-cell foam. But make sure the installed R-value at least meets the IECC requirements for your climate zone. And make sure that no vapor impermeable layer (such as peel-n-stick membrane) is later applied on top of the roof deck to avoid the deadly "sandwich".

    Then you need to decide whether an intumescent coating or taped ½" drywall makes more sense as an ignition barrier.

  11. Michael Chandler | | #11


    I am proposing a much lesser drywall solution than the full wrap your contractor was proposing. I am assuming that you can air-seal the ceiling from above and then just put flooring in a modest storage area and stick frame some walls and ceiling in from the foamed area so you wouldn't have to shave the foam.

    12'x12' works for drywall and plywood size fairly well and you can hold in the framing from the gable walls and down from the ceiling and only use the collar ties for drywall. Keep it simple, it's just for fire and air proofing, then you have your drywall ignition barrier. Paint the ceiling below and the drywall storage area and you can skip the ignition barrier paint and the vapor barrier paint on the foam.

    I tend to focus on what will get things done fast and cheap and meet the goal. this is what I have found to meet that standard. you can get closer to perfection and you can spend more money, but this will get the primary job done inexpensively.

  12. GBA Editor
    Martin Holladay | | #12

    I disagree with Michael Chandler, who said you could skip the vapor-retarder paint on the cured open-cell spray foam.

    In your climate -- northern Massachusetts -- skipping the vapor retarder would be a mistake. You don't want your roof sheathing to accumulate moisture.

  13. Rich Cowen | | #13

    We have a lot of stuff ... so putting the floor on just a portion of the attic is not gonna work :) And with the basement too cold for the bicycles, they have to go somewhere this winter!

    Also I will probably have the contractor add a window to the attic for ventilation (and a view of the forest). Without the window there will be no way to air out the house after the foam is sprayed!

    Another vendor called me at 8am today... and after a brief conversation about the attic he recommended that we use a closed cell product -- Heatlok Soy -- which according to Demilec, has been approved for use without an intumescent coating.

    If the cost is reasonable than this may turn out to be the least expensive solution...

    Martin -- shold I trust the fire testing results claimed by Demilec? see

  14. GBA Editor
    Martin Holladay | | #14

    This is a complicated technical question.

    Here are excerpts from the lead story in the November 2007 issue of Energy Design Update:

    "The only problem with intumescent paints used as spray-foam ignition barriers is that they may not meet code requirements. For any material other than the six materials listed in R314.5.3 to obtain approval as an ignition barrier, it must meet the “specific approval” criteria set forth in IRC section 314.6: “Foam plastic not meeting the requirements of Sections R314.3 through R314.5 shall be specifically approved on the basis of one of the following approved tests: NFPA 286 with the acceptance criteria of Section R315.4, FM4880, UL1040 or UL1715, or fire tests related to actual end-use configurations.”

    Whether or not any intumescent coatings on the market have passed the code hurdles listed in R314.6 is a matter of technical dispute. Intumescent coating manufacturers often cite the fact that their products have passed a crawlspace fire test (SwRI 99-02) developed by the Southwest Research Institute.

    According to Mason Knowles, former executive director of the Spray Polyurethane Foam Alliance (SPFA), “Using this test procedure [SwRI 99-02], some SPF suppliers have obtained evaluation reports from ICC-ES that allow the use of their spray foam with specific intumescent coatings in specific applications, or even bare foam in some applications. There is some controversy over this test procedure.”

    ... When SwRI 99-02 is used to justify the use of an intumescent coating on spray foam installed in an attic, critics note:
    - The test procedure is not one of the tests listed in the code (R314.6).
    - Results from a crawlspace test may not apply to attics, where intumescent coatings are often installed on a slope.
    - An interpretation by the ICBO Evaluation Service has muddied the waters by ruling that products that perform better than exposed kraft-faced fiberglass insulation in a test for flash-over and burn characteristics are acceptable for use in crawlspaces and attics.

    One SwRI 99-02 critic is Neal Ganser, the president of Corbond, a manufacturer of spray polyurethane foam. In an article in the August 2007 issue of SprayFoam magazine, Ganser wrote, “Currently, there are several intumescent paints on the market that claim to be ignition barriers and are sold as such for attics and crawlspaces. This claim seems to be based on a Southwest Research Institute (SwRI) Test Procedure 99-02, which is not among the code-accepted test criteria. The SwRI test is configured as an 8 foot by 8 foot underfloor crawlspace, not an attic. … This means that the test is conducted in a vastly different space than the attics for which the product is intended.” Ganser also wrote, “Attics and crawlspaces are located in very different areas of a building and they may act quite differently at the onset of a fire. As such, SPF applications in these two distinct areas will likely require different test approaches to indicate the value of various fire-retardant coatings.”

    In an April 11, 2000 letter, Brian Gerber, a senior structural engineer at the ICBO Evaluation Service — the precursor to the International Code Council Evaluation Service (ICC-ES) — provided a widely quoted ruling on ignition barrier tests for spray foam installed in crawlspaces and attics.

    Gerber wrote, “At the July 9, 1999 hearing, the Evaluation Committee agreed with the proposal to permit recognition,in ICBO-ES evaluation reports, of foam plastic insulation without an interior covering on exterior walls of attics and crawlspaces under Section 6.7.5 of ICBO ES Acceptance Criteria for Foam Plastic Insulation (AC12), based on successful completion of one of the following three tests:
    1. Comparative crawlspace tests where the performance of the foam plastic is compared with that of 3 ½ -inch-thick kraft-faced fiberglass batt insulation. The time to flash over and the time to burn through the wood-framed floor/ceiling must be less for the foam plastic assembly than for the assembly with fiberglass insulation. … In all three cases, testing will be satisfactory to justify foam plastic installed, without covering, on walls of both attic and crawlspaces, either on the interior or exterior side of wall framing.”

    When asked about the letter, Gerber explained to EDU, “We have procedures acknowledging that paper-faced insulation could be exposed in a crawlspace or attic, so the procedures call for comparing the fire performance of an installation of paper-faced insulation against an installation with foam plastic, or block foam. The laboratory will make observations of the heat and smoke, and will determine if the comparison is favorable for foam plastic.”

    Some builders have reacted with astonishment to Gerber’s contention that kraft-faced fiberglass insulation can be left exposed in crawlspaces and attics. As most builders know, IRC R320.1 prohibits kraft facing, which is highly flammable, from being left exposed. As explained in Technical Bulletin No. 27, published by the Insulation Contractors Association of America, “Standard foil-faced and kraft-faced batts do not conform to the requirements of any model code for exposed applications. Their facings have flame spread indices greater than 25. The only acceptable application of standard foil-faced and kraft-faced batts is when the product shall be behind, and in substantial contact with, an approved ceiling construction material as required by the building codes.”

    Any testing procedure that determines how fast a product ignites compared to exposed kraft facing has chosen to set the bar remarkably low. In an e-mail to EDU, Bob Brenk, the president of Aldo Products, wrote, “ICC-ES has published reports with a few manufacturer’s products (mostly 0.5-pound density open-cell SPF products, a fundamentally different product from 2-pound-density closed-cell foam) saying their products can be used without an ignition barrier, although even these test reports can be confusing and (to some) seemingly self-contradictory. These products mentioned in such test reports have passed [SwRI] 99-02 when using the paper-faced fiberglass batt as the baseline test but probably have not been tested against a baseline of ¼ -inch plywood.” Brenk emphasized that the SwRI test for AldoCoat 757 compared the AldoCoat product to ¼ -inch plywood, not to exposed kraft-faced fiberglass.

    One recipient of Gerber’s April 11, 2000 letter — an insulation manufacturer employee who prefers to remain anonymous — told EDU, “A house of cards has been built based on this code interpretation.”

    In the spray-foam industry, opinions vary on the effectiveness of intumescent coatings used as ignition barriers. Mason Knowles has written, “While I believe the intumescent coatings probably are effective as an ignition barrier, I am not so sure that bare foam is safe in the conditions being allowed now.”

    Unlike Knowles, Ganser has expressed doubts about intumescent coatings. “In burn tests performed at UL, Class I foam covered by a code-specified ignition barrier of ¼ -inch oriented strand board (OSB) structural panel was compared to Class I foam with a high quality intumescent coating at various mil thicknesses up to 20 mils. At its best, the intumescent was only 2/3 as effective as the OSB as an ignition barrier. Had we compared it to the 3/8" gyp, also specified in the code, perhaps that divide would have been even greater. We further compared these results with 1/8-inch of a cementitious spray-on material (which is rated by UL as a full thermal barrier over SPF at ½ inch) and found the 1/8-inch application to outperform the OSB three times over. Good ignition barriers do exist, but they don’t look like intumescent coatings, in spite of our wishes for a simple-to-install product that actually works.”

    Both Knowles and Ganser worry that a catastrophic fire in a foam-insulated building will tar the reputation of spray polyurethane foam. According to Knowles, “The SPF insulation business was severely curtailed by a series of ‘flash’ fires involving exposed spray foam in buildings in the mid ’70s. Regardless of the relative fire properties of other building materials such as wood, spray foam has a ‘reputation’ in the construction industry as being extremely flammable if not covered. If a fire is started in an attic and there is no ignition barrier, our concern is that the foam will be perceived as contributing to the extent of the fire and the whole industry gets blamed.”

    Ganser picked up the same theme in the SprayFoam article. “Some formulators have successfully argued before the product evaluation services that their testing indicates that SPF performs just as well when completely uncovered,” Ganser wrote. “Again, the supporting tests were done in an underfloor crawlspace, yet these foam products are being sprayed, uncovered, into attic spaces. Could we be headed for disaster in an actual building fire that moves to an attic? Could the known flame-spread characteristics of SPF in an exposed condition endanger lives? … It is time to again start policing ourselves or we will get policed by others.” "

  15. Rich Cowen | | #15

    Wow, thanks so much, I didn't expect such a complete answer! However the heatloc soy 200 product is brand new ( and they say that the test was not the old SWRI test, but an "NFPA286 assembly" with no intumescent coating applied to the foam. So am I ok going with it, and leaving sections of the foam open?

    See also:

    One further question: if we use closed cell foam I believe we are supposed to spray 5 inches on the roof, to be in compliance with the code. So we still have the thermal bridging issue I raised in my first post -- do I need to worry about it much?

    It seems to me that a possible fix (in the long term) would be to spray just 4" of closed cell foam now. Then, in 5 or 6 years, when the roof may need to be replaced, add a 2" layer of exterior foam insulation. Has this been done in a way that can protect the roof deck from moisture? Or is this the "deadly sandwich" that Robert Riversong was referring to earlier?

    Feedback greatly appreciated!

  16. Riversong | | #16


    If you're concerned about thermal bridging, then I would suggest installing rigid foil-faced foam board under the rafters (after whatever is put in between), strap and drywall. Then it's done, you don't have to worry about doing it during a roofing replacement, and you'll avoid the deadly sandwich.

    Also, what's the point of air sealing the attic and then putting a big hole in the gable, called a window, which will inevitably leak air. And why do the bicycles care about being cold?

  17. GBA Editor
    Martin Holladay | | #17

    The letter from Robert Naini refers to an NFPA 286 assembly test. According to my understanding, any assembly test is only valid for assemblies that exactly mimic every detail of the assembly that was tested.

    In other words, if a roof assembly with a 12:12 slope was tested, then the test results cannot be applied to a roof assembly with a different slope.

    If an assembly was tested with 6 inches of foam covered with 5/8 inch drywall, than the test only applies to assemblies with 6 inches of foam covered with 5/8 inch drywall -- not to an assembly with 7 inches of foam covered with 1/2 inch drywall.

    If you change any aspect of the assembly, you can't use the assembly test to prove anything. So you need to get information on the assembly that was tested.

  18. Rich Cowen | | #18

    Martin --I got it.

    As for the window, remember that we eventually want to finish the space, maybe in 2-3 years. If we finish the rough opening for the window after the foam is put in, then the framing people will have a more difficult time and will have to remove a lot of the foam. If we put the rough opening in now, and seal around it, (the window might not go in until summer) then we should have no problem air sealing the window.

    I am assuming the bikes will rust in the cold, damp basement!

  19. Michael Chandler | | #19

    to be more clear, I am not saying that he should skip the vapor retarder paint but rather that the paint should be applied to the air-sealed drywall separating the foam from the living space rather than on the foam above the drywall.

  20. Rich Cowen | | #20

    Well, the spray foam is in so I am overdue for an update. The foam job (closed cell, 5 inches on roof deck and 3" on gable ends) was completed the Monday after Christmas and we returned the following evening to a very different house.

    Instead of the temperature of the attic dropping from 75 in the day to 50 at night as a vented attic, the attic temperature now varies from 80 to 90 degrees F. There are no heating vents in the attic, so most of this heat is coming from leakage and radiant heat from the sheet metal on the main heating trunk.

    Meanwhile, the 2nd floor of the house is a toasty 73 degrees and the downstairs temperature, where we have the thermostat, is down around 59! Those are the temperatures when the daytime high is around 35 and the low is around 20.

    So I guess it is time for us to address the second problem in the house, the poorly insulated fieldstone basement!

    Here is what I have been doing so far:

    1) Back in the summer and late fall, we replaced the leaky basement windows with modern, sealed vinyl windows.

    2) Nine days after the spray foam was installed in our attic, a second company came in (as part of the utility company weatherization program) to do 8 hours of free air sealing. This involved a blower door test, filling old holes in the basement ceiling with spray foam, and for large holes, installing and sealing in pieces of 2-inch rigid foam. In the upstairs, two or three windows with leaky frames were caulked, along with leaky spots on the first floor along the exterior baseboard, and then numerous access holes in the kitchen (for the fridge, gas range, etc.) were sealed. Leakage of the house was reduced from about 3500 CFM to around 3275 CFM. I am told the leakage is now in the "average" range. What goal should we aim for here??

    3) While the weatherization crew was here, they found one of the main reasons our bathroom (in the front of the house on the first floor) was only 55 degrees: the heating duct feeding it was a broken heap of scrap metal only partly inserted into the basement ceiling. So I took the larger duct pipe feeding the bathroom and laundry room and reworked it so that it now feeds only the bathroom. We properly insulated and foamed this duct so finally we have a decent flow of hot air into the bathroom.

    4) The bathroom floor was still a cold spot, though. This is because the bathroom sits right above the section of basement closest to the bulkhead door. This door was never insulated -- and the 7 x 4 foot bulkhead it opens to has an uninsulated ceiling, completely outside the envelope of the house and fieldstone wall. So today a contractor came by to fabricate a second door at the interior side of the bulkhead, which consists of two sheets of foil faced polyiso board, separated by an air gap and backed by half inch plywood.

    Too early to measure a change in basement temperature, but we do notice that the heat is kicking on a whole lot less. However when it does kick on it still delivers 70% of the heat to the second floor, and probably 10% of the heat to the attic where all this heat is retained.

    Obviously, we have more to do to make our house comfortable. What do you all think of this plan:

    1) remove the sheet metal ductwork in the attic and replace with a better sealed, insulated main trunk.

    2) add a lot of air flow to downstairs (our furnace can support this) by cutting an additional duct chase from attic to 1st floor.

    3) insulate the basement ceiling (fiberglass was recommended)

    4) insulate the rim joists (closed cell was recommended)

    5) do one of the follwing:
    - insulate the exterior of the fieldstone wall once the spring thaw is over OR
    - insulate the interior of the fieldstone wall

    6) possible replace (and insulate under) the basement floor
    Feedback welcome. The choice of what to do might also depend on the amount of moisture coming into the basement, so I can elaborate on that if people would like. Also, I can post some pictures of the attic if anyone wants them!

    Thanks again for all of your helpl

  21. Craig Dimitroff | | #21

    I realized I'm late to this discussion thread, but I noticed that 2 GBA advisors noted the need for a vapor barrier on open-cell foam used on the underside of a roof in Massachusetts to prevent wintertime condensation on this surface from moisture carried through the open cell foam via vapor diffusion. They also both seemed to support the use of closed-cell foam in this application since it is a much better vapor barrier.
    Howerver, one commentor raised the issue of creating the situation where the roofing might be sandwiched between the foam vapor barrier on the inside and a vapor barrier of ice & water blocking roofing membrane on the outside. It's my understanding that you shouldn't ever sandwich a building material, like wood, between 2 low perm vapor barriers.
    Does anyone have input on this situation? Whether just the first 3' feet of roofing above the gutters is covered or perhaps the entire roof surface with a product like Grace ice & water shield? This is the situation I face with the lower portions of my Gambrel roof (64-degree pitch, 3' ice/water shield) and a front roof section (10-degree pitch, completely covered with ice/water shield & then architechtural asphalt shingles). would like to use spray foam but don't water to do something that may damage the roof over the long term. house is located in northeast NJ - i've been told i'm in climate zone 5A (looking at the map, I seem to be right on the line between zones 4 & 5). thanks.

  22. GBA Editor
    Martin Holladay | | #22

    Craig Dimitroff,
    Any time you install a vapor-permeable insulation directly against cold sheathing, you are taking a risk. Vapor-retarder paint lowers the risk by slowing the vapor flow from the interior during the winter.

    Another way to lower the risk is to choose an insulation that is not vapor-permeable -- for example, closed-cell spray polyurethane foam.

    You wrote that you are worried about "the situation where the roofing might be sandwiched between the foam vapor barrier on the inside and a vapor barrier of ice & water blocking roofing membrane on the outside."

    But I didn't recommend an interior vapor barrier -- only a vapor retarder.

  23. Craig Dimitroff | | #23

    Thanks for your response.
    Instead of the vapor-retarder paint over open-cell foam, would you ever recommend using 5/8 drywall (with it's added fire resistance benefit) or a foam board product (with it's counter-thermal bridging benefit) over the open-cell foam as a vapor retarder? I'm not sure of the vapor perm ratings of all of these products.
    If closed-cell spray foam is a vapor barrier would this be something to avoid if the roof had a vapor barrier on the outside? I assume the Grace ice & water shield membrance is a vapor barrier and that the builder's felt/tar paper (or whatever it is) often used under asphalt shingles is simply a vapor retarder.

  24. GBA Editor
    Martin Holladay | | #24

    Yes, you can install drywall on the interior side of open-cell spray foam. You can also install interior rigid foam on the interior of open-cell spray foam, but if you like the idea that the roof assembly will be able to dry to the interior, any rigid foam on the interior will reduce the roof assembly's ability to dry inward.

    If you install closed-cell spray foam against roof sheathing that is covered on the exterior with Ice & Water Shield, you want to be absolutely sure that the plywood or OSB is dry on the day that the spray foam is installed. Many people worry what will happen to such assemblies when the roof finally begins to leak. It's a valid worry. If you are that type of person -- one who worries -- don't build a roof that way.

    A ventilated unconditioned attic is best if you want to be able to detect roof leaks early and repair them easily.

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