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Insulation upgrades

humm9er | Posted in Energy Efficiency and Durability on

Hi Everyone.

Thank you in advance for your feedback, you have all been so helpful.

Zone 5A (coastal Mass) house.

I have an older 3200 sq ft home with a mix of well-pointed fieldstone foundation and poured concrete. The 1000 sq ft attic is currently air-sealed in most places, and has a mix of fiberglass batts and cellulose for about R-30. I have 10 recessed lights in attic, which have the LED airtight retrofits with gaskets, but are not covered in cellulose. The recessed fixtures are IC-rated. The remainder of the home has vented cathedral ceilings (sigh).

I have a System 2000 boiler and am averaging about 450 gallons/year of oil on heat and hot water.

During the past 100 year freeze earlier this month, I had some frost on underside of roof sheathing. I spent hours digging through cellulose and airsealed some wall plates and open wall cavities that the previous homeowner’s airsealing and insulation contractors clearly missed. There is one 30′ exterior wall plate in the eaves I cannot reach to airseal from inside the attic.

Mass Save (a national grid rebate program) came to the house, and pitched me on pulling up the existing attic insulation, double-checking and airsealing anything I missed, adding 2″ polyiso to a 1′ attic kneewall and blowing enough additional insulation all over my IC cans and around the entire attic to get me to R-49 (this amounts to an additional about 7-12″ of cellulose they say, depending on existing levels). With the 75% off rebate, my cost is $500.

Question #1:

Is it worth pulling the exterior soffit/fascia board to seal that 30′ exterior wall plate? I have a hard time quantifying how much air loss that will stop. Mass Save says they wont do that, it would be up to me.

Question #2:

I know quantifying ROI is tough, but will I see substantial benefits going from R30 to R49 and really burying the recessed lights in cellulose? I was estimating about a 5-year payback?

Pertaining to the basement, my rim joist is air sealed with 2″ XPS and spray foam. I have forced hot air ductwork in basement. I wanted to add 2″ foil-faced polyiso to my poured concrete foundation, which amounts for about 70% of the foundation. Mass save rep says I’m wasting my time unless I also deal with the 30% fieldstone.

Question #3: Is insulating 70% of my basement walls with reclaimed polyiso (about $400 cost to me) not going to provide much benefit?

I have no capillary break between fieldstone and sill plate. I see no signs of rot where I foamed the rim joist in the section, and I understand that fieldstone will wick far less moisture than poured concrete.

Question #4: Long-term, Is jacking up house, slipping capillary break under sill, and then running that vapor barrier to floor, and spray foaming that, best option for airsealing and insulation the fieldstone? Any other practical way to aireal and insulate fieldstone?

Thank you!!

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  1. dbaerg | | #1

    Hi Justin,

    Is this house a Cape? You mentioned kneewalls and cathedral ceilings; so, I'm wondering.

    If you have enough air leakage to cause moisture problems in cold weather, insulating without air sealing will make the problem worse - the attic will be colder and, therefore more condensation will occur.

    The recessed fixtures are a likely point of air leakage. Did you have a blower door test done? Did your advisor find significant leakage at the recessed fixtures? Even if they are "air tight", they can still be improved. There is likely leakage through the wire channel - caulk this from the attic. And the seams in the can likely leak a bit - seal them with aluminum foil duct tape. And, of course, put a bead of caulking between the can and the ceiling from below.

    Having a skilled technician do a blower door test will give you a rough idea of how much air leads through that top plate you are concerned about. It's still not a defiinitive answer to question 1 but it's better than a guess.

    I doubt you will get a 5 year payback going R30 to R49 in your attic. That upgrade is marginally worth it where I work (northern Ontario - much colder with higher fuel prices). Do it to make your house more comfortable, reduce ice dams and expect a 3-5% reduction in your oil bills.

    Re question 3 70% is better than 0%. Go ahead and insulate the poured concrete foundation.

    I hope this helps.



  2. Expert Member
    Dana Dorsett | | #2

    Heat loss is a U-factor x area type of thing. It's definitely NOT a waste of time to fix 70% of the total area.

    Only 2" of polyiso won't get you to MA code minimum (= R15 continuous insulation), but it's still WAY better than nothing. Depending on how much above grade exposure you have on the foundation the basement losses (even at cooler basement temps) can be 20% or more of the heat loss. Fixing 70% of that with even R12 (2" polyiso) would result in more than a 10% reduction in overall fuel use, even if the 30% that's fieldstone is left uninsulated.

    You don't usually need a capillary break between fieldstone and the sill in this region. Fieldstone in MA is almost always high density igneous rock with very little wicking capacity, orders of magnitude less wicking than poured concrete or limestone.

  3. GBA Editor
    Martin Holladay | | #3

    The air-sealing work being offered by Mass Save (for just $500) is absolutely worth the money -- and the upgrade from R-30 to R-49 is being thrown in for the same price. So do it! It's a bargain.

    There is no easy way to estimate the value of pulling the soffit and fascia to gain access to the area for air sealing. Do it if you can afford the cost -- skip it if you can't.

    The best way to air seal the interior of a fieldstone foundation wall is with closed-cell spray polyurethane foam. Not cheap -- but effective.

  4. Expert Member
    Dana Dorsett | | #4

    "I know quantifying ROI is tough, but will I see substantial benefits going from R30 to R49 and really burying the recessed lights in cellulose? I was estimating about a 5-year payback?"

    The ROI in higher wintertime indoor relative humidity comfort is often pretty immediate when air sealing and covering over recessed lighting.

    An inch of closed-cell foam on the field stone followed by 2.5-4" of blown fiberglass (not batts, which would leave too many voids) in a 2x3 wood studwall would get you up to code-min performance on that part of the wall. Using cellulose instead of fiberglass might work, but could be risky, since it would draw up any bulk water moisture from even a minor flood (like a clothes washer that overflowed or something) like a sponge and take forever to dry. Blown fiberglass dries a heluva lot faster, and wicks a lot less.

    Like air sealing and insulation over the recessed can in the attic, air sealing the fieldstone and foundation sill & band joists with a shot of closed cell foam will improve wintertime indoor moisture levels.

    Going with 2.5-3" of polyiso on the poured concrete portion wall rather than 2" would get you to code-min there.

  5. humm9er | | #5


    Thanks! No not a cape...a colonial with an addition.

    The LED recessed light retrofts are gasketed and hug the ceiling drywall tight. I dont think they are leaking...but I could build airtight boxes in the attic before they blow the cellulose if we think that's worth it? Can I just just 1" Polyiso to build the boxes and sprayfoam them to the attic floor? They can then blow cellulose over the airtight boxes...


    Ok, bump up to 3" polyiso if I can? On the poured concrete portion, I have about 3' above grade. Sounds like Mass Save was dissuading me since they don't get paid to hang insulation on basement walls (not "approved").


    I'm inclined to agree with you. My only worry is if they somehow dont get all the airleaks and then burry my attic in 24" of insulation. I'll have to really be sure they are thorough on the air sealing...

    I can pull that rake/soffit myself and hit the wall plate with spray foam. I'll put it on the summer project list.

  6. humm9er | | #6

    Closing the loop on this, before I have Mass Save air seal and you all think it preferable to build enclosures in the attic for each IC can, and then blow cellulose over that? Or just blow cellulose right over the cans?

    Assuming I mostly solved for the air leaks with the gasketed LED retrofits at the ceiling plane, which approach is better from an insulative perspective?


  7. GBA Editor
    Martin Holladay | | #7

    Installing LED retrofit kits with gaskets cuts down on air flow through a recessed can, but doesn't stop the air flow completely. If I were making the decision, I would retrofit some type of airtight cap over each recessed can.

    But it's fussy work, and the decision depends in part on how many recessed cans you have to address, how easy or difficult the access is, and who's doing the work (and at what cost).

  8. humm9er | | #8

    I'll do the work.

    10 cans

    Do you prefer 1" foil polyiso, xps, or other to construct the boxes, which I will spray foam to the floor? I read that the tenmat covers leak air.

    Thank you Martin!

  9. GBA Editor
    Martin Holladay | | #9

    I like foil-faced poyiso, because it's easy to tape. For more hints, see this article: Recessed Can Lights.

  10. humm9er | | #10


  11. humm9er | | #11

    Okay, I have built 10 airtight boxes out of foil-faced polyiso and spray foamed them to the attic floor over all my recessed lights.

    I have a bath fan in the attic (ceiling of 2nd floor bathroom), which is exhausted through the roof -- I dont love this design, but redoing it would be cumbersome. Can I build an airtight box around the bath fan as well? I see no IC sticker or anything, but could leave an airgap if there is concern. The light itself in the fan is a 9 watt LED.

    Once I finalize that, I will have Mass Save come in and airseal anything I missed and blow more cellulose to get me to R-49.


  12. GBA Editor
    Martin Holladay | | #12

    Yes, you can build an airtight box around your bath exhaust fan if you want.

    Another approach (working from below, in the bathroom) is to remove the ceiling grille and caulk the gap between the drywall or plaster and the metal fan housing. Then, from the attic side, just blow your cellulose or fiberglass insulation right over the top of the fan. (This latter approach works best for newer fans. As Bill Hulstrunk explains, "If it is an older combination fan/light that takes a standard screw-in incandescent bulb, it's better to treat as if it were a non-IC recessed light fixture, and enclose it in a box before insulating.")

  13. humm9er | | #13

    Thank you Martin! Will do.

    I am going to also tackle the 70% of my basement that is poured concrete, and will use 3" foil-faced polyiso.

    I will tape all seams with nashua foil tape, and use canned "great stuff" to air seal the top and bottom of the sheets where they meet the foundation. Lastly, I will have the bottom of the sheets stop about 1' from the floor, leaving a gap there to avoid potential moisture wicking issues.

    I plan to use 4" tapcons and 2" fender washers to hold the insulation to the wall. I've seen the fancy plastic insulation washers, but they are very pricey! I can either glue my metal surface-mount wall outlets to the foam, or tapcon it through the foam to the wall -- any recommendation there?

    Is there any issue with the plan above, or something I need to do differently? Thank you!!!

    Lastly, Dana if you're reading, you have mentioned a reclaimed supplier in MA -- is it the one in Worcester or Framingham or elsewhere?

    Thank you everyone!!!!

  14. Expert Member
    Dana Dorsett | | #14

    Both Nationwide and Green Insulation Group are good sources, with constantly evolving (but usually pretty good) inventories of goods on hand. There was a smaller outfit operating out of Winchendon a handful of years ago that I've sourced foam from, but I'm not sure if they're still around. There are others. I'm not buying foam on a regular enough basis to keep tabs on all of them. For foil faced polyiso you're best bet is probably Green Insulation Group, which seems to have a steady supply of factory-seconds (typically cosmetic dings in the facers or bashed corners) at about half the distributor pricing for perfect goods.

    With polyiso be sure to keep the bottom edge off the slab, or above the high-tide mark if there is a flooding history to the basement.

    To meet fire codes you'll still need a thermal barrier between the foam and basement. Instead of 4" TapCons and fender washers, 5" masonry screws and 1x4 furring would give you something to mount half-inch wallboard onto. Alternatively, gluing the foam to the foundation then overlaying it with 1/2" OSB though-screwed to the foundation with 4.5" masonry screws works.

  15. humm9er | | #15

    I'll stop the insulation 1 foot above the basement floor to be safe. That's still 6 feet of insulation height-wise.

    For the past 2 weeks, my basement has stayed right at 55 degrees, with a relative humidity range of 40-50. A piece of plastic taped to the wall for 1 year has shown no condensate. Is that a "normal" humidity range for this time of year in MA?

    I see no evidence of moisture issues on my concrete walls, but read a cautionary article on BSC about using foil-facers on basement walls because they trap vapor behind them. Provided I airseal all seams with tape, and airseal tops/bottoms of polyiso with a bead of canned spray foam, there should not be any moisture issues correct? In Summer, this should slightly raise my basement temp, but lower the relative humidity too, right?

    One other factor is that my exterior basement walls are insulated with what appears to be 2" XPS or EPS. There is still 2-3 feet of foundation above grade, which will still solve for/allow drying to exterior I believe?

    Thank you everyone tremendously!!

  16. Expert Member
    Dana Dorsett | | #16

    Air with a relative humidity of 50% @ 55F has a dew point of 37F, which is fine. Raise the temperature of that air to a fully conditioned 70F and it's RH drops to 30%, which is the low end of the human-healthy & comfortable range (but still in the range.) Your upstairs space might have a higher or lower dew point temperature, but even your high-end 50% RH @ 55F doesn't raise any alarms.

    If the exterior polystyrene insulation is only below grade, it has only minimal affect on the drying capacity of the foundation, but it does limit the rate that ground moisture can reach the walls, and raises the temperature of the concrete helping it dry toward the interior. If the new interior insulation is stopping a foot from the floor, moisture wicking from the footing will dry reasonably to the interior (as long as the water table stays below the footing), and won't affect the moisture content of the foundation sill. Above-grade exposure of 2-3' (or even half that) offers good drying capacity toward the exterior as well, as long as there is reasonable surface drainage, and limited spash-back from the roof overhangs. (That 2-3' of exposure is also a huge heat leak for an insulated foundation wall!)

  17. humm9er | | #17

    My in-house humidity levels hover around 28-35% at 65 degree or so, much lower.

    Ok, so no worries with foil-facers to interior, but keep an eye on moisture.

    I have exterior roof gutters piped to buried drains that run away from house to daylight. I also have an interior french drain running the entire perimeter of the poured foundation, tied to a sump which also runs to daylight. I've never seen that sump turn on in 3 years of living here.

    Do you advise I stop the insulation higher up than 1 foot from floor, or is 1 foot good?

    My sill in the poured portion of basement has a foam gasket between it and the top of foundation. I have 2" XPS which I can spray-foamed along the entire rim joist, then rockwool or fiberglass batts up against the airsealed rim joist and along the basement ceiling.

    I plan to leave the 2" horizontal top of the concrete foundation, where it meets the foam gasket and the sill, "exposed" for now to monitor moisture. I would spray foam the bead where the polyiso meets the edge of the concrete though. Is that all correct? I can spray-foam it all in with closed-cell in the future...

    Thank you Dana!

  18. GBA Editor
    Martin Holladay | | #18

    Q. "Do you advise I stop the insulation higher up than 1 foot from floor, or is 1 foot good?"

    A. Ideally, all of your basement wall is insulated. If you are worried that moisture might wick up the polyiso, you can stop the polyiso 1 inch off the floor. But 1 foot is too much -- it just means that your wall isn't fully insulated.

    For more information, see How to Insulate a Basement Wall.

  19. Expert Member
    Dana Dorsett | | #19

    If the wall has exterior polysytrene down to the footing, it's still an insulated wall, but the exposed above-grade concrete and exposed foot of interior at the bottom is a significant thermal bridge.

    With the 2-3' of exterior exposure there is no reason to NOT insulate over the top of the foundation and foundation. Monitoring the moisture with the foundation sill exposed to the interior won't tell you anything meaningful, but you have huge drying capacity toward the exterior.

    There is no good reason to not insulate the entire height of the wall even if the wall has a flooding history- just stop the polyiso above the historical high tide mark. Even if the high-tide mark was 8" above the slab and you stop the polyiso a foot above, filling in the bottom foot below the polyiso with unfaced EPS of equal thickness would prevent moisture wicking, and would be flood-tolerant. Rock wool batting could also be used, albeit with some risk of frost formation in the rock wool during exceptionally cold winters. Rock wool wicks some moisture, but sealing the bottom edge of the polyiso with an appropriate tape would keep it out of the foam.

    My basement has a history of flooding with the slab sometimes several inches below the local water table during the spring thaw or heavy rain weeks. I stopped the polyiso 4" above the high tide mark when I insulated it a decade or so ago. It's doing fine.

  20. humm9er | | #20

    I have mounted the 20 3” thermax to the basement walls. I have taped all the seams where the boards meet.

    I will run a bead of spray foam at bottom of boards and tops to airseal.

    Should I also tape all the “exposed” edges? Meaning the tops of the boards, the bottoms, and where I made cutouts around windows.

    Thank you!!

    Also, I did insulate a 18’ section attached to garage ... i can’t see sill there, but assume it has a foam gasket like rest of sill. But other side of that foundation wall does not have exterior exposure. Should this be ok or could I have a drying/moisture issue?

  21. humm9er | | #21

    Quick follow up as I'm in the middle of this project presently and don't want to get something wrong. Aside from air-sealing where the thermax meets the wall with foam, and taping all the seams, I do NOT have to cover all the edges with foil tape too, right?

    And lastly, I have a ton of extra 3" thermax...rim joist is already sealed with 2" XPS and spray foam. No issue adding thermax on top of that ("doubling up") and foaming it in, right? There would be an air gap between the layers owing to the irregularity of the foam, but I would spray foam around the thermax perimeter.

    Thank you!!

  22. Expert Member
    Dana Dorsett | | #22

    No you don't have to seal the edges with foil tape, just tape the seams for air tightness. The only cut edge you need to worry about is the bottom edge, and then only if it's resting on a potentially damp surface.

  23. humm9er | | #23

    Thank you Dana! I brought the 3" thermax down 2" off the slab. Got it from Green Insulation Group.

    I'm now debating the full plunge, closed cell spray foam on the fieldstone. I have 360 sq feet of fieldstone walls...and no gasket/capillary break between stones and sill.

    Dana, you mentioned that MA igneous rock and/or brick (I have a mix) cant wick moisture to the sill the way concrete does? I only have about 8-12" of above-grade foundation exposure outside around the "old" foundation. I love the idea of spray foam, but worry a bit about it being irreversible, and moisture.

    Thank you again everyone so much!!!

  24. Expert Member
    Dana Dorsett | | #24

    The wicking of brick varies with material type & density, but igneous rock has fairly weak capillary draw.

    Take a 2-pronged wood moisture content meter and measure the foundation sill/beam on the old part of the foundation where the exterior exposure is minimal, and compare it to locations on the same side of the house where the exposure is greatest. The % m.c. is probably within a few percent everywhere with no discernable pattern as to whether there is 8" of exposure vs. 12". There may be some differences between the sunny side of the house and north face or shaded parts.

  25. humm9er | | #25


    I will get a moisture meter and report back. Anything else I should be assessing in the fieldstone foundation/old sill as I weigh whether it's a candidate for spray foam? I'm not opposed to jacking up the house but that definitely complicates the project...

    In terms of the foundation wall that connects to my garage (ie, no exterior portion), I did affix thermax there too. There is a sill gasket as that's "new foundation." Given it's subjected to zero direct rain water (it's in the middle of the house so to speak), is it reasonable to conclude I shouldn't be too concerned with excess water issues there? I just wanted to ask given I just insulated the only open face of that wall.

    Thank you again, everyone's feedback as been so helpful. I hope to pay it back to others where I can.

  26. humm9er | | #26

    I have several updates and additional questions on this which I've posted to the blog. I would love feedback for those willing to provide.

    Thank you.

  27. humm9er | | #27

    So I have continued my insulation journeys.

    I DID remove my exterior soffit to airseal that 1 wall plate (what a pain)

    I added gutters and buried drains to daylight.

    Mass Save is coming to further airseal and add cellulose taking me from R30 - R49 on about 600 sq ft of attic.

    Question: My bath fan/light's older, but nutone says it's okay to cover with insulation. It sounds from Martin and others that building an airtight box with a 3" airgap is the smarter play before burying it in cellulose. That will also eliminate any possible air leaks. Agree?

    Also, I struggled with winter moisture/frost in the attic and assumed it was air leaks, which is odd because I've gotten it pretty tight. Well, my dryer vent is 6' below a cathedral ceiling soffit vent, which ties into the attic that has the moisture. I assume that stack effect/air movement would suck that dryer air up into the soffit no problem?

    HUGE moisture source. I plan to reroute that dryer vent ASAP...but of course there are no good alternate venting routes.

    I have been monitoring basement wood moisture levels too in prep for sprayfoam. Will be back separately on that item...

    Thank you!

  28. user-2310254 | | #28


    Have you considered replacing your existing unit with a heat-pump dryer. HPD are pricey, but they don't require venting.

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