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Total energy efficiency — ROI and what else to do

humm9er | Posted in Energy Efficiency and Durability on


Very much appreciate everyone’s feedback. I would like to give a quick overview of the improvements I have performed to my new (to us) home and look for feedback but additional DIY improvements to save energy.

House is 3300 sq feet, originally built pre-1700, with extensive refit performed by previous owners within the past 20 years, including: new double paned Andersen windows, new concrete foundation poured around 2/3 of the structure when addition was added, house brought down to studs, walls insulated with bats, basement ceiling insulated with bats, cellar floor poured concrete, oil heat system upgraded to system 2000 with indirect hot water tank. 50% of attic space (over addition) insulated with bats on top of which is 14″ of blown cellulose. Other 50% of attic over old portion of home had 1.5″ foam board between joists under attic floor and nothing else.

Since we moved in several months ago, I have:

Air sealed old portion of attic as best I could with spray foam, including using aluminum and firesafe caulk to seal a 6″ air gap around chimney (must have been a huge source of heat loss)

After air sealed, rolled R-30 unfaced over floor and foam board

Insulated rim joist (what an awful job around the old portion of foundation!) using 2″ pink foam board and spray foam, sealing all plumbing / electrical exits around foundation with spray foam

Sealed all 7 basement windows with window / shrinkwrap plastic

Sealed bulkhead and insulated access door on foundation wall with heavy plastic and 1″ foam board.

Air sealed all duct joints and register outlet perimeters with foil tape — discovered many significant leaks.

Wrapped hot water lines to tank with foam insulation.


Air sealed all exterior doors with sweeps and gaskets, as well as air seal attic door which I also insulated with 2″ xps foam on inside of door headed up the attic stairs.

Insulated all exterior facing outlets with foam gaskets and used plastic child-proof prongs in all exterior facing outlets not being used

I also plan to imminently:

Seal all 36 recessed light fixtures with retrofit LEDs to minimize air flow to attic/roof.
Wade into new portion of attic into cellulose to air seal wherever necessary / wherever I can and add rock wool insulation over any recessed cans I can access.

I have pondered adding 1 or 2″ xps foam board to new foundation cement walls where I can (I can’t do so on old fieldstone and portion of my basement is a crawlspace). Those walls face north and span about 50% of my foundation. Worth it? Also, if so, go all 8′ of height or only 4′ which would take me about 2′ below ground level?

Any other relative low-cost DIY projects with a decent return? Suggestions on other things I can do?


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  1. Expert Member
    Dana Dorsett | | #1

    In zone 4 or higher, 2-3" of foundation insulation on the poured concrete is definitely "worth it", especially if very much of the concrete is above grade on the exterior.

    Foil-faced polyiso has comparable or better average performance to XPS at any thickness in this application, is easier to reliably air seal (using high quality foil tape), and is quite a bit greener, since it uses less polymer per R, and a blowing agent with only 1/200 the global warming potential of the HFC134a used for blowing XPS. It's also more fire-safe, with a higher kindling temperature, charring in place rather than melting and spreading flame even while burning. (Dow Thermax polyiso is fire rated and would not even need a thermal barrier against ignition in most jurisdictions.)

  2. humm9er | | #2

    Thanks Dana -- all 8' of Concrete or just the portion above grade and below ground a foot? I've seen both recommended.

    Any other relatively high-return improvements I'm missing in terms of overall energy savings?

  3. Expert Member
    Dana Dorsett | | #3

    Depends a bit on your US climate zone, which you have not specified. In climate zones 1 & 2 there isn't a huge rationale for insulating even the above-grade portion. In zone 3 insulating at least the upper half of the wall usually makes sense, or at least 2' below grade. You could fine-tune it based on your local deep subsoil temperatures, but as a DIY it's probably going to make sense to go all the way to the slab with R9-R10 polyiso even in zone 3. (IRC code min is R5 continuous for zone 3 basements.)

    In zones 4 & higher going down to the slab is a given. In zones 5 & higher R15 would be code-min.

    The labor involved is about the same whether it's R5 or R10 or R15, but the fastener costs and insulation costs go up a bit with thicker foam. If you're planning to finish the room into living space it's often easier/better to install 1"-1.5" of foam against the foundation as the exterior side of a non-structural 2x4 studwall, with R13-R15 unfaced batts. If taking that approach don't skip the foam- you need it to protect against groundwater moisture even in zone 1-3 and for dewpoint control in winter in zones 4+. And don't install interior side vapor barriers. For colder climates use IRC chapter 7 for a guide on the foam-R relative to the insulated 2x4 framing, which will mitigate the potential for wintertime moisture accumulation in the studwall:

  4. user-1072251 | | #4

    Have the fieldstone foundation spray foamed with closed cell - sticks great to rocks, keeps out moisture and drafts. The more comfortable your basement, the more comfortable your house.

  5. Expert Member
    Dana Dorsett | | #5

    Bob: Spraying a couple of inches of closed cell foam on the rest of the foundation may be a good idea, but doesn't fall under the rubric of "...additional DIY improvements..."

  6. humm9er | | #6

    I am in New 5.

    I would not be finishing the basement ever. Okay so foil faced 2" polyiso from floor to ceiling on my concrete north-facing walls. Do I have to frame and drywall on top of it? That prospect does not thrill me since I don't intend to really use the space.

    Given my original foundation predates 1700 and the historic nature of my home, I am reluctant to use sprayfoam on the old foundation or frankly anywhere. My historic preservation friends have already yelled at me for contemplating it in the attic. I guess aside from insulating the rim joist as I have in that area, there is not much more I can do there.

    I have 50% forced air and 50% baseboard, powered by an oil-fired system 2000. I insulated the baseboard pipes where I could, along with the lines to the hot water tank. Worth foam insulating all lines coming off the boiler? I read that in terms of "bang for your buck" it makes the most sense to insulate the first 10 or so feet off the boiler. I have a LOT of pipes coming off the boiler so tackling them all would be costly...if a huge savings I will do it of course.

    Also should I insulate the ducts? I've read mixed reviews there...consensus seems to be sealing the joints is most important and any errant heat radiating off the ducts makes its way up and into the house anyway but let me know if incorrect.

    Lastly, given the design of the old foundation, insulating the rim joist in that area was tricky (lots of pipes / wires coming through, irregular shapes, etc). I did my best to affix the 2" foam board tight to the sill, but in some cases there may be some air pockets behind the board. The entire perimeter of the foam board insulation was WELL sealed (I went overboard) with great stuff so I am very confident basement vapor cannot pass through...but I wanted to confirm that an air pocket here and there between the 2" foam board and the sill won't pose a moisture/rot problem.

    As always, thanks so much for everyone's input learning a lot here!

  7. charlie_sullivan | | #7

    " foil faced 2" polyiso from floor to ceiling on my concrete north-facing walls. Do I have to frame and drywall on top of it?"

    As Dana noted, "Dow Thermax polyiso is fire rated and would not even need a thermal barrier against ignition in most jurisdictions." I think that's a great choice for a basement you don't plan to finish.

    As for insulating pipes and ducts, I agree that air sealing the ducts is more important. After that, theres's some validity to the idea that heat that comes out of pipes in the basement goes up through the floor into the house ... but that's only true if a) the basement is warmer than the first floor, or b) there's lots of air leakage in the basement, up through the house, and out the attic. And even if it's true, it's only some of the heat. Some of the heat goes into the ground, especially with an uninsulated slab (not sure if that's the case for you), or an uninsulated wall (sounds like you've got one that is going to stay uninsulated.

    Pipe insulation doesn't cost much to buy, but it can be a lot of work to apply around complex joints. I'd start by insulating the straight runs.

    Depending on whether your historic preservation of the stone foundation means keeping it visible all the time, or just having it be possible to open up the wall and see it, putting up polyiso along it but not attached, e.g. spaced 1" away, might be an option.

  8. Expert Member
    Dana Dorsett | | #8

    In zone 5 code min is really R15, not R12, so it would take 2.5" of polyiso to get there. If you use fire rated Dow Thermax it doesn't need to be covered. That's how Marc Rosenbaum did it at his former residence (albeit at less than code-min):

    Be sure to either keep the bottom edge off the slab, or install a capillary break between the concrete and the cut edge of the polyiso, since (unlike polysytrene) it will wick moisture in, taking forever to dry.

    In MA there are multiple vendors of reclaimed roofing foam which makes it pretty cheap to do an all-foam solution, even after factoring in the cost of a thermal barrier against ignition. I did mine with 3" fiber faced iso and 1x4 furring through-screwed to the foundation with TapCons, hanging wallboard on the furring. But if you don't mind it taking up space it's probably easier to trap the foam to the foundation with a 2x studwall on which to hang the wallboard. The two largest are Nationwide Foam in Framingham, and Green Insulation Group in Worcester. (google 'em). There are smaller operators too, but those two would almost always have something suitable in stock.

    On the rubble foundation section you installed a 10mil plastic or EPDM sheet over the rubble foundation and sprayed the foam only on the sheet goods would the historical hystrionics subside? How about an air gap to a foam-clad studwall on the interior?

    If the boiler and hydro air handler is in the basement and the foundation has not been insulated HELL YES the ducts need to be insulated (both supplies and returns.) If the basement is insulated and air sealed it's still worth insulating the supply ducts, but the returns don't need to be. The passive heating of the duct loss is probably keeping the basement above 55F even in the dead of winter. At an average mid-winter delta-T of say 25F between the basement and exterior every square foot of ~R1 uninsulated above-grade foundation is losing 25 BTU/hr. If that's 2' of above grade foundation on a 200' perimeter that's an average loss of over 10,000 BTU/hr. Multiply that by ~2000 hours of winter that's 20,000,000 BTU, which is worth about 165 gallons of oil burned in a System 2000. Even at this year's lower oil prices that's sorta-real money.

    If you insulate only the ducts the basement will run colder, but the loss will be lower due to the lower temperature difference between interior & exterior. If you insulate the entire foundation to code-min the basement will be much warmer (probably never dropping below the 60sF) but the heat losses will be MUCH lower, since you've decreased the U-factor of the basement by more than an order of magnitude, more than making up for the now higher temperature difference. It also raises the average temp of the first floor floor a degree or two, which is a subtle but noticeable difference in overall comfort.

  9. humm9er | | #9

    Hi Dana and Charlie,

    Thank you both for your very thoughtful replies.

    Let me recap and be sure I have this all correct:

    Okay, on the cement walls, foil-faced polyiso, at least 2", 2.5"+ if possible, attached with tapcons, with bottom edge 1" off the slab/basement floor. I will have to cut around windows and electric outlets, etc with a drywall saw. I have a French drain around the perimeter of the basement and I would not describe the basement as damp -- there are 2 sumps but they apparently have only turned on once after the mother's day storm of 2006 here in MA.

    No moisture concerns with this project right?

    Should I foil tape where boards meet or edges where boards meet concrete?

    I will tackle this first and then mull options with old foundation. When I address that project I will post here for input. Basically, I want to be able to monitor the fieldstone to be sure nothing needs repointing.

    I will insulate all straight run water lines to/from boiler with foam, and focus on the 90 elbows further down the road when I'm really bored.

    Regarding duct insulation, am I correct that insulating the concrete walls will provide greater ROI than insulating the ducts? I will plan to tackle duct insulation, but it's expensive and I need to prioritize since I have a number of other projects I have recently tackled. For duct insulation, I assume the foil-faced wrap is what you all advocate? I am estimating that it will run me $300+ in duct insulation.

    With the System 2000, the previous owners consistently burned 600-700 gallons of oil/year on heat/hot water. After air sealing attic, insulating 50% of attic which was not insulated with r-30 bats, air-sealing basement, insulating rim-joists with 2" XPS and spray foam, and air-sealing all 20 recessed cans facing attic, I am curious what type of reduction in oil use I'll see (assuming that we keep heat temp the same etc). Anyone have a guess?

    I am also curious how much more oil reduction I will see with insulating 70% of basement (concrete portion) with 2" polyiso and then with duct insulation. Can anyone guesstimate / ballpark all things being equal?

    Just trying to gauge impact on all this so I can plan/budget appropriately.

    Thanks so much everybody I am learning a lot.

  10. Expert Member
    Dana Dorsett | | #10

    Yes tape the seams of the facers with decent quality aluminum duct tape (eg Nashua 324a, sold in box stores), and foam-seal the top & bottom edges to prevent slow convection in that micro-space.

    Without knowing how much of the foundation is above grade or how leak the basement is currently it's hard to estimate the fuel savings with any accuracy. In my case, by adding 3" polyiso to 90-95% of the foundation walI I was expecting maybe a 10-12" reduction of total fuel use based on some guesswork on the air leakage, but it actually delivered something like a 16-18% reduction, probably due to higher than estimated initial air leakage. If you are not air sealing or insulating the field stone /rubble foundation amounting to 30% of the total area, you may be leaving a lot of low-hanging fruit on the air-leakage vine. Poured concrete is inherently more air-tight than field stone, CMU or brick foundations.

    By insulating even 70% of the basement you should be achieving more fuel savings than just insulating the ducts, and it's often an easier retrofit. You also buy more comfort by insulating the basement instead of the ducts.

    The heat losses off the heating distribution plumbing is miniscule in comparison to the losses off the ducts. The total surface area of the ducts is just SO much larger!

  11. humm9er | | #11

    Okay awesome. I bought 1 roll of R6 foil-faced duct insulation to estimate my total requirements and I would need 15 rolls just to do the 8" round ducts off the main duct. Including the main rectangular duct would swell my requirement dramatically.

    Am I correct that ROI-wise insulating the concrete walls with poly-iso is a smarter initial approach than insulating the ducts? The ducts only run in the "new basement" where the concrete walls are, not in the fieldstone area.

    I ask because cost of duct insulation and poly-iso for walls appears to be about the same in my situation.

    I did upgrade all 36' cans to the LED retrofits and definitely notice less cool air...however I still need to go back and insulate the cans with rockwool for maximal effect.

    Thank you!

  12. Expert Member
    Dana Dorsett | | #12

    Insulating and air sealing the foundation walls lowers the total load more than just insulating the ducts, and it puts the ducts inside conditioned space. But unless you're insulating the entire foundation wall it may be hard to estimate.

    Is there an air-tight partition wall between the new and old basements?

  13. humm9er | | #13

    My basement footprint is complicated:

    See below for rudimentary drawing. Basically, the big rectangle at the top is the nice cement area facing north...there is a cement wall and drywall splitting the new basement from the old (the 2nd line you see at bottom of the rectangle at top).

    There is a door on bottom concrete wall that enters a walkway which is framed by concrete walls about 4' high...on either side of that walkway is heavy-mil plastic laid down over crawlspace areas...on the right of the crawlspace the 1st floor runs right into a 1' rock wall ...that area is all foam sealed and the rim joist is also sealed with 2" XPS foam etc. So there is really not much more "wall" space there to seal.The left area of crawlspace has the massive 12' x 12' stone chimney and there is an earth/rock mound that basically goes up to the sill so again, not much to insulate there unless I get into spray foam.

    Once you walk past the chimney, you go up 3 stairs and enter a comparatively smaller say 16 x 12 fieldstone area with a nice concrete floor.

    |--------------- ------------------------------------|
    |xxxxxxxxxx| | xxxxxxxxxxxxxxxxxxxxxx |
    |xxxxxxxxxx| | xxxxxxxxxxxxxxxxxxxxxx |
    |xxxxxxxxxx| | xxxxxxxxxxxxxxxxxxxxxx|
    |xxxxxxxxxx| |-----------------------------------|

    I could certainly close the door between the basements and air seal the wall with spray foam wherever there are outlet or duct holes, yes.

  14. humm9er | | #14

    Okay that looked great before I hit submit...but you should get the gist, the X areas are crawlspace separated by a walkway. I can close off a door between cement area and crawlspace / fieldstone area.

    only part of basement with proper walls are cement area and fieldstone area labeled FS

  15. humm9er | | #15

    Sorry if that last post describing my basement layout was confusing. I appreciate your input.

    Also, the R6 foil-faced batt duct insulation is expensive! I got a few rolls to start with the 8" round ducts off the main trunk in the far area of the house that seems the coldest.

    Can I use paper faced R-13 just on the main duct / rectangular trunk portion? This is heat-only ductwork. I'm afraid using the foil-faced product on that area is just totally cost-prohibitive. Like $700 worth of material. I will use the foil-faced product on all round ducts.

    I will be back with questions on the polyiso on the walls when I dive into that project after the new year. Thank you everyone for your help!

  16. Expert Member
    Dana Dorsett | | #16

    Wrapping ducts with batts rarely works well in the long term, and if the ducts are also being used for air conditioning they get wet and grow mold.

    If you can partition off that section of the basement from the rest with an air tight insulated studwall and insulate the poured foundation so that all the ducts & air handler are inside the pressure and insulation boundary of the house it'll (usually) be money better spent.

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