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Is there value in flash and batt with today’s IECC requirements?

andy719 | Posted in Energy Efficiency and Durability on

I’m asking that question as a homeowner, not builder or sales rep.  Also, specific to new construction homes.  I am building a new 4000sf, two-zoned heat pump all electric house in Maryland (zone 4A).  I am using a spec home builder, and my options don’t include exterior insulation.  The one option they have been pushing me toward is flash and batt on the above grade walls only, and maybe the garage ceiling under living spaces.  The typical walls are vinyl siding, house wrap, either plywood or OSB (haven’t asked to confirm), 2×6, R19 batt, and drywall.

So, I am new to this site and FHB, but I have crammed many houses reading and listening and came away with a few observations.  Exterior insulation is best, but I don’t have that option here.  Due to the studs, the increase in whole wall R-value I get with 1 inch of closed cell spray isn’t much.  1 inch is fine for my climate zone with regard to condensation though.

My jurisdiction is pretty strict about blower door testing to verify the required 3ACH50 requirement.  The upgrade cost to flash and batt will add about $50 to my monthly mortgage.  If the base house cost already includes air sealing enough to get below 3ACH50, will I really get any benefit from the optional flash and batt?  Would the answer be different if I were to fill the cavities with open cell instead?


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  1. Expert Member
    Michael Maines | | #1

    Andy, flash and batt can be worthwhile in thick assemblies or when the goal is to meet specific building code targets, but it won't make financial or environmental sense if you can get to 3 ACH50 another way. More info here:

  2. andy719 | | #2

    Thanks. I read that article and your earlier article about flash and batt. I guess I interpreted it as full cavity open cell makes more sense than nearly full cavity closed cell.

    Just to be clear, the consensus here is that if I get 3ach50 with tape and caulk, then diminishing returns says R19 fiberglass is a better investment than 1" flash and batt or full cavity open cell?

    1. Expert Member
      Michael Maines | | #6

      Andy, without doing the math, I believe you are correct--"if I get 3ach50 with tape and caulk, then diminishing returns says R19 fiberglass is a better investment than 1" flash and batt or full cavity open cell?" Both from cost and environmental impact viewpoints. But as Akos mentioned, high-density fiberglass (R-21) would be better than R-19, which is hard to install properly. Open cell foam would perform well and its blowing agents are benign (H2O) but the resin still has higher embodied carbon than fiberglass and its stickiness makes future renovations or repairs more difficult, and its R-value is no different than fiberglass. Its main advantage is that it makes a good air seal. But there are more flexible ways to achieve a good air seal.

  3. Expert Member
    Akos | | #3

    The one spot flash and batt would be worth it is in the garage ceiling. There the extra air sealing would make a big difference on indoor air quality.

    For the walls, your best return if possible, is getting mineral wool or high density fiberglass batts installed. These are slightly higher R value, but more importantly more solid which generally gets you a higher grade insulation install.

  4. walta100 | | #4

    You do understand IECC produces a model code that your local elected officials will adopt sometimes with a few changes often tens of years behind the model.

    The current IECC model calls for R20 wall or R 13 +R5 for zone 4. This is for the whole wall and the framing will cut the R value if the R19 bat. This is the minimum for any “green” house.

    I will say it again spray foam is an easy and expensive way to solve last minute problems the designer was too lazy solve with a pencil.

    It is difficult build a thin R20 wall without exterior insulation.

    You may want to read this article.

    In my opinion 3 ACH is a very low bar and passing that test should not be a problem if the contractor is making any effort at all.


    1. andy719 | | #5

      Thank you Walter. I am fully aware of the code adoption process. The state of Maryland has adopted the 2015 IECC and specifically stated no local jurisdiction shall amend this standard to reduce the requirements.

      The IECC states walls in zone 4 shall either have cavity insulation of R20 or cavity 13 plus 5 continuous. It doesn't say you need whole wall R20 as is calculated with regard to the framing losses.

      Like I said, I don't have the option for double studs or any other changes to the framing. The price i received from spec builders was about 70% of the cost of custom home builders. Better wall designs won't save that kind of money in energy use.

      Thank you for the comment about 3ach being easy though. It gives more credence to the comment from Akos about using mineral wool just to keep the insulator honest with regard to installation quality. I know the builder is okay with mineral wool since we are already using it in some interior walls for sound proofing.

  5. Expert Member
    Dana Dorsett | | #7

    >"The typical walls are vinyl siding, house wrap, either plywood or OSB (haven’t asked to confirm), 2×6, R19 batt, and drywall."

    That doesn't even meet the code minimum.

    An R19 batt only measures R19 at it's manufactured loft of ~6". When compressed to 5.5" in a 2x6 wall cavity it only performs at R18, fully 10% below code minimum.

    Purpose-made R20 fiberglass batts perform at R20 @ 5.5". R23 rock wool performs at R23 @ 5.5", R21 fiberglass is R21 @ 5.5".

    With vinyl siding the sheathing dries easily toward the exterior in a zone 4A location, so there is no vapor permeance benefit to a flash'n'batt. The additional R-value's performance of the closed cell foam is mostly robbed by the thermal bridging of the framing, and there are cheaper ways to air seal the sheathing layer.

    1. andy719 | | #8

      Yeah, I was aware of the R19 issue. I'm thinking they just need to update their promotional materials. I didn't want to press them on that until I decide what major items I want.

      I'm still not sure of the application of that article with regard to what I'm asking. It is geared toward colder climates that would ever consider 5 inches of closed cell which is obviously significantly more expensive than open cell of an equivalent depth or flash and batt.

      1. Expert Member
        Dana Dorsett | | #9

        >"I'm still not sure of the application of that article with regard to what I'm asking."

        The point is that whether 1" or 5", the potentially higher thermal performance of high R/inch insulation is largely robbed by the thermal bridging of the framing. The improvement of a 1" flash 'n batt on whole wall R is negligible- it's academic- even hard to measure, despite adding ~R2 or so to the center-cavity R value.

        Since it adds effectively no thermal-performance, and offers little to no moisture resilience benefit in a climate where the inherently back-vented vinyl siding alone is sufficiently protective of the sheathing to allow Class-III vapor retardency (=standard interior latex paint) as the interior vapor retarder, there aren't any good reasons to go with flash-foam.

        Note: An inch of closed cell foam has as much polymer as 4" of half pound open cell foam, and doesn't air seal as effectively as 4" of open cell. Even HFO blown closed cell foam isn't exactly "green", with a fairly hefty CO2e footprint due to the high polymer per R :

        R20 cellulose would be a lot greener (it's sequestered carbon, with a negative CO2e footprint), and it would be even more protective of the sheathing due to it's hygric buffering capacity.

        1. andy719 | | #12

          Following up. The insulator came back and said he knows the prescriptive code requirement, but the builders' production homes have been RESchecked for the whole house performance using R19.

          They will use R21 at a cost 1/5th flash and batt, so it would add about $10/month to the mortgage. I'm good with that if it uses a little less energy and helps a little with the comfort level. I guess I'm just asking you fine gents if I should believe that is the case, or assume I'm just throwing money away by doing anything between the studs above the standard R19?

          1. Expert Member
            Dana Dorsett | | #26

            >"They will use R21 at a cost 1/5th flash and batt, so it would add about $10/month to the mortgage. I'm good with that if it uses a little less energy and helps a little with the comfort level. I guess I'm just asking you fine gents if I should believe that is the case, or assume I'm just throwing money away by doing anything between the studs above the standard R19?"

            The standard R19 is really R18 when compressed to 5.5" in a 2x6 cavity (true fact) and that's only when PERFECTLY installed in a completely AIR TIGHT stud bay, including the drywall side, and all wiring & plumbing holes in the framing sealed. R19s have almost no air-retardency- the moisture & heat transfer hit from that ultra-low density fiberglass from any air leaks are magnified compared to denser fiber insulation (like blown cellulose or R21 fiberglass.) The weight per square foot of an R19 batt is the same as an R13 batt- it's simply a fluffed-up R13. At R19 (really R18) density that fluffed up R13 only an air filter, and easy nesting for rodents etc.

            By contrast, an R21 will perform at R21 in a 2x6 framing by, and is FAR more air-retardent.See:


            So while it may not make sense from a net-present-value-of-future-energy-savings point of view the energy accounting, there is additional value in the improved resilience and lower air infiltration when denser material (or a full cavity fill of o.c. foam) is used.

  6. Mwheeler602 | | #10

    I am a homeowner and not an expert but I'm all about the numbers. $50 a month over 30 years is $18000 over 15years is $9000. Lets say 3% interest so that another $9000 or $4500. So you're All in at $27k of 13k. Is it really worth it? I know that's not what this websites about or the answer you're looking for. I couldn't sit back and tell you there's another part of the equation you should consider.

    1. andy719 | | #11

      Mwheeler, I mentioned the monthly cost rather than the total because there is some hopeful expectation of reduced electricity bills. If the electric bills were reduced by $50 a month, then it would be a free option that is marketed as a comfort upgrade. Energy efficiency upgrades have a different cost model than upgrades that are strictly cosmetic.

    2. Expert Member
      Dana Dorsett | | #30

      >"I am a homeowner and not an expert but I'm all about the numbers."

      The value of insulation can't really be reduced to the net present value of future energy savings. Durability, indoor air quality, and the ability to protect the structural wood from moisture also come in to play. The R19 fiberglass batt is one of the crappiest products pretending to be insulation that is still on the market. The only place I personally would recommend an R19 is under a staple-up plated radiant floor over a conditioned crawlspace or basement, and then only if there is a reasonable air barrier keeping aerosolized fibers the fiberglass from entering the basement air.

      Then there is the performance/durability over time. R19s (and their low density 2x4 cousins, the R11) are more prone to rodent damage and degradation over time, with measurable degradation in as little as 15- 20 years. That just doesn't happen very often with R21s, and is all but unheard of with dense packed cellulose or open cell foam.

      If you're a short timer planning to leave before the decade is up that might not matter to you (in which case none of it matters- even the R19s aren't likely to "pay off" in energy savings over short time spans vs. leaving it uninsulated), so if the only "green" part of "green building" that matters to you is the greenback, go with the cheapest legal option and don't look back. (Don't pose the question on a green building forum either.) Code minimums are simply the crummiest house that is legal to build, and have nearly nothing to do with building better/greener/healthier house.

      But for '...all about the numbers..." guys, if going with a low density batt solution the most relevant number beyond cost to be "..all about..." is the cfm/50 final blower door test number. INSIST that it be tested by a third party, even if you pay extra for that. IRC code-max air leakage is 3 air exchanges per hour @ 50 pascals (which has to be calculated from the cfm/50), so if it doesn't meet that the builder needs to fix it and re-test on their dime. As a rule of thumb, in most houses getting the cfm/50 to less than 1 cfm per square foot of conditioned space is the threshold of where the heat and moisture within the house can be readily controlled, and the HVAC systems (if correctly sized & designed) will actually provide comfort to all rooms in the house. Almost any <3ACH/50 house would clear that low hurdle, but a shocking number of tract homes built in areas where the air leakage isn't enforce still fail, even though it's dead-EASY to hit with even modest efforts at air sealing directed and monitored by the builder during construction.

  7. Expert Member
    Akos | | #13

    R19 batts gives you ~R17 whole wall assembly R value (you loose around R1 for medium install, R2 for sloppy install).

    R21 batts bump you up to R18 but is harder to get a bad install, so you'll probably be around R17 to R18.

    In my area of 2F design temp, on 4000sqft house, so roughly 4000sqft outside walls, you loose in the winter time
    -17k BTU for R16 assembly
    -15k BTU for R18 assembly

    My guess is that it probably won't save you the extra money the insulation costs in energy savings.

    I would still go for better insulation though, the cost delta in terms of house cost is noise and you do get a slightly better wall.

  8. andy719 | | #14

    Bringing up my old thread to ask one final question on wall insulation value. We are still waiting for permits, so I have had time to dwell on questions. I have asked builder to price dense pack cellulose to settle my curiosity. I'm wondering if you all think it is worth putting in over our current batt selection and at what pricepoint?

    Tract home base price is for R19 batt filled walls (they do rescheck whole house value rather than prescriptive and apparently this gets approved.) House is in Maryland (4A) with requirement for 3rd party blower door and 3ach50 max. Walls are 2x6 studs with vinyl siding, no exterior rigid insulation (not an option builder would consider).

    Upgrade to R21 was roughly $0.40 / sf (currently selected option)
    Flash and batt was offered at roughly $2.25 / sf and turned down based on what I've learned here.

    What should I expect for a fair price increase for dense packing walls, also assuming a little more effort has to go into drywall work with this? Is dense pack definitely preferable to R21 batts in a new 2018 IECC house that meets air leakage requirements? If so, at what price point would you say it's too expensive, just stick with the r21 batts?

    Thanks again everyone.

    1. Expert Member
      Dana Dorsett | | #28

      Do they even offer a code-min R20 option? R20 batts are being manufactured in much higher volume than the somewhat denser R21s, and by all rights should be at most a small upcharge from R19s, and unlike R19s performs at it's rated R in a 2x6 cavity.

      The installation labor is the same, and the material cost isn't a huge up-tick from R19s, yet it's a better product overall- more air-retardent than an R19, about as dense as a standard R13 (but like R21s are 5.5" thick at it's tested loft compared to 3.5" for R13s, or 6.25" for R19s- ergo the performance loss from compression.)

      Dense packing is labor intensive and likely to cost more if the builder doesn't have a regular contractor doing cellulose, but R20 cellulose batts for 2x6 framing also exist, and 90% more air retardent than an R19 stuffed into a 2x6 bay. I suspect the contractor would charge about as much as the R21 f.g., but it has several worthwhile properties that fiberglass does not have. Cellulose has substantial hygric buffering capacity, and will safely wick & redistribute minor bulk water incursions without damage or loss of function, and shares the seasonal moisture burdens with the structural wood, lowering the average moisture content of the wood. The borate fire retardents are toxic to the gut flora of wood boring insects needed to digest wood, and thus protects the wood from carpenter ants, wasp & bees, termites etc. It's also sequestered carbon, with a negative carbon footprint. R20 denim/cotton batts have similar qualities, but are less green (the agricultural practices for producing cotton are pretty rough), and usually more expensive than cellulose batts.

      Installing cellulose or denim batts is more or less the same labor content of installing R19s, and unlike R19s the risk of airborne shards of insulation fiber affecting indoor air quality in the event of air leaks are nil.

      1. andy719 | | #31

        I wish I realized that was something that even existed when I started asking about insulation changes. The builder has their cookie cutter package. They are open to almost change, but I have to bring it up. They then just pass along the request to their insulation sub for a quote. There is no option menu on insulation though. At this late stage though, I think they might start refusing me if I ask about another type of product.

  9. charlie_sullivan | | #15

    Well installed cellulose is better than poorly installed R21 batts, but well installed R21 batts would be better than poorly installed cellulose. If it's hard to find someone to do cellulose in your area, it might be even harder to find someone who can do it well. I'm in Northern New England, supposedly the region where dense pack cellulose is well known, but even here the quality of the work depends a lot on who you hire.

    I would ask about having a "Grade I" installation, with an inspection by you or a third party before drywall.

    1. andy719 | | #16

      Thank you, but I guess I'm asking for an opinion on competent R21 batts vs. competent dense pack. I'm in the DC metro area, so there are plenty of installers, but the builder only uses one. It is a large local company that installs fiberglass, cellulose, or spray foam. They are already putting loose fill cellulose in the attic as their standard insulation method.

      The builder's specs just have the blanket statement that they install to manufacturers instructions. I checked the JM website, and their instructions didn't say anything about grade of install. So, I checked the 2018 IECC, and it says this, "In exterior walls, batt insulation shall be cut neatly to fit around wiring and plumbing, or insulation, that on installation readily conforms to available space, shall extend behind piping and wiring." I know I get my own close in inspection, and the county requires a specific insulation inspection.

      1. Expert Member
        Dana Dorsett | | #17

        As a rule blown insulation has a better fit than batts, and dense packed offers some forgiveness on less than perfect air sealing. But if the AIR SEALING people are also competent it doesn't much matter.

        In zone 4 spending the budget money on competent air sealing plus rainscreened siding is worth more than competent dense pack vs. competent R21 batts.

        When all else is equal, dense packed cellulose (or cellulose batts) are greener than fiberglass from a carbon footprint point of view, and R-for-R the favorable thermal diffusivity of cellulose (a thermal mass effect) offers a somewhat subtle improvement in overall comfort & efficiency, though not enough to affect code prescriptives.

        1. andy719 | | #18

          Thank you Dana, I was hoping you or Akos would chime in again. The air sealing company is the same as the insulator. I'm hoping they do a decent job all around. There will be OSB with a housewrap WRB under the siding. Their details don't appear to show any additional vertical furring behind the siding.

          The insulator just sent in pricing. They offered up a combo of dense packing cellulose in the major cavities, and spraying open cell foam in the rim/band joists (full 5.5" fill). That pricing was an even $1 per sf above the R21 fiberglass batt cost, so I would have to cut a check for another $4k for that.

          It sounds like your advice would be to save that money now, and use it for extra solar panels later on (or something else anyway). Is that correct?

          1. andy719 | | #19

            Does anyone (outside of the cellulose industry) think this added cost makes any economical sense?

          2. Expert Member
            Akos | | #20

            I think the spray foam at the rim boards is a good idea. This is a big leak in most houses. Never mind the energy savings, sealing this joint up makes a huge difference in comfort, your basement, thus the floor on your main floor, will be much warmer.

            Not sure if the dense pack is worth it unless you are in a noisy area. In lab test, dense pack VS batt walls are about the same STC, but in the real world with leaky assemblies, dense packed houses tend to be way quieter.

  10. andy719 | | #21

    Akos, can I get the same effect then by doing a rigid foam cut and cobble later on, for the basement rim? I wouldn't have the foam on the 1st/2nd floor rim or attached garage rim, but it would save money and make it easier to attach a deck and possibly add an ERV or ventilating dehumidifier if needed later on, without having to remove any open cell spray. Or do you think that wouldn't have nearly the same effect as the open cell sprayed on two levels of rims?

    I am definitely going to be in a quiet area, so no noise concerns.

    1. Expert Member
      BILL WICHERS | | #22

      Spray foam is great in the rim joist area because it seals all of the many little gaps that are there. You can cut and cobbly and do a good job that way too, but it's pretty tedious. I did cut and cobble on my own home's rim joist, with 2" blocks of EPS in each bay -- and almost every bay needed a slightly different size block. Another pass is then needed to caulk the top plate to the top of the foundation wall. Spray foam can do all of that in one shot.

      I would go with the spray foam in the rim joist option. You will save yourself a whole lot of work this way compared to doing a cut and cobble project later. You're also likely to get a better overall result, and it will be there right from day one. This is a plus.

      Note that building a quiet home doesn't just mean keeping noise from outdoors from getting in, a quiet home also provides sound isolation between rooms. It's really nice to be able to watch a movie at night in one room while people are sleeping in another room and not worry about disturbing anyone. I would, at a minimum, use 5/8" drywall throughout the home for this (residential standard is 1/2"), and it's a pretty cheap upgrade. Putting some insulation in interior walls, at least in critical spots, will also help with sound. After "thicker drywall and some batts", all of the next steps up in sound control start getting expensive and significantly altering the way the walls get built.


      1. andy719 | | #25

        Interesting notes, thanks. They have quoted me basically the same price to either flash and batt just the rims and garage ceiling under bonus room, or to do open cell in rims and upgrade R21 batts to dense pack with no garage upgrade (standard r30 batts). I have advice from two people on here to do the spray foam on the rims and garage ceiling. I'm just nervous about closed cell foam after seeing all the propaganda about smells and pulling away from framing.

        1. Expert Member
          BILL WICHERS | | #29

          Remember that you hear stories from people who have had bad expieriences far more often than you hear stories about people with good expieriences. As I recently said on one of these Q+A threads, "no one ever comes back complaining that their spray foam install went perfectly and now their heating bills are too low".

          I've done closed cell spray foam in my own home, in an unvented cathedral ceiling. I've done many designs with it commercially in industrial facilities. I've never had a problem, but I've always used expierienced contractors. I have one contractor I personally like to use because he is very fussy, and takes a lot of personal pride in getting as flat a finished surface as possible with the spray foam application. Doing a good install is his art, and that's the kind of installer you want. If you have a good installer, it's very unlikely you'll have any problems. Just don't ask them to do the install on a cold (under about 50F) day, and make sure the areas being sprayed aren't wet and you should be in good shape.


  11. AntonioB | | #23

    Hi Andy, I've been following this discussion and just have a question. Pure curiosity on my part. I hope you don't mind. In your part of the country what is needed in the way of a vapor barrier/retarder in your wall design? How are you handling that issue?
    Working in the Pacific Northwest I've used flash & batt with closed cell foam acting both as vapor retarder and air barrier. In our mixed climate it works well because it puts the vapor control near the center of the wall section, allowing the inner half of the wall to try inward and the outer half to dry outward, while keeping the barrier at a point where the temperature never reaches the dew point.

    1. andy719 | | #24

      Class III, so just latex paint.

  12. andy719 | | #27

    Thank you Dana. Yes, it was advice from you and Akos that made me request the upgrade to R21 a few months back. We have just been waiting for months for civil and permits so I've had time to overthink. I had settled on the R21, but now I was just asking if dense pack is worth triple the upgrade price as R21? Sounds like no.

    I am interested if you also agree spray foam is worth it for the rim joists and garage ceiling only, or if you think code compliant air sealing is good enough for those too. I don't know the exact details of the air sealing. I asked what part they skip if they spray foam anywhere. They said the tape and caulk is the same, they just omit picture framing cavities with canned foam wherever they are going to spray.

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