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New construction insulation options

Justin_ | Posted in Energy Efficiency and Durability on

We’re having a new home built in southern NH, roughly 2100 sf. with southern exposure to the rear of the home. The builder is including ZIP System R-Sheathing R6.6 with 6″ R21 fiber glass batts in the standard price. I’ve been quoted about $5k to add 2″ of closed cell foam in the wall cavities on the back side of the sheathing (keeping 4″ of fiber glass in the remaining space). They are also putting R49 in the ceilings, R30 in the slopes (our plan has a few slopes on the 2nd story). They build to better than energy star standards, taping seams, caulking etc. I believe they are using a blown-in insulation product in the attics, not sure what type. Our plan has a two car garage under, so the usable space in the unfinished basement is only about 500-600 sf, I would like to eventually finish this space for added living area. The standard windows are vinyl low-e dual pane. We’ve not made any final decisions yet, I have a couple of questions for experts first.

Are there any moisture concerns given this hybrid-wall construction? We’ll have roughly 3″ of foam for a very air tight wall. With adding fiber glass on top of the closed cell foam, doesn’t that have the potential to trap moisture in the wall cavity between the foam and fiber glass? Will they need to maintain a more traditional vapor barrier under the drywall?

I’ve read that insulating under the basement slab can save a great deal of energy. While I’d like to have this done I’m not sure its worth the added cost. Is it worth having the entire slab insulated, even under the garage area? My thought is when I do finish the basement I’d frame walls and use a spray foam insulation against the foundation walls. If the basement slab was already insulated under, I think we’d be in pretty good shape. The garage area isn’t heated, but I have to imagine it wouldn’t hurt insulating under. In that same train of thought, any guesses as to how much ICFs in the basement would add to a project like this?

I am also looking into adding an HRV, cutting into the existing supply/return of the gas furnace, most likely the HRV would not have dedicated ducting unless someone can convince me otherwise. I’d also like to add solar later when budget allows, undecided about PV or hot water. These both would be projects I’d likely take on my self.

Any help or input is appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    There is but a MINISCULE performance uptick when replacing high-density fiberglass cavity fill with high-R high-dollar closed cell foam. At a 20% framing fraction (typical for 24" o.c. framing) the performance upgrade of replacing 2" of R3.8/inch fiberglass with R6.5/inch foam is a modest R1.6 of increase, after the thermal bridging of the framing is factored in. (It moves from R21.6 whole-wall to R23.2 whole-wall.) That's a very expensive R1.6!

    Applying the money to a single inch of EXTERIOR foam would do far more than the 2" of foam between the studs, since it would add R6.5 to the whole-wall performance.

    In the US climate zone 5 section of southern NH the R6.6 sheathing doesn't quite meet the R7.5 minimum required by code to be able to skip the interior side vapor barrier. (See: ) As little as 3/8" XPS siding underlayment would bring it within code, but an inch of exterior polyiso (R6) or an inch closed cell spray would give it huge margin on dew-point control, and leaving the interior side comparatively vapor-open using only standard latex paint as the vapor retarder would make for far better moisture-resilience. The proposed 2" of spray foam would get you there too, but only at a very substantial expense. Most closed cell foam would run ~0.6 perms @ 2", which has at least some drying capacity (unlike 0.05 perm polyethylene), so it'll never be a moisture trap, but it's not nearly as good as keeping the inside face of the sheathing above the dew point of the interior air and leaving the interior side at 2-3 perms (latex paint.)

    Insulating the slab isn't a huge energy savings, but some. As importantly, at NH deep subsoil temps insulating under the slab improves moisture performance, particularly summertime mold-hazard issues. Assuming natural gas heating fuel there's a long term financial rationale for R8 (2" of EPS) on fuel savings along, but if oil or resistance electricity R12 can still be cost effective.

    If the garage exterior walls are not insulated, isolate the garage slab from the rest of the slab with a couple inches of EPS and don't insulate under the garage slab. If the garage walls & door are insulated, insulate under the garage slab.

    Building the foundation walls with even the most minimalist of insulating concrete forms can sometimes be less expensive as insulating the walls to R15 with spray foam later. It's also a lot greener: Closed cell foam is almost universally blown with HFC245fa, with a global warming potential (GWP) of greater than 1000x CO2, whereas the EPS used in ICFs is blown with pentane, at ~7x CO2. (Polyiso is also blown with pentane, but XPS, whether pink, blue, or green is blown with stuff as bad or worse than HFC245fa). When insulated with ICFs, the foam runs down to the footing, and the slab-foam meets the wall-foam, floating the slab for a more-perfect thermal-break. (Even without ICFs, if you do nothing else, put 2-3" of foam between the slab edge and the foundation wall when pouring the slab as a thermal break.)

    For a sanity check as to where the financial break-even points see table 2, p10 of this document:

    You are either on the cool side of zone 5 or the warm side of zone 6, and the values given are whole-assembly R, with all thermal bridging accounted for. You'll note the recommended R30 starting point for walls is about R7 ahead of your the 2" foam between the studs approach, but if you added 1.5" of exterior polyiso over the ZIP-R sheathing you'd be there.

  2. DickRussell | | #2

    Well, I'll touch on a couple of things, to get the discussion going. Others ought to chime in. First, I think for a new house, it's a matter of what philosophy you want to follow, energy-wise. In our climate zone (I'm also in NH), I always advocate going the general "superinsulated" route, because with new construction it doesn't cost more than a nominal 5% extra but pays off very well in terms of energy savings and especially comfort.. Amortized over the life of the mortgage, and offset by annual energy savings, the monthly cost for PIT & energy might be very little more and quite possibly less than for little more than code-minimum construction.

    Something in the superinsulated realm can can be achieved with either conventional double-wall construction or somewhat more exterior insulation than what the Zip R-sheathing 6.6 provides. You can search here for a blog on how much exterior insulation is required for out climate zone; I don't think 6.6 cuts it, although it does reduce the thermal bridging of the framing considerably.

    As to sub-slab insulation, definitely put it in for the part under living space, which you may later finish off. Up here, a considerable amount of heat is lost through an uninsulated slab. If you finish it off later, that slab will be far more comfortable at room temperature than it would be at not far above ground temperature. Make sure the edges of the slab are thermally isolated from the foundation walls. As far as the garage goes, if it isn't heated then you could leave that slab unheated and actually pick up a little ground heat in winter to moderate the temperature there considerably. Be sure to insulate any garage walls also. It's cheap to do while the walls are open. Make sure you have a very tight air barrier all around the living space, with particular attention to the boundary between garage and living space. You also may want to add a good layer of polyiso foam under the framing of the garage ceiling before it's sheetrocked, and insulate the ceiling cavity. Otherwise you'll have an uncomfortably cold floor above the garage.

    As far as the windows go, look into decent triple pane glazing. The inner glass surface will be notably warmer in winter than conventional double pane. Consider too using casement type windows rather than double hung; they admit far less air leakage. Not all windows have to be operable; those can be fixed glass units.

    You certainly ought to insist on very tight construction, with verification by blower door testing. Having a tight house will save energy and keep the interior air from being uncomfortably dry in winter. But then you do want mechanical ventilation (HRV), for controllable ventilation at a proper rate. Don't count on "natural" leakage to provide fresh air for the family; that can't be built in properly, no matter what any builder might tell you. Anyway, you don't want your walls to be an uncleanable air filter.

    But, as I said, what you have built is a matter of what philosophy you follow. You can insist on something at the "pretty good house" (PGH) level, or the barely modified code-minimum that the builder feels he can slap together for you. What do you really want?

    Edited: Ah, while I typed, Dana posted!

  3. Justin_ | | #3

    I'm really looking for the most cost effective efficiency. I was leaning towards the additional spray foam not so much for R value but for a tighter wall overall. Is anyone familiar with the exterior sheathing product "ZIP System R-Sheathing"? It would add 1" of foam to the exterior of the studs and provide a thermal break. Would the addition of 2" of foam need to be on top of the sheathing or between it and the studs? I imagine it's a bit of a wash in terms of cost to go this route, the materials might be cheaper but the construction complexity of window and door openings would increase, not to mention the builder might want to charge a premium if they've never built using an additional layer of exterior foam, but I can ask.

  4. Expert Member
    Dana Dorsett | | #4

    That happens to me a lot,when ticking away at a longer post, Dick- welcome to the club! :-)

    But hell-yeah, what Dick said- retrofitting to high-R is far more expensive than building to high-R, and it's more than just about the net-present-value of the presumed future energy savings- it's more comfortable! An R21+6.6 stick framed (R21 whole-wall) is a decent uptick over the R20 or R13+5 code min, but nothing quite like the performance of R30-R40 whole-wall assemblies that aren't too difficult or expensive to build using exterior foam. Closed cell foam in wall cavities is largely wasted, very low bang/buck, whereas exterior foam thermally breaking the framing delivers the real deal: True high-R performance, not merely some fanciful center-cavity R value.

  5. Expert Member
    Dana Dorsett | | #5

    Justin: Standard ZIP sheathing and 2-4" of exterior polyiso is probably more cost effective than ZIP-R sheathing + spray foam. ZIP-R isn't really for high-performance builders, it's merely an easy way for the average homebuilder to build an R13+5 without the additional labor costs of adding exterior foam. As I noted earlier, it's insufficient exterior R from a dew point control point of view for 2x6 construction in your climate zone. So with 2x6 you're not saving much, if anything using ZIP-R, since you'd need at least a thin layer of some other R on the exterior, or a flash-shot of closed cell on the interior of the sheathing to meet code and avoid wintertime moisture issues.

    Insulation only adds its full rated R value performance when it is exterior to the framing, since the framing otherwise robs it blind. Any expensive high-R foam between the studs is wasted! Save the high-R foam budget for the exterior of the sheathing, under the siding. Open cell foam is as-tight or tighter than closed cell foam, and may be cost-competitive with the high-density R21s. A full 5.5" of open cell runs trimmed flush to the stud edges runs about the same raw cost as 2" of closed cell foam the contractor quoted, and you wouldn't have the cost installing the thinner batts.

    Air sealing is more than just the cavity fill- the seams between doubled up top plates, between the bottom plate & subfloor, subfloor & band joist etc are all significant. Sealing the sheathing seams is just the beginning. Air sealing is about the cheapest performance upgrade you can buy, but it helps if the crews are aware and trained on the subject.

    Martin Holladay has included a bunch of detail-links & tips on how to build with exterior foam on this blog post:

    It's not unreasonable to build the foundation with an R16-R20 ICF, and align the exterior foam of the wall above co-planar with or slightly proud of the exterior EPS of the ICF which preserves the continuity of the thermal break. A bottom-of-class R16 ICF has 2" of exterior foam, and putting the sill plate 0.5-1" in from edge of the concrete would allow you to put 2" -2.5" of polyiso (R13-R16) on the exterior. With a 2x6 24" o.c. studwall w R20 cellulose or open cell foam you'd then be in the R28-R31 range for the above grade walls with a fairly easy to seal & build structure.

  6. Justin_ | | #6

    Dana, I'm a bit confused by your statement "it's insufficient exterior R from a dew point control point of view for 2x6 construction in your climate zone. So with 2x6 you're not saving much, if anything using ZIP-R, since you'd need at least a thin layer of some other R on the exterior, or a flash-shot of closed cell on the interior of the sheathing to meet code and avoid wintertime moisture issues." - What do you mean by flash-shot, and how does that differ from the ZIP-R + 2" of closed cell as I described? It sounds like ZIP-R + 2" closed cell means I won't have moisture problems, but it just isn't the best bang for my buck, correct?

    So as for insulation choices, I'd like to narrow down a few options...

    #1. ZIP R, 6" of fiber glass - included in the standard home price
    #2. ZIP R, 2" closed cell + 4" fiber glass - $5k upgrade, not the best "bang for buck", middle of the road performance? no moisture problems?
    #3. ZIP R, 6" open cell - might be most cost effective, better performance than option 2?
    #4. Standard sheathing + 2" exterior rigid + 6" fiber glass - highest labor cost, best performance, no moisture issues, best air sealing?

    Thanks again everyone,

  7. Expert Member
    Dana Dorsett | | #7

    A flash-shot of closed cell is ~1" nominal or less, which is half the amount of foam. Even 1/2" of closed cell foam has a permeance of about 2 and an R value of about R3, and inch would be about R6 /1 perm, and it still presents a non-wicking condensing surface. That's enough additional non-air-permeable total R on the exterior side of the assembly that the condensing surface would average above the wintertime dew point temperature of the interior air, and condensing events would be brief and non-damaging even without an interior vapor retarder. At 1-2 perms between the sheathing and interior the sheathing can still dry toward the interior for 9-10 months out of the interior.

    With 2" of foam you'd be at about 0.5 perms, which is a pretty stiff vapor retarder, comparable to an asphalted kraft facer. It would still be enough to allow seasonal drying toward the interior, but at 1/4-1/2 the rate of a flash-foam job.

    And, at a buck a board foot you'd be saving ~$1 or more per square foot by going with the minimal flash-foam, and you'd have a more resilient assembly to boot.

    Option #3 would not have sufficient exterior R to avoid accumulating wintertime moisture in the exterior side of the open cell foam unless you added a "smart" vapor retarder on the interior side of the assembly. It's thermal performance would be comparable to #2.

    #4 would be a significantly better thermal performance point, has the highest amount of dew point margin too, but is not inherently air-tight (none of them are). You still have to detail both the sheathing/framing and rigid foam as air-barriers. If you use fiberglass, either highes-density "Cathedral Ceiling" batts or new-school blowing wools (Spider, Optima, L77, etc.) at 1.8lbs minimum density are preferred. (If batts, rock wool is probably a better choice, and are priced comparably to high-denisity fiberglass. It's far more fireproof, than fiberglass, and has fewer air-quality issues.)

  8. Justin_ | | #8

    ok, so option #2 would be slower to dry than a flash shot, #3 requires a separate vapor barrier and should be cheaper? I've asked them to price out 2 & 4 inch rigid options for me, I'll inquire about rock wool also.

  9. Expert Member
    Dana Dorsett | | #9

    If you take the open cell foam option, be sure to specify a SMART vapor retarder like MemBrain or Intello not poly sheeting. Poly is dirt cheap, but super vapor-tight, and creates as many moisture problems as it solves. The cost adder of smart retarders adds up:

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