Central NH Energy Retrofit

Discussion in 'Remodel Forum & Blog' started by bostondiyer, Jun 17, 2020.

  1. bostondiyer

    bostondiyer New Member

    Joined:
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    Location:
    Westwood, MA
    Hi all,

    I'm buying a home in central NH (Climate Zone 6) that was built in 1975. There are issues with water infiltration through the siding that I will need to address shortly after closing on the purchase - most likely, the siding will need to be replaced all around the house, and it's possible I will need to replace some rotted structural members and insulation as well. I also need to replace all of the windows anyway, so I want to do all of this at once.

    The interior is in good shape for the most part, so I don't expect to be ripping out much, unless I find mold. There is a finished basement (it's a walk-out basement) with carpet, so I plan on ripping out the carpet and insulating the floor with XPS and glued-and-screwed OSB.

    The makeup of the wall assembly is currently unknown. Once I open up the siding, what am I likely to find for a wall assembly underneath given its age and location? Is it worth planning on replacing the insulation with something better, even if it's not damaged? And what are the considerations for a wall assembly in this zone?

    Side note - heating is currently electric, but I plan on installing a 3-zone heat pump for heating and cooling.

    Thanks
     
  2. bostondiyer

    bostondiyer New Member

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    May 12, 2014
    Location:
    Westwood, MA
    From what I gather, NH had no statewide building code until 2002. The town where I'm buying the house definitely didn't have one. So I won't know what the wall assembly is like until I open it up. And it's probably worth it to upgrade what's there.

    Any advice on a retrofit wall assembly in this climate zone?
     
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  4. Dana

    Dana In the trades

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    First, scrap the XPS plan. XPS is blown with a mixture of extreme greenhouse gases (the predominant component is HFC134a, with a 100 year global warming potential of ~1400x CO2), making it by far the least green insulation material in common use today. Those HFCs are banned for this purpose under the Kigali amendment to the Montreal protocol (which the US hasn't yet signed onto, and ALL XPS in the US is currently using the HFC blowing agents, primarily for the marketing benefit they get out of the temporary R5/inch long term (really medium term) performance benefit compared to it's fully -depleted R4.2/inch performance as the HFC bleed out. XPS is only warranteed to R4.5 inch @ 20 years- I'd expect it to be in service for much longer than that.

    The CO2e footprint of XPS is almost 10x that of EPS of similar density. It's the same polymer, but uses low impact hydrocarbon blowing agents (isopentane, ~7x CO2 @ 100 years), most of which leaves the foam while at the factory where it recaptured, not vented to the atmosphere:

    [​IMG]

    So use EPS instead. It's financially rational on a lifecycle basis to put as much as 2" of EPS under the subfloor in a basement over-slab finish floor.


    To figure out what do when residing, what is the current framing size & R-values of this house? (2x4/R11? 2x6/R19?) Was this built prior to the era of polyethylene vapor barriers on the interior side? (That's usually easy to figure out by inspecting at electrical boxes or plumbing penetrations.)

    Are the windows low-E? You'll need to figure out the window flashing details too.
     
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  5. bostondiyer

    bostondiyer New Member

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    Location:
    Westwood, MA
    Based on pictures I took, I'm 95% sure they're 2x4 walls. I have some reason to believe they are foil-faced batts, but have no idea what specific R value. Probably whatever was typical around 1975. I don't believe there is any vapor barrier other than the foil face.

    I'm planning on replacing all of the windows at the same time as the siding, so I'm completely open to recommendations as to what type to buy.

    As far as siding, I'm considering replacing what's there with ZIP System R-sheathing plus HardiePanels - does that make sense?
     
  6. Dana

    Dana In the trades

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    In 1975 most mid-range or custom houses were getting R13s. To have sufficient dew point control at the sheathing layer for an R13 wall in an IECC zone 6 climate (most of NH) takes a minimum of R7.5 on the exterior. To bring it up to the current IRC 2018 code minimum with R13 in the cavities takes a minimum of R10 on the exterior. See TABLE R1101.1.2

    The foil facers on the batts are potentially a problem, since with low-permenance foam on the exterior it creates a moisture trap. Any bulk water that finds it's way in there from flashing defects, etc. can't leave very quickly, sometimes taking years to get out. Can you verify that they are indeed foil faced, not kraft?

    ZIP-R is a very expensive way to improve the R-value of the house. ZIP-R with sufficient R-value to bring it fully up to IRC code isn't sufficiently structural to work without keeping the original sheathing in place. Most homes in 1975 were sheathed in plywood, and even with the failing siding the plywood may not have to be replaced.

    In most of NH a double- low-E double pane glass such as Cardinal's loE 180 +i89 (used by several vendors) offers nearly the performance of a triple-pane if filled with argon (not an expensive upgrade from air), yet a high solar heat gain coefficient, at a price point well under even low-end triple panes. Other glass manufacturers have similar double low-E double panes. Look for double-pane windows with a U-factor in the U0.20 - U0.22 range, with an SHGC north of 0.45 for the south facing windows. Large unshaded west facing windows would want an SHGC under 0.3, and there are double low-E windows in the low U0.2s that can deliver that too. The second low-E coating is a hard coat low-E (usually indium tin-oxide) on the indoor surface of the glass- the surface in contact with the room air. While it's possible to scratch and mar that coating, it's pretty tough unless you're cleaning the windows with Brillo pads or something.

    If mounting new windows in a wall with significant exterior foam it's generally best to build out a window buck and mount the window "outie", with the glass roughly co-planar with the siding or foam than co-planar with the structural sheathing, putting the housewrap on the exterior side of the foam, with the window flashing directing bulk water out to the housewrap, properly lapped. If it's a replacement window going in the old window framing, putting the housewrap between the foam and structural sheathing, using a crinkle type housewrap (eg Tyvek Drainwrap) to avoid trapping moisture directed to that plane by the window flashing.
     
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  7. bostondiyer

    bostondiyer New Member

    Joined:
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    Location:
    Westwood, MA
    Hi Dana,

    Thanks for the advice - the window info is very useful.

    Now that I've closed on the house I was able to take a closer look as well as take some measurements. It appears the house has 2x6 walls with an vapor barrier against the interior finish (it looks like clear poly). I thought it was foil at first glance because of the way it reflected light, but it appears I was wrong. So it appears to have 2x6 walls with unfaced batts and clear poly on the inside of the house. I can open up a few walls to confirm this with certainty if it's needed.

    I've discovered a major issue with the plan to add foam to the exterior - the house is a contemporary style gable roof structure that doesn't really have eaves or gutters. See below for more detail. It's hard to see how I could add 2+ inches of foam to that.

    The first floor is cantilevered over the basement on the sides where the gutters would be, which is I think how the architect chose to manage bulk water (and probably to reduce ice dam risk) in lieu of eaves and gutters. But given the evidence of water intrusion behind the siding, I'm at something of a loss of how to deal with this without completely rebuilding the roof and adding eaves. But I think it has to be addressed before I start spending money on windows and other items. Do you have any recommendations?

    [​IMG]
    upload_2020-7-26_22-47-36.png
     
    Last edited: Jul 26, 2020
  8. Dana

    Dana In the trades

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    There are various ways of extending the roof a few inches, most easier to execute if re-roofing as part of the same project. Getting a foot of overhang at both the eaves and at the rake of the roof does quite a bit for protecting against bulk-water incursion, but might violate the architectural aesthetic of some contemporary houses, or even some more traditional Cape Cod type houses. There are commercial 4"drip edge products that may work for dealing the eaves, but you have to think more about it on the rake, and what's done on the rake may modify how you approach the eaves.

    In a "chainsaw retrofit" deep energy retrofit it's common to through-screw faux rafter extensions for more projection at the eaves, but that's still usually done during a complete re-roofing. It's common enough in NH to see sheet metal roofing along just the eaves of houses (a common band-ait approach for shedding ice-dams), that could be worked into the existing roofing lay-up without a complete roofing job.

    Is it a vented attic, with soffit vents under those very slim eaves combined with a ridge vent?

    How much insulation is in the attic?

    BTW: I'm going off the grid for a few days, but I'll check up on this forum by the weekend.
     
  9. bostondiyer

    bostondiyer New Member

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    Location:
    Westwood, MA
    There's enough room for a small attic/crawlspace, but there's definitely no access from within the house - not even a scuttle. I ordered an extension ladder, so I'll be able to check for soffit vents in a few weeks. I can confirm there is a ridge vent, though.

    Below is a picture of the side of the house with a rough sketch of where the 2nd floor is. There's a cathedral ceiling for the other ~30% of the house. I included a picture of the cathedral ceiling with the skylight in the hopes it can give some indication of the amount of insulation.

    [​IMG]

    [​IMG]


    Ultimately I'm all about function over form, so I don't care if eaves mess with the look of the house, but I do have concerns about cost effectiveness. Right now the roof is only 5-10 years old. I can't help but think it might make the most sense to add some gutters and call it a day for now, and some years down the line when it's time to re-roof I can address adding eaves and further insulating the exterior. But I'm open to suggestions as to other approaches if there's a cost-effective way to address it now.
     
    Last edited: Jul 27, 2020
  10. Dana

    Dana In the trades

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    With a <10 year old roof it's not worth ripping it up just to add insulation (unless there's literally no insulation in the attic or cathedralized ceiling.) It's worth assessing the consistency and quality of what's there using IR imaging (or even a cheap IR thermometer, though that's a lot slower.) When the sun is beating on the roof the ceiling temperatures should be fairly even along parallel lines equidistant from the ridge, except at rafters where it could easily be 2-3 degrees warmer. (In winter the rafter lines will be colder than between the rafters.)

    It should be possible to slip 4" commercial drip edge under the roofing felt at the edges of the roof to cover over a couple inches of wall foam. It comes in both galvanized (which can look pretty ugly when exposed, or a few other power-coated paint colors (white is pretty popular and innocuous.)

    [​IMG]
    [​IMG]
     
  11. bostondiyer

    bostondiyer New Member

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    Location:
    Westwood, MA
    I've been considering all of this and have learned a lot more about my house after opening up some walls. As it turns out, the walls are 2x4 and not 2x6, and have a combination of poly vapor barriers and foil faced insulation, due to a variety of interior wall finishings (wood and sheetrock).

    I have also discovered the house has a significant rodent problem.

    Since the windows and siding needs to be replaced anyway, there is likely some moisture damage to some of the sheathing, and adding substantial eaves to the roof may not be feasible until the roof needs to be replaced, here's what I'm considering:
    • Address each wall one at a time from the exterior
    • Remove sheathing, windows, and existing fiberglass insulation. Replace fiberglass with rigid foam insulation between the studs, with a 1/2" gap around the edge of the foam sealed with Great Stuff to ensure a complete air seal that also provides a less habitable in-wall environment for rodents
    • Replace windows with high-efficiency new-construction windows
    • Replace siding with HardiePanel fiber cement siding
    This plan would allow me to do the work myself without disturbing the interior. I would consider spray foam, if not for the fact that spray foam contractors are not going to be interested in coming out for one wall at a time without charging an exorbitant premium. I would also emphasize the importance of the foam in minimizing/reducing access to the home - there appears to have been an extensive infestation within the walls in the past, including moving between floors. I would consider only using in-wall foam on the first floor for this purpose, and using rock wool on the upper floors, if that would make more sense.

    Does this sound like a good plan? And what would you think about also using a small layer (1/2" to 1") of continuous foam on the exterior, possibly in place of the sheathing?
     
  12. Dana

    Dana In the trades

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    Bad idea. Even the cheapest virgin stock rigid insulation costs 2x what fiber insulation costs per R, and cutn'n'cobbled sealed in place with can-foam a lot more labor to install. Installing expensive high R/inch foam between thermally bridging studs wastes the vast majority of it's potential R-value benefit. Do the math, (or read the math.) In a 2x4 assembly it adds at best about 10% improvement, whereas installing even ONE inch of continuous foam on the exterior of a fiber insulated studwall increases the thermal performance by about 50%.

    An IRC code minimum 2x4 wall in climate zone 6 would be R13 cavity fill + R10 continuous insulation, or the performance equivalent thereof. But even if you got to R100 in the cavity, the thermal bridging of the framing would keep it from hitting that performance point without at least some continuous insulation on the exterior.

    It's fine to work from the exterior, which makes it easy to air seal the wallboard to the framing. Install a bead of caulk between any doubled up framing (headers, top plates, jack studs, etc) with a bead of polyurethane caulk. Carefully fit high density R15s (fiberglass is fine, rock wool is better) leaving no compressions or voids, and put a bead of caulk between the new sheathing & framing for a tight air seal on the exterior, reinforced by purpose made tapes at the seams. Even all of that is less work and less risky than a cut'n'cobbled cavity foam job.

    Using foam board in lieu of sheathing requires that someone engineer a bracing solution for racking strength, to ensure it doesn't blow down in a Nor'easter with a snow load on top. Foam board is less rodent-proof than OSB or plywood too.

    To keep the average temp at the sheathing (or interior side of the foamboard) above the average wintertime dewpoint takes about R7.5 foam for R13 cavity fill, about R8.5 for R15. That would be 2" of EPS, or 1.5" of polyiso. At those thicknesses the foam board would come close to meeting (o exceeding, if foil faced) Canadian code definition of "vapour barrier". If there is a polyethylene vapor barrier on the interior of your assembly the most that would be reasonably safe to install would be 1" of UNFACED EPS, which has roughly the same vapor retardency of interior latex paint. That is enough to allow the wall assembly to dry toward the exterior as long as there is at least 1/4" of air gap (aka "rainscreen") between the siding & foam for both the siding and the rest of the wall assembly to dry into. An air gap is an excellent capillary break to keep rain/dew moisture from wicking into the assembly, and lets gravity take care of any bulk water incursions from wind driven rain or window-flashing, etc. The gap can be established with either furring at every stud , which is the best way to hold the foam in place anyway. I believe there is a fastener schedule for mounting Hardie to furring over foam somewhere on their website.
     
  13. bostondiyer

    bostondiyer New Member

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    Westwood, MA
    Got it. All of that makes sense.

    Is there any problem with having differential levels of insulation on different walls of the house? There are areas below 4' cantilevers that would be easy to better insulate from the outside, which could accommodate an unlimited amount of continuous exterior foam insulation. Only thing I'm not sure about is how to handle the vapor barrier.
     
  14. Dana

    Dana In the trades

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    It's fine to have different walls at different thermal performance levels. (In reality the performance levels are never perfectly uniform.)

    For the walls under the cantilever it will be fine if you meet/exceed the minimums prescribed in TABLE R701.7.1 supporting the use of only Class-III vapor retarders. The table calls out R7.5 minimum for 2x4 walls in zone 6, but that is still be low current code minimum thermal performance (which is R13 + R10 continuous insulation in zone 6 ) . Even if there is a polyethylene vapor barrier on the interior side, if you pad that table by a few R (go for R10-R12 not R7.5) the sheathing stays warm enough in winter that even with minor air leakage to the interior no moisture accumulates. Only bulk water incursions would put it it at risk with R10+ on the exterior, but under a cantilevered overhang you'd have to be spraying the walls with a hose to find any bulk water leaks getting into the wall cavities. At 2" even cheap used/reclaimed roofing polyiso runs about R10 (derated for age & climate), 2" of new foil faced polyiso would run R12, and 3" of EPS would run R12.6.
     
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