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Community and Q&A

Summer cabin retrofit

Mike Wanless | Posted in Energy Efficiency and Durability on

We bought a 70’s tongue and groove cathedral roofed cabin in Meyers Ca. I think the winter is a region 5. The current shingles are unfortunately in too good a shape to tear off but ultimately i will put 6″ of cc foam and a cold raised seam roof with some sort of screen to prevent spark entry. I have spent the last 60 days of spare time addressing the down slope water issues by installing a french drain and water proofing the foundation and removing the mostly fallen fiberglass from under the 4′ centered beam crawl space. Battling the mice, rats and the idiots who rolled out a half assed vapor barrier over the top of the detritus and rats nests. We are now clean and I have fitted two layers of 1 1/2 cc thermax with a openable 5″ cap for termite inspection. I am adding 3 1/2 inches of thermax to the stem wall cavities and 1 1/2 to the rim joist and will then spray foam with 1″class i fire retardant cc for air sealing. I also threw in a piece of drain tube to set up for a radon remediation system if the radon test comes back positive. The new vapor barrier will hopefully be tight enough for good suction. Any comments or suggestions would be appreciated

The next phase i would like to insulate the exterior. The current system from the outside is 1/2 sheathing, 31/2″ fiberglass batts with kraft paper, drywall. There is no wrb. So i would like to add 2 layers of 1 1/2 cc and strapping and cedar siding. My question is what wrb and i guess the rigid foam if taped is the air barrier and will i be running a risk of trapped moisutre . I dont think this area is humid even in the summer. We may convert to a gas stove which i am have some thoughts about for winter humidity generation. Again, thoughts and comments are appreciated.
cheers mike

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Replies

  1. Andrew C | | #1

    Sounds like you've made some good improvements. I don't know what the costs would be, but if you are confident in your vapor barrier and in the tubing that you've put in for passive radon abatement, I would consider adding a rat slab over the vapor barrier. It just makes things nicer and eliminates the vermin.

    If you had to convert the passive radon abatement system to active (with a fan outside the conditioned air space), do you know where you'd run the tubing? Make sure you've positioned the radon tubing termination in the crawlspace floor where you want it before you finish your crawlspace work.

    Oh, and add a light that can be turned on near the crawlspace entrance.

    Those are just thoughts, it sounds like you are thinking about the right things.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Mike,
    I'm not sure what "cc" stands for in this sentence: "I would like to add 2 layers of 1 1/2 cc."

    My best guess is that you are talking about two layers of 1 1/2-inch-thick rigid foam of some type -- polyiso, XPS, or EPS. Sometimes "c.c." means "closed-cell," but that expression is usually used for spray polyurethane foam -- so I'm not sure what you have in mind.

    To answer your questions:

    Q. "What WRB?"

    A. Most builders choose plastic housewrap (for example, Tyvek or Typar) as a WRB. It's possible to use rigid foam as a WRB, but I'm not really a fan of that approach. For more information on your options, see these two articles:

    All About Water-Resistive Barriers

    Using Rigid Foam As a Water-Resistive Barrier

    Q. "The rigid foam, if taped, is the air barrier. Will I be running a risk of trapped moisture?"

    A. Assuming you are planning to install two layers of 1 1/2-inck-thick rigid foam on the exterior side of your wall, this type of wall is designed to dry to the interior. As long as it is detailed correctly (that is, with good exterior flashing to deal with rain), this wall won't trap moisture. The kraft facing on your fiberglass batts is a "smart" vapor retarder, so this type of wall can dry inward if necessary.

    For more information on this type of wall, see these two articles:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

  3. User avatar
    Dana Dorsett | | #3

    El Dorado county is listed as zone 4B by the US DOE, not zone 5B. The adjacent Alpine county is zone 6B. Unless this place is significantly higher than most of Meyers, it's probably still zone 4.

    Standing seam metal is a good choice for roofing. In forest fire zones using something less flammable than cedar for the siding such as fiber cement, stone veneer, or masonry might be in order. Consider installing metal shutters to keep the windows from exploding under the intense radiated heat of a nearby inferno too.

    Using a peel'n'stick self healing membrane on top of the structural roof and going with 5-6" of reclaimed roofing polyiso above the structural roof deck would make it as air-tight as closed cell foam on the interior, and would get you to code-minimum performance on a U-factor basis, allowing you to keep the aesthetic of the t & g ceiling. An OSB nailer deck through-screwed to the structural t&g deck with pancake head timber screws would allow you to use #30 felt above the nailer as the exterior weather resistant barrier(WRB) or use Huber ZIP roof sheathing for the OSB with it's factory applied WRB. That's probably going to come in cheaper than 6" of closed cell under the roof deck. It's a heluva lot greener too, since reclaimed foam uses no new polymer, no new blowing agent.

    Alternatively, in zone 4B you can get to IRC code minimum with only 2.5" of foil faced low density polyiso (such as Thermax) or 3" of 2lb fiber faced roofing polyiso above the roof deck for dew point control on 9" of half pound open cell foam (or 8" of rock wool or high density fiberglass) below the roof deck. At 9" open cell has the same amount of polymer as 2.25" of closed cell and is just as air-tight, and it's blown with water vapor, a very low impact blowing agent. In zone 5B it would take 3.5" of foil-faced or 4" of roofing polyiso and 7.5" of half-pound open cell under the roof deck.

    It's fine to use 3" of reclaimed roofing polyiso on the exterior of the walls too, and would increase the size of the order to make shipping cost from more remote areas "reasonable". At least one reclaimer ships pretty much anywhere in the US from regional depots (Nationwide Foam: ), but there may be local salvage outfits and dedicated foam reclaimers competitively price nearer you. Reclaimed foam is typically 1/4-1/3 the cost of virgin stock goods (eg: $15-$25/sheet for 3" fiber faced polyiso instead of $60-$75.)

  4. Mike Wanless | | #4

    Thanks and sorry for the delay, fighting the mice and sealing the subspace are taking up most of my time short term. We were down to 24 degrees so i am pushing hard to get tidied up underneath.

    I would love a rat slab but the ground ungulates too much, besides i like the look of the operating room white poly. It's pretty comfy with the "geocloth" padding underneath. Definitely will add some lighting as time allows. I do have a plan for the active radon outlet if needed.

    I meant using foil faced poly iso = cc

    On this part of the left coast we apparently have something called Title 24 that dictates construction standards and our own climate zones. I tried to zero in on the DOE rating but could not get specific enough California rates us as a Zone 16, the most extreme which make sense as we have numerous ski resorts near by. Title 24 has a prescriptive requirement of R38/21(roof/walls) or R49/29 for "package D or C. It took me 4 hours to find out D is if you have natural gas and C is if you don't! We do so that is why i think we need 6" on top of the roof deck so we can retain the tongue and groove visually.

    I am thinking that seeing how there was nothing before, tarpaper should work great and i "like" it better than house wrap and the problems you have outlined. With this much insulation on the roof do were really need to peel and stick the whole thing?

    i would love to find recycled polyiso. There was a guy in idaho who had some seconds and over runs that seemed to help the economics. good excuse for a road trip!
    cheers, mike

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Mike,
    Q. "I think we need 6 inches [of rigid foam] on top of the roof deck so we can retain the tongue-and-groove visually. ... With this much insulation on the roof, do were really need to peel and stick the whole thing?"

    A. If your ceiling consists of tongue-and-groove boards, you don't necessarily need to install peel-and-stick over the entire roof sheathing, but you do need a high-quality air barrier directly above the tongue-and-groove boards to prevent problems with this type of assembly.

    A peel-and-stick product like Grace Ice & Water Shield (installed directly above the tongue-and-groove boards) is one possibility.

    Another possibility is a layer of OSB or plywood sheathing with taped seams (installed directly above the tongue-and-groove boards).

    A third possibility is a high-quality synthetic roofing underlayment, installed with taped seams.

    A fourth possibility is one of the European air-barrier membranes (for example, Solitex Mento) installed with taped seams.

    A fifth possibility, notably inferior to the above options, is to use the lowest layer of rigid foam as the air barrier. This approach requires the rigid foam seams to be taped with a high-quality compatible tape. The main disadvantage of this approach is that rigid foam may shrink over time, breaking the bond between the rigid foam and the tape.

    For more information, see How to Install Rigid Foam On Top of Roof Sheathing.

  6. User avatar
    Dana Dorsett | | #6

    The peel & stick is to guarantee air & water tightness over time.

    CA zone 16 covers a wide range of altitudes and annual heating degree-days (HDD) . The DOE zones are broken out by county, and defined by HDD.

    Insulation prescriptions for Title 24 presume that the insulation is thermally bridged by framing (for roofs the framing fraction is presumed to be 7% of the total area.). With continuous insulation the same performance can be met at a lower R, since there is no thermal bridging. The IRC recognizes that, and allows a U-factor path to compliance (where the thermal bridging is calculated) as well as a simple insulation-R path. In general the IRC has been somewhat higher-R than Title 24 for roofs.

  7. Mike Wanless | | #7

    You guys and this site are great.
    I believe that the roof has a layer of wood sheathing on top of the tongue and from the last roofing.After tear off I hope we can tape it,and/or as you suggest, will go with peel in stick or another membrane. My question from the various tables is, that it looks like i need 5-6 inches of polyiso on the roof or can i get away with less . For the walls would 2" be enough(an iffy R13 from the fiberglass)to get to R29 minus the thermal bridging elimination? I really like the idea of staggered sheets but would 2 1" sheets be silly? I would like to use the "right" amount for environmental reasons and i am about to install a window that i would like to set up for the ultimate exterior insulation.
    Thanks again

  8. User avatar
    Dana Dorsett | | #8

    At a typical framing faction of 25% an R13-ish cedar clad 2x4 framed wall comes in at around R10-R11 "whole wall", after factoring in the thermal bridging and various material layers. Adding 2" of polyiso gets you maybe R22, but it's still under R25 whole-wall.

    A couple inches of Thermax is what, R13? That derates R10-R11 derated for an exterior side application under your mid-winter conditions. Add R13 for the fiber insulation and another R2 for the combined wallboard/ exterior sheathing / siding and you're at R27 center cavity at the fully rated R, or ~R25 if derating the polyiso for mid-winter conditions.

    But that's still very decent wall for your climate. R22-R25 whole-wall is usually good enough to hit Zero Net Energy with a PV array that still fits on the roof in a zone 4A climate, provided the rest of it is similarly upgraded. Take a peek at the zone 4 row of Table 2, p10 of this document, noting the whole-assembly R recommendations for walls, roofs, crawlspaces, etc.

    Those are roughly the economic limits of what was financially rational in 2009, but also pretty close to what it took to get to Net Zero then. Since then PV efficiency has improved (taking less rooftop real estate per unit of energy) and fallen in price, and cold climate heat pumps have improved efficiency while the prices have remained flat. So while Table 2 is still roughly in effect, the performance limits of financial rationality have moved down a bit. (The higher insolation of the sunnier eastern slope of the Sierra at altitude helps too- with more kwh out per watt of PV, than at sea level in Santa Cruz.)

    If you bump the exterior foam to 3" of reclaimed roofing iso is you'd be at about R30 whole-wall, but 2" is still enough. That makes the wall thickness the same as a typical 2x6 framed wall. If you want to double layer it, 1.5- 2" of cheap reclaimed foam with a 0.5-1" of foil faced polyiso on the exterior layer works. The foil facer makes itt easier to air-seal the foam (with foil tapes), and a low-E aspect of the aluminum facer with an air gap between it and the siding adds another ~R1 of performance.

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