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Community and Q&A

Shed roof addition on pier foundation

Scott Guerin | Posted in Green Building Techniques on


For a 18 x 50′ addition, we priced a hybrid closed and open cell spray foam (~$9,500) for the entire structure but are now leaning toward mineral wool throughout with a contiguous-as-possible smart vapor barrier like MemBrain underneath a service chase. This is more to be being environmentally friendly than for cost.

We’ve raised the addition off the ground due to wet site conditions but I’ve yet to find a solid detail on insulating the floor and whether to vent the shed roof at the eaves and peak assuming we go with mineral wool.

Other inputs:
1. We’re zone 5 in the mid-Hudson valley in NYS
2. Planning on 1 or 2 miniplit condensers and heads
3. Not yet sure of an HRV
4. Will have wood stove (Morso 7110)

A typical section detail is attached, it’s a bit out of date as early on we thought we’d use Zip-R panels and spray foam.

In the floor assembly, does the vapor barrier (assuming one is desired) stay on the inside like for a wall?

Any thoughts on this or pointers to best practices for this kind of shed roof and raised floor?

Tx in advance,


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I’ve yet to find a solid detail on insulating the floor."

    A. Read this article: "How to Insulate a Cold Floor."

    Q. "I don't know whether to vent the shed roof at the eaves and peak assuming we go with mineral wool."

    A. Mineral wool is air-permeable. With air-permeable insulation like mineral wool, it is a code violation to install the insulation between rafters unless there is a ventilation channel between the top of the insulation and the underside of the roof sheathing.

    For more information, see "How to Build an Insulated Cathedral Ceiling."

    1. Scott Guerin | | #9


      A lot to digest! Thank you so much...esp for the Cathedral Ceiling article and al of its links...

  2. Eric Habegger | | #2

    No expert here but I think I can answer the moisture barrier in the crawlspace from my own experience. The moisture from ground level moisture can effectively be prevented from damaging the house by installing a water proof tarp made for crawl spaces. There are numerous outfits marketing them just for that purpose. Before I installed mine humidity down there in the rainy season got up to 95%. So far this season the max has been 50% with the barrier installed.

    There are some important points to consider. You need to wrap the piers separately with poly before you lay the main bulk of the poly on the ground. The pier wrap will overlap the main poly wrap and must be taped at the point where they intersect at the piers. Also you should shingle the main poly strips that lay on the ground so that whatever direction that the elevation changes, if any, doesn't make the water work its way through that junction by the force of gravity. You should then tape those junctions as well. Finally, adhere poly to the foundation walls at a point above where the dirt touches the those walls on the "outside". It should be like a skirt all around the perimeter on the inside of the foundation walls that reaches down to cover about a foot on the dirt also. You should shingle that skirt so that any water seeping through the concrete foundation that is behind that skirt goes "under" the tarp laying on the ground. So you should tuck the skirt where it overlaps the ground under the main area tarp. Finally tape all the junctions between pier, skirt, and main poly sections.

    That should cure 99% of any moisture problems. You can still do other remedies right at the floor level but for myself I haven't had to do anything. I hope that helps.

    1. Scott Guerin | | #10

      Eric, thanks so much for your thoughtful reply.

      The piers were cast using BigFoots and are in the ground, a moisture barrier was then put in place and held down with a couple inches of gravel.

      There is no crawl space just the floor raised ~24" off the ground.

  3. Expert Member
    Malcolm Taylor | | #3


    How much space do you have between the underside of the floor system and grade? It would be a lot easier to sheath the bottom of the joists than to have to piece in sheathing between every joist. Better still, put a continuous layer of foam on before the sheathing and use batts instead of spray foam. It will both yield better results and be cheaper.

    1. Scott Guerin | | #11


      You are so right, it'd be a lot easier but there will be only ~12" clearance from the gravel to the underside of the joists. I think it's too little to get a person below safely. Also, I am not happy about the solution (drop in sheathing) since it also lets the bottom of the joist hang down into the cold or hot air. I've been thinking about a kind of jack jig, like a ceiling sheet-rock brace, or using eye screw lags to pull a sheet up to the joists and then toe nail them up to the joist...

      Not sure what you mean by "...put a continuous layer of foam on before the sheathing and use batts instead of spray foam. " No matter what, we'll be foaming the floor sheathing for air and moisture barrier and letting moisture evaporate into the house...


      1. Expert Member
        Malcolm Taylor | | #12


        You may find this discussion of a similar build useful.

        It might also be worthwhile checking you code about clearance to the floor. Our's has requirements for access and the use of treated-lumber that close to grade.

        I was suggesting that if you could get at the underside of the floor (or as I suggested in the discussion I linked to, built the floor upside down), you could place a layer of foam on the underside of the floor assembly before putting on the protective plywood surface. You could then fill the joist spaces with batts, or have them spray-foamed - although with continuous foam below there wouldn't be much benefit. The main advantage of getting a layer of foam either above or below the joists is the reduction of cold spots due to thermal bridging at the joists - something spray-foam in the cavities doesn't address.

  4. Expert Member
    Dana Dorsett | | #4

    With 2x10 (9.25" deep) floor stacking R15 +R23 rock wool batts would get you R38. The nominal loft of those batts only adds up to 9", but I doubt the additional 1/4" of fluffage is going to be a problem If concerned about that 1/4" you can cut 1/4" fan-fold siding underlayment into 14.5" wide strips and staple it to the underside of the subfloor between the joists. A 3/4" OSB or plywood subfloor is already a class-II vapor retarder, so there's no need to install a vapor barrier.

    With the unvented roof bumping it to 2x12 rafters (11.25" deep) would allow you to get you to code min R49+ with just 4" of closed cell foam + R30 rock wool (nominally 7.25" loft) without moisture accumulation issues, and does not need interior side vapor retarders. Alternatively dropping to 2x8 rafters /R30 rock will and 4" of polyiso above the roof deck under the EPDM membrane gets you there. ( Used and factory seconds polyiso is availble from multiple sources in the hudson valley for very low money compared to closed cell spray foam:

    The wall assembly as detailed doesn't meet code minimum thermal performance, with 2"/R11 closed cell + R3.6 ZIP R. R23 rock wool batts would exceed code min but would need an interior side vapor retarder if you keep the ZIP-R. Without the ZIP-R , using OSB or CDX sheathing it won't need the interior vapor retarder as long as the siding is backventilated (rainscreen assembly, or vinyl.) Dropping to 2x4 /R15 rock wool and R6 ZIP-R walls would be fine, but any ZIP-R is a fairly big upcharge. A 2x4/R14 + 2" reclaimed polyiso wall might be cheaper despite the higher installation labor cost.

    A single 1-ton Mitsubishi FH12NA or 3/4 ton cold climate Fujistu 9RLS3H would probably cover your heating load at -5F (or whatever the local design temp is) for the whole 900', with room to spare on the cooling load too (unless it's a lot of west facing glass.) A pair of them would be excessive. A 1-ton Fujitsu 12RLFCD would probably cover it too, with better heat/coolth distribution if it's going to be broken up in to multiple rooms.

    1. Scott Guerin | | #14

      Dana and all,

      Thank you very much for your inputs. Green Building Advisor is the gift that keeps on giving! Lot's to digest across all these comments and more from last night. I'll repost updated details and will have questions I'm sure.

    2. Scott Guerin | | #27


      I'm getting a quote on putting AirKrete into an unvented 18' x 42' cathedral ceiling (I saw a post of yours on the technique elsewhere). See images in posts lower down. The material would seem to reach R-46 in my 11.875" deep joists, A notch below the R-49 code mandate. If this is a viable material, do I need a smart membrane underneath?Is it a vapor barrier? Any guidance appreciated! I was leaning toward R-25 of closed cell spray foam + R-30 mineral wool but the $6500 quote is pretty hefty!!


  5. Eric Habegger | | #5

    "A 3/4" OSB or plywood subfloor is already a class-II vapor retarder, so there's no need to install a vapor barrier."

    I beg to differ. Scott is not going to be too happy with you if he follows that advice and he starts to get cupping wood floors. It happens too often and wood flooring manufacturers all recommend that poly be installed over dirt before any wood flooring is installed. I'm assuming you meant that plywood or osb is a replacement for "any" kind of moisture barrier. That of course isn't true. If that isn't what you meant let me know.

    1. Expert Member
      Malcolm Taylor | | #6


      I'm pretty sure Scott was asking if he needed a vb in his floor assembly, not under the addition. He is building on piers, not a crawlspace, so I'm not sure a vb on the ground makes much sense either.

      1. Eric Habegger | | #7

        Sorry if I'm being dense. Maybe I just skipped ahead to the fact that Scott apparently was going to build a crawl space when he chose to build on piers, even if he didn't use so many words. That's what I still think. What did you think? Maybe Scott can clear it up. Good practice usually requires a VB over dirt, especially in areas that have wet conditions.

        1. GBA Editor
          Martin Holladay | | #8

          Here is a link to an article that discusses various options: "Crawl Spaces vs. Skirts."

        2. Expert Member
          Malcolm Taylor | | #13

          If he is building on piers, then the perimeter needs to stay open.

          In the codes I'm familiar with, If he closes it in whether it is with a skirt or stem-walls, it is a crawlspace - with all the attendant requirements of insulation, ventilation, access, etc.

          if he is building a crawlspace then whether he supports the intermediate loads on piers or stem-walls makes no difference.

  6. Eric Habegger | | #15

    Ok, I (think) I finally get it from Martin's "Spaces vs Skirts" article and Malcolm's reply. Scott, I think you are very wise to build on piers if your add-on will be built on wet ground. The only thing I would add is that it sounds like you might live in an area in danger of freezing pipes in winter. So it sounds like you should at least have a skirt and perhaps insulation on the perimeter attached to the skirt to protect that area. So I think its wise that you put in the VB because if you decide to build a skirt it will limit the natural ventilation that would otherwise occur and lead to dampness problems that would propagate to the underside structure of the house.

    1. Scott Guerin | | #16

      Intentionally, there is no plumbing in the addition. However, we are planning to add a critter barrier using wire fencing as skunks and possums like to burrow under such structures

      Tx again

  7. Scott Guerin | | #17

    We've made rapid progress! 2 x 6 construction and 11" roof trusses with fiberglass insulation and a vented roof. It's far from green-perfect but I'm thinking of an electrical service layer on north and east facing walls which will be coldest. The pier construction and raised floor worked great to keep it all above moisture and snow.

    1. Expert Member
      Malcolm Taylor | | #18


      Thanks for the update. It's aways nice to know how these projects turn out - and i'm glad this one seems to be going well for you.

  8. Scott Guerin | | #19

    An update and Questions!

    We've roughed in the electrical and put in the Anderson windows.

    There are several fixed vertical glazed thermopanes that will be installed into the rough openings. I'm doing that work and will add a bevel to the opening and flash with stretch tape. Any details come to mind on GBA? Ref Door-sidelight and Windows-RO-South Side

    Does the lower edge of the Zip panels need taping? ref image

    Next, I'm seeing some possible fails in the flashing around doors. It doesn't look like stretch tape was used at the jams. Should it have been to wrap the ZIP panel to the 2 x 6 frame? Ref Door-Porch image South-facade-windows

    Before insulating, should I caulk the lower sill? How about at corners etc?

  9. GBA Editor
    Martin Holladay | | #20

    Here's a link to an article that will answer your questions on door jamb flashing: "How to Flash an Exterior Door."

    If you want to build your own fixed window, you can. In addition to needing the IGU (insulated glazing unit), you'll need to build the window frame, which needs to include a sloped sill, jambs, and a head, ideally put together with rabbeted or dadoed joints, glued together. You'll need glazing stops or rabbets to hold the IGU, and butyl glazing tapes and wooden stops.

    If this type of work is new to you, you can just buy a manufactured fixed window.

  10. Scott Guerin | | #21


    While "How to insulate a Cathedral Ceiling" is a great article, over the years so many conflicting replies are embodied in it! Especially re Cellulose which I'm considering (vented or not). I can readily admit that I am in the figuring-out-the-details-having-not-thought-them-all-through-ahead-of-time stage...sigh. Nonetheless, the main question is the roof insulation approach.

    First image, "Section" shows the basic conditions. Heated area is 42' long x 18' wide x ~11' high. We're installing a 4 zone 36,000 BTU Mitsubishi minisplit and will have a 45,000 BTU wood stove as backup (and pleasure!) Will use Intellowrap+ on walls and ceiling. Exterior will be siding on furring strips over Zip sheathing to create the air gap for drying.

    Second image: "Thermal Break" the walls I'm doing will use the thermal break technique referenced above and R-30 Rockwool. I did a mockup this weekend and it's fantastic, love the Rockwool and def get the knife they sell to cut it it too. Totally DIY once I get the nail gun.

    Third Image: "Upper Eave Cellulose" If I put the Doug McEvers fiberboard (BTW is that like Cellotex?) up into the I-joist, it takes about 2" of depth out of the cellulose fill and I only hit R-36 of the 49 I need (the fiberboard adds a point or two I guess.) Thus, I'm thinking to add 2 x 4 purlins to the I-Joist and fill them with R-15 Rockwool to get to R-48. I suggest the Rockwool inlay as the cellulose would not fill the purlins very well due to the egg crate condition. Attaching them to the joists in parallel is an option I suppose. BTW I was originally showing them at 24" oc but now think to go back to 16" due to the weight of the insulation and risk of bowing out the GWB fire rated later.

    Fourth image: "Lower eave" shows the ~14" overhang and how I continue the cellulose and Rockwool assembly there. Venting the walls air gap and roof looks a little sketchy but I'll mock that up next weekend.

    Everyone is so generous with their knowledge and I'm sure I'm missing something obvious on the roof detailing.

    In summary

    1. Vented or unvented? Safer vented but comments and article say unvented is OK, but BSC research says emphatically "no."
    2. Cellulose alone assuming parallel joist extensions?
    3. Cellulose + Rockwool using purlins?

    Comments and inputs much appreciated!

    Sorry to have gone on so long...


    1. GBA Editor
      Martin Holladay | | #22

      Q. "While 'How to Build an Insulated Cathedral Ceiling' is a great article, over the years so many conflicting replies are embodied in it!"

      A. Focus on the article, Scott, not the comments. I wrote the article, and the article includes good advice. Over the years since it was written, I've even edited the article to clarify ambiguities and sharpen the advice. The comments are a totally different type of content -- any inexperienced person with a strong opinion has the right to post comments on this site. We don't screen comments to remove bad advice, so I wouldn't build a building based on advice from JackRabbit77 or BlueWave902.

      Q. "Vented or unvented? Safer vented but comments and article say unvented is OK."

      A. Of course an unvented approach is OK -- but only if you have either (a) included an adequately thick layer of continuous rigid foam on the exterior side of your roof sheathing, or (b) you follow the ratio rules for the flash-and-batt approach (an approach which requires an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing). Otherwise, you can't install fluffy insulation like cellulose or mineral wool between your rafters.

      Q. "Cellulose alone assuming parallel joist extensions?"

      A. That will work, but only if you include a vent channel between the underside of your roof sheathing and the top layer of your insulation.

      Q. "Cellulose + Rockwool using purlins?"

      A. I don't think you mean purlins -- I think you mean interior 2x4 strapping, installed 16 inches on center, perpendicular to the rafters. That will work, as long as you include a vent channel between the underside of your roof sheathing and the top layer of your insulation.

      1. Expert Member
        Malcolm Taylor | | #23

        I think the first section shows a low slope roof - which realistically limits the options to an unvented assembly.

  11. Scott Guerin | | #24

    Thank you Martin,

    Going with Dense Pack Cellulose alone with parallel joist extensions and a vent channel above the cellulose and below the sheathing. Below the cellulose is a installer-provided netting that holds the stuff in, then I'll add Intello+ and the 1/2" GWB.

    @dougmcevers suggested using "Fiberboard" for the vent channel board, is that MDF? Cellotex? How permeable should it be?

    Thanks again in advance to all!

    1. GBA Editor
      Martin Holladay | | #25

      Malcolm has a good point -- if this is a low-slope roof, you can't install a conventional vent channel. You need to choose from a different list of approaches.

      Here is the link to the article that explains what you need to know: "Insulating Low-Slope Residential Roofs."

  12. Scott Guerin | | #26

    Martin and Malcolm,

    Sry to go on but one last time:

    We were trying to avoid foam but obviously missed the bus on the design requirements for cellulose or adding rigid above the sheathing.

    So final decisions seem to be:

    1. All closed cell spray foam >7-8" thick =>R 49


    2. From "Insulating a Cathedral Ceiling" 4" closed cell to R-25 and R-30 mineral wool below with Intello+ membrane and GWB inside, drys t the inside.

    Cellulose seems like a non-starter unless a non-vented Dense Pack Cellulose is viable...

    Tx . again! will keep the thread in the loop on costs performance etc.

  13. Scott Guerin | | #28

    Climate zone 5, mid Hudson Valley:

    I gave in and sprayed closed cell to ~R-50 direct to the Zip sheathing underside. I have an EDPM roof so the sheathing is sealed in, fingers crossed. A very recent article pointed to a vented solution using thin-ply vent baffles and spray foaming onto them. Given the I-Truss' top member, it would have been easy to do that using 1" rigid-or ply to span the 16" o.c. truss web. I'd recommend that to others.

    Last question: Since my I-Truss is about 12" deep, I have ~4" of air space below the foam to the bottom of the joist. *Does that need to be filled or can it be dead air space?* See the photo.

    A side note to any readers:
    The 19 x 42' ceiling took two days for set up and spraying. The sheathing was very dry. Cost $4,400 + tax.

    I did the walls using thermally broken studs and r-30 Rocksul. A total DIY job with a helper that took three weeks of intermittent work and about 200 man-hours. Going to Intello+ it this week coming.

    In planning, I urge a service chase approach on top of the thermal studs as cutting and fitting the batts around the wires was very time consuming and even with care, certainly does a less effective job of insulating. Cost of materials ~$2,500 + $1,200 in labor and my free time!

    1. Expert Member
      Malcolm Taylor | | #29


      Rather than a service cavity just for wires, another approach is to locate all horizontal runs at the bottom of each cavity and vertical ones attached to the sides of the studs. This uses more wire, and still means you have to cut around fixture boxes, but makes insulating easier.

      Considering that the typical two man crew installs the batts in the houses I do in less than a day, I'm not sure including a service cavity dedicated solely to wiring ever makes much sense.

      1. Scott Guerin | | #30


        That's an excellent idea...for the next project...haha, this is likely it for me...

        What do you think of leaving the cavity between sheetrock and spray foam open? I am so sick of insulation it'd kill me to fill it in!

        1. Expert Member
          Malcolm Taylor | | #31

          The only penalty is an energy one. If you are sick of insulating, and happy with the R value of the foam, relax, do something fun instead. Watch Game of Thrones or chase the neighbour's cat around your yard.

          1. Deleted | | #32


  14. Scott Guerin | | #33

    It's been about 7 months to get to where the end is in sight!

    Intello+ vapor barrier installed DIY. Was easy but tedious. Used more tape than recommended but I think it was because of a less than perfect layout of the wrap. I used 1.75 rolls of Intello (~1,200 SF total) and 8 rolls of tape and would have needed 10 but we cut the last roll in half for the floor and top seals (worked fine than way). 8000 staples!

    Would have been easier to do the wrap before demising walls and a partial attic and its joists were put in as the wrap had to be cut and taped around piecemeal (see photo).

    "Pro" insulating tip. They spray foamed the ceiling right over the wood stove flue opening (anyone else remember Earl Scheib car paint jobs? Better mask those window and mirrors yourself! So I have to sawzall that foam back a few inches away from where the flue will go. Next time, block it off!!

    Walked through the space with my sheetrocker...all good after a couple other nailers were added.

    There's a utility corner - where minisplit lines and power come in that's a mess to seal and insulate, working that this afternoon after a nice glass or two of Hahn Cabernet and a sandwich for Father's Day!

    Sheetrock, wood stove, fixed glazing, flooring, final electrics, and finish carpentry next...

    1. Expert Member
      Malcolm Taylor | | #34

      Nice work Scott. Glad things are going well.

      Rather than wrap the whole interior before demising walls or lofts, we typically use a 16" roll of membrane (or poly) to cover the intersection of these elements and the exterior walls and the ceiling. You can then complete all the framing all at once, and avoid damage to the vapour-retarder.

      I doubt it makes any difference to your project, but it's a good idea to place the joints in the membrane over solid backing, rather than rely on the tape alone. Our code requires it be installed that way.

      1. Scott Guerin | | #35

        Super smart as usual Malcolm! As far as tape joints over studs, mine tended to do so 80% of the time or better. Seemed tight at "floating" seams anyway as the R30 Roksul pushed against it as I sealed the tape...

  15. Scott Guerin | | #36

    Brief update: Electrical is in, on, and inspected. Sheetrock is up and taped, and I've started the exterior battens for air gap and the siding. Mitsubishi minsplits are roughed in for install in the next couple weeks. Morso 1710 stove will be installed once place is painted. Current per SF cost = $128 with about $12/SF of DIY on top of that. I'm doing the interior finish carpentry so figure another few bucks/SF on top of that.

    A couple of observations and questions:

    1. The place is pretty tight and I'm not required to do blower test as it's an addition and now that it's rocked...too late anyway, right? I guess it'd show me outlet leaks (I used the Madison Electric Flanged add-ones which worked great to tape to the Intello+)

    2. The walls are a DIY Bonfiglioli R-30 installation using mineral wool with Intello+ wrap and the place is stuffy as heck. I have locations planned and accessible for ventilation but didn't spring yet for an HRV or the Luminos HRV point sources...I'm hedging my bets.

    3. The ceilings were a design cop-out as since it's low slope and not designed deeply enough to enable an airbaffle approach. I went with closed cell spray to R40+ across the entire roof system including attic space. There was enough room underneath the foam to add 4" of fiberglass but I was really sick of doing the insulation. I may do the attic that way down the road and will report on its effect on snow melt.

    1. Expert Member
      Malcolm Taylor | | #37

      Looking good Scott. Hope you come back with finished photos!

  16. Scott Guerin | | #38

    Very nice, and clear, rainscreen, flashing and other details (from a builder's perspective) can be found at Hammer and Hand here:

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