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Community and Q&A

Open truss post frame roof

kcov | Posted in Green Building Techniques on

I am building a cabin on Ky lake and want to do it post framed. The floor plan calls for vaulted ceiling over the kitchen and living area. I want to do exposed scissor trusses on 8’ centers. My thoughts were to span the trusses with vertical 2x6s on 24” centers and fill with batts then cover the bottom side between the trusses with tongue and groove pine. So what would I cover the top side 2x6s? I was thinking osb then 2” rigid foam then layer of 1/2 plywood screwed to the 2x6s then foil bubble wrap then metal. Doesn’t seem to be anyone around here, southern Ky, that cares much about good insulation and sealed house. I don’t have any education other than applying different parts of roofing projects I have seen online and what I have seen in person. Please point out problems with this idea or a better way. I’m not going to do sips.

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  1. Expert Member
    Michael Maines | | #1

    Kcov, start by reading this article, which should answer most or all of your questions:

  2. GBA Editor
    Martin Holladay | | #2

    First of all, can you tell us your name? (I'm Martin.)

    Your description is confusing, but it sounds like you have many things wrong.

    You absolutiely need an airtight ceiling to prevent interior moisture from entering your roof assembly and causing moisture problems. That means (a) You can't just install your finish ceiling between your rafters -- the ceiling has to be continuous and airtight. (b) You definitely don't want to have tongue-and-groove boards as your finished ceiling unless you have an air barrier (usually taped drywall) above the tongue-and-groove boards. Again, this drywall ceiling has to be continuous -- not installed in strips between your rafters or trusses.

    Fiberglass batts are a poor choice for a cathedral ceiling, but they can work, as long as (a) you have an airtight ceiling, (b) your fiberglass batts have a minimum R-value of R-49 or R-38 (depending on your local code), and (c) your fiberglass batts are installed impeccably, without voids, and (d) you have a continuous ventilation channel in every single rafter bay, extending from your soffit vents to your ridge vent. These ventilation channels need to be installed between the top of the insulation and the underside of the roof sheathing.

    More information here: "How to Build an Insulated Cathedral Ceiling."

  3. kcov | | #3

    Ok, my name is Kevin.
    I am going to try to attach a photo of ceiling/roof I am going to build. So i will try to make a picture of what I was trying to say starting with the outside layer being the roofing metal. The next layer would be reflective bubble wrap to divert any condensation on the underside of the metal. next layer would be plywood, then 2"xps, then tongue and grove osb that would be nailed to 2x6 purlins that would be on edge nailed to the trusses. Then the underside of the 2x6 would get batts or I could do blown in then covered by tongue and groove pine running between the trusses. I haven't seen sheet rock used in any roof systems in my research so far.

  4. kcov | | #4

    so as you see in the picture, the vinyl backed insulation would be replaced with the tongue and groove osb and the all the other layers that I described.

  5. GBA Editor
    Martin Holladay | | #5

    First of all, the bubble wrap is a waste of money. For more information, see "Stay Away from Foil-Faced Bubble Wrap."

    My advice is to take the money you were planning to spend on bubble wrap and invest the money in thicker insulation.

    Second: You are planning to build a roof assembly that combines exterior rigid foam and fluffy insulation. There are rules that apply to the minimum R-value for the rigid foam layer in this type of assembly. Here is a link to an article that explains the rules: "How to Install Rigid Foam On Top of Roof Sheathing."

    To apply these rules, we need to know your geographical location or climate zone. You tell us that you are building in "Ky lake." Is "Ky lake" the name of the lake? Or is "Ky" an abbreviation for the state of Kentucky?

    If you are building in Kentucky, your are in Climate Zone 4. In Zone 4, all you need is a minimum of R-10 of rigid foam for this assembly to work. Two inches of XPS will meet this minimum. However, you should know that green builders try to avoid using XPS because it is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see "Choosing Rigid Foam."

    For your plan to work, you really need a perfect air barrier at the OSB layer. That means that you'll need either a peel-and-stick membrane above the OSB, or you'll need to carefully seal the OSB seams with a high-quality tape like Zip System tape.

  6. kcov | | #6

    Ky lake runs in Ky and in Tn, I will be just south of Ky in Tennessee but I am building in the county and there are no inspections or codes I have to meet for my building, just septic. That said, I want a well insulated house done properly. I understand the complaints on bubble wrap and the idea was to use it as a moisture barrier for the back side of the metal. I also have 2 warehouses, one with bubble wrap under the metal roof and one with nothing under the metal roof. the one with the wrap is no doubt at least 10 degrees cooler than the one with nothing, its like an absolute oven right now. So however small and likely not the best choice, it does do something and appears to be how it is applied to the structure.

    So say i just throw out the bubble wrap and use tar paper over the last layer of wood. I can go to 2x8 purlins and add 8" or whatever it is that goes into an 7.5" space and increase the insulation there.
    Thanks for the advice, I will start looking into the peel and stick or zip tape.

    Would spray foam on the underside of the osb provide the perfect air barrier or does it need to be on the top side of the osb?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Say I ... use tar paper over the last layer of wood."

    A. Yes, that would be appropriate. "Tar paper" (more accurately, asphalt felt) is a roofing underlayment. Roofing underlayment is required by building codes under your roofing, and installing it is a good idea.

    Q. "Would spray foam on the underside of the OSB provide the perfect air barrier?"

    A. Spray foam is often used as an air barrier. As long as the installer does a perfect job, and the spray foam is thick enough -- generally a minimum of 1 inch or 1.5 inch for closed-cell spray foam, or a minimum of 2 or 3 inches for open-cell spray foam -- you'll end up with a close-to-perfect air barrier. For more information, see "Air Leakage Through Spray Polyurethane Foam."

  8. kcov | | #8

    Martin, how does a wood stove work in a tight house? Can you pipe fresh air to the stove and open it when you use it?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "How does a wood stove work in a tight house? Can you pipe fresh air to the stove and open it when you use it?"

    A. For a thorough discussion of this question, see "All About Wood Stoves." (Scroll down to the section of the article under the bold-font heading, "Can you put a wood stove in a Passivhaus?")

  10. walta100 | | #10

    From the photos it looks like you have established your air, thermal and water barrier on the outside wall and roof. They are what they are. Given their location I see few reasonable options for improvement, as any new insulation needs to be close contact with the old insulation.

    The following is a rant, please do not take it as a personal attack. I understand people want to get there building up fast and cheap only after it is up do they stop to consider how and how much it will cost the heat and cool the building.

    Is it just me or do I see this same question on this and other forums. “How can I make my pole barn into a hi performance building for a song”. Ok that not how they write it but how I read the question. The owners of these building always manage to find some other way describe their building other than “pole barn” Let’s admit it a pole barn! Pole barns are designed and built with 1 goal in mind and all other considerations are abandoned for this goal. How to build at the lowest cost per dry square foot possible.

    Is insulated pole barn an oxymoron like jumbo shrimp?

    Rant off

    I do think it could be a great article “How can I make my pole barn into a hi performance building for a song”

    If your pole building is like most it will leak more than enough air for a few wood stoves. The only way to know is to have the building tested with a blower door.


  11. Jon_R | | #11

    Even with a design where they are optional, I'd do the following for increased moisture safety:

    a) a vent channel under the metal roofing (above the foam)
    b) some ability to dry in both directions (for example, unfaced EPS foam+wood, no peel and stick)

    Similar for walls.

    With these options and the proper ratio of rigid foam to fiberglass, you should be fine with taped sheathing and no drywall or spray foam. Not far from your plan - drop the bubble wrap, add a vent and use EPS.

  12. kcov | | #12

    Walta, the photo is not my building. It is of the truss and purlin system I want to use to have exposed ceiling. I currently live in a post framed house and my electric bill is usually a little over $100 a month and it was poorly built for heating and cooling purposes. Just as many traditionally built homes in my area as well. That said, if I take my time and educate myself on techniques as described on this website, a post framed building can be as efficient as any other building. I can get the structure built and enclosed in a week relatively cheap, then take my time to finish everything. Just my idea. I may start a blog on the process as efficient affordable housing seems to be very limited. I appreciate everyones comments very much.

  13. kcov | | #13

    Martin, how does this sound as a roof package? Trusses with 2x8 horizontal purlins covered with tongue and groove osb covered with grace ice and water, covered with 2 layers polyiso alternated and top layer taped. Then horizontal lathe screwed down through the foam into osb then metal screwed to lathe. Trusses wouldn’t have a tail so I could lap the grace over the side wrap and use the lathes to create the overhangs. I could then do bibs into the 2x8 purlins from the underside and then cover it with tongue and grove pine.. I don’t know how the price of doing it this way will add up but it’s what my research has me favoring. Does this sound like a practical plan?

  14. GBA Editor
    Martin Holladay | | #14

    I don't know how thick your polyiso will be. All you tell me is that you'll have "2 layers polyiso."

    In Climate Zone 4, with this type of roof assembly, at least 31% of the R-value of the total roof assembly must come from the rigid foam layer. This is explained in the following article: "Combining Exterior Rigid Foam With Fluffy Insulation."

    If your two layers of polyiso add up to R-13, you'll have R-13 in your rigid foam layer, and R-26.8 in your fiberglass layer, for a total of almost R-40. The rigid foam meets the minimum requirement, so that stack-up would work. Double check with your local code enforcement official, however, to make sure than an R-40 roof is legal in your area. Many jurisdictions in Zone 4 insist on a minimum of R-49.

    If your two layers of polyiso add up to R-26, even better.

    I realize that you plan to make the Ice & Water Shield your primary air barrier. But it wouldn't hurt to include a secondary air barrier between the fiberglass and the tongue-and-groove boards, in case sloppy workmanship results in a few unplanned air leaks through your roof assembly.

  15. kcov | | #15

    Just looked up 1” poly is r6 and 2” is r13 so 19 rigid and bib in 7.5” 2x8 purlin is an r30 for a total of 49. I will be in zone 4 so this is to code.

  16. kcov | | #16

    So you think I need an air barrier over the bib? You said fiberglass in your post which was my original plan but see that bibs would be better. Also I don’t have codes to meet with building this structure.

  17. theolman | | #17

    Where are you getting the trusses?
    Thanks, John

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