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Community and Q&A

Metal shed roof / Ice & Water Shield / Insulation over and under

user-5254480 | Posted in General Questions on

I am doing a remodel in the Seattle area – zone 4C. This question is related to roof build up and insulation.

– 2:12 shed roof, high side faces south
– Joists are 20″ oc and 12″ deep
– Decking is 5/8 plywood
– Roofing material will be metal standing seam

During construction we applied some ice and water shield over the entire the decking – mainly to temporarily keep the decking dry and prevent internal water intrusion until we were ready to install the roof.

Our original plan was to lay the metal over the decking without insulation; however we altered our plans and our now going to do 2″ build up of insulation over the decking. We wanted to get a thermal break for the decking and wanted to get more R overall.

Under the decking we will build a 2″ vent space in the joists with Majvest (stapled and caulked) the entire bay right up to the ridge and low edge. The ridge and low edge have bird blocks vents to the soffit.

Under the vent space we will use 10″ of Roxul or a fiberglass batt.


1) Should we remove the ice and water shield before laying down the insulation? I read a few posts about breath-ability issues with the decking. My take is that since have a 2″ vent space running end to end it should breathe well from below. Plus, the added top insulation helps minimize any top condensation.

2) What insulation is recommended for over the decking? If we left the ice and water shield down I was thinking that permeability of top insulation becomes less of an issue. We are using cork of the majority of external walls, but I’d like to save some money on this roof section if possible.

3) What insulation should we use for the joist bays? Roxul does not come in a 10″tx20″w batt. Its 23″w and 7 1/4″t. I’d have to cut all of the bats to get them to fit and then layer another 3″ to get maximum insulation pack.

4) Is building the venting path in the joist bay a solid approach? If not what else do you recommend?

5) Is it common to put a layer of ice and water shield over the top side insulation?


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  1. GBA Editor
    Martin Holladay | | #1

    You're not going to be able to remove the Ice & Water Shield once it has been installed, so you'll have to leave it in place. Once it's glued, it's glued.

    If you want to install rigid foam on top of the roof sheathing -- and that's generally a good idea -- then you shouldn't install a ventilation channel under the roof sheathing. If you introduce outdoor air under the roof sheathing, then the rigid foam above the roof sheathing is on the wrong side of the outdoor air. It's pretty much useless. If you have the vent channel, the rigid foam might as well be hung on a clothesline in your back yard with giant clothespins for all the work it would be doing.

    After you have installed your 2 inches of rigid insulation (or whatever you need to achieve R-10) above the roof sheathing), the rest of the code-required insulation needs to be installed under the roof sheathing and in direct contact with the roof sheathing. This is an unvented roof assembly, so don't try to vent it.

    Here is a link to an article that discusses these issues: Insulating Low-Slope Residential Roofs.

  2. user-5254480 | | #2

    Thanks Martin. I wish I could buy you dinner. Your replies are consistently thoughtful and most useful :-)

    We can block off the existing vent ends easily as we are still all open below. And seal them tight. Sold.

    We tested pulling off the ice/water shield and it looks like we can get is off easily (it never got hot enough for the under glue to bond). Remove or not?

    What type of insulation do you recommend for the top side?

    What type of insulation do you recommend for the bottom side (give we will now have 12" deep with no vent)? The 20" OC is a bit of a challenge. I did measure again and the majority of the joists are in fact 16" oc, just a few 20" where they had to work around existing framing.

    Thanks again.

  3. Expert Member
    Dana Dorsett | | #3

    In zone 4C you may need a hint more than R10 of exterior foam for dew point control on 10" of rock wool, and certainly more if it's 12" of rock wool. The 10" of rock wool is about R43, the code presumes R49 total, anticipating R39 in the cavity. It's the ratio of foam-R/total that determines the average temperature at the roof deck, and it's the average temperature that determines the moisture risk. At 12" of rock wool or high density fiberglass you'd be looking at more than R50 in just the cavity, and would need at least R25 above the roof deck.

    But is it a true 12" TJI or are you talking milled 2 x 12 rafters (=11.25" nominal.)

    If the latter, 11.25" of damp sprayed or blown-in-mesh cellulose would be about R41 a bit closer to the ratio, and has quite a bit of moisture buffering capacity (an order of magnitude or more higher than mineral or glass fibers), which makes it a pretty safe bet. Taking that a bit further, a layer of Intello Plus or Certainteed MemBrain behind the ceiling gypsum would limit moisture uptake without reducing the trying rate. Intello is imported, and a bit tough to find (other than online), but MemBrain can be found through many distributors (even some Lowes stores in my area are starting to carry it.)

  4. GBA Editor
    Martin Holladay | | #4

    Q. "What type of insulation do you recommend for the top side?"

    A. Work with your roofer. Either EPS or polyiso can work, but remember that polyiso has reduced thermal performance in cold temperatures. Not all types of foam work with all types of roofing.

    For more details on this type of roof, see the article I linked to: Insulating Low-Slope Residential Roofs.

    You may also find some useful hints in this article: How to Install Rigid Foam On Top of Roof Sheathing.

  5. user-5254480 | | #5

    Dana - these are true 12" TJI. Does that help/hurt/influence your previous recommendation?

    If I did use comfortbatt, I could layer R15+R30 for R45 with a net depth of 10 3/4".

    What's the context of "in zone 4C you may need a hint more than R10"? The cyberregs docs stated 10. If I go with cork will that help with R degradation overtime?

    Thanks for all of the help.

  6. Dana1 | | #6

    The governing factor of risk to the roof from interior moisture drives is it's average winter temperature. Colder= longer season of moisture adsorption. If you put more R on the interior side, the roof deck runs colder, put more on the exterior, the roof deck runs warmer. But it's not a fixed R, it's the ratio that determines the average roof deck temp.

    IRC chapter 4 prescribes R10, but that is at the presumptive TOTAL of R49 prescribed in chatper 11. That limits the cavity-fill to R39. That's about 20% above the roof deck. If you have less than 20% the R value above the roof deck the roof deck runs colder, which means it also runs wetter, and more prone to rot. You can't put R10 on the outside with R75 in the cavity or you'll very likely run into problems eventually, because the roof deck will run significantly colder.

    With R45 in the cavity and R10 above the roof deck (R55 total) you would need an air tight class-II vapor retarder on the interior side to be protective, since you'd only have 18% of the total R value above the roof deck. If you don't fill the cavity up, you would still need an air barrier in contact with the interior side of the batts to prevent convection, even if the batts actually fit, but they don't. Batt widths are designed for 1.5" nominal dimensioned rafters & joists, not 0.5" thick TJI webbing, on 16" or 24" on center spacing, not 20". To insulate it with batts you'd have to find batts 19-3/4" wide (not a standard product), and you would have to cut them to fit snugly at the TJI flanges to avoid convection bypass channels/gaps. As a rule, TJIs call for blown or sprayed insulation.

    At a full 12", cellulose would come in between R42 & R45, and would fill the space completely with no compressions or voids. Damp sprayed mid-density cellulose can be simply sprayed into place without netting, dry blown or dense packed would require mesh. Assuming R45, you'd need at least R11.25 to hit the 20% total-R ratio. If you only put R10 of that, installing a smart vapor retarder becomes much more important.

    At 3" you would have an honest R12.5 if using Type-II EPS, or R11.5 if using the cheap Type-I EPS, either of which is enough for R45 cavity fill. At 2" with polyiso you would have a labeled R12-R13, but it's mid-winter performance in a 4C climate would only be R11-R11.5, but that's still going to be enough, since the performance during the shoulder seasons will be higher than the labeled R.

    So that's what I'm recommending: Either 3" EPS or 2" polyiso, and a full cavity fill of cellulose, not rock wool. MemBrain would then be optional (but in the long run it's cheap insurance.)

  7. user-5254480 | | #7

    Thank you very much Dana.

    I verified the the TJIs. They are 11 7/8" 360 series. And as you point out there are several areas where the spacing is not 16 or 24. I new better understand the blown/sprayed aspect of TJIs and top side to bottom side ratio needs - thanks.

    I am working on a plan using 2" ISO and MemBrain. We will seal up the ends of the existing bays where we originally planned for the venting to prep for the cellulose.

    Do think we should pull of the ice and water shield on the decking before putting the ISO down?

    Thanks again for your thoughtful help.

    I am going to post another question related to an additional flat roof with low slope ISO build up that is already installed. I think I need your sage advice there as well.

  8. Dana1 | | #8

    Leave the Ice & Water Shield. Roofs can't dry toward the exterior anyway without very special treatment, and 2" of polyiso by itself would impede drying toward the exterior about as much as the Ice & Water Shield does. As a self-healing membrane, the Ice & Water Shield improves overall air tightness.

  9. user-5254480 | | #9

    Thanks Dana.

    Another question for you after reading Martin's comments on his "Insulating Low Slope..." article.

    If I go with 12" of cellulose as you recommend - can i install cans in the ceiling?

  10. GBA Editor
    Martin Holladay | | #10

    You shouldn't ever install recessed can lights in an insulated ceiling assembly. The recessed cans leak heat, leak moisture, and displace insulation.

    Your options are:
    (a) Choose a different lighting scheme -- wall sconces, track lighting, pendants, etc -- that minimizes ceiling-mounted electrical boxes, or uses shallow airtight electrical boxes in the ceiling.

    (b) Choose new pancake-thin LED fixtures installed in shallow airtight electrical boxes.

    (c) Build a utility chase under your airtight insulated ceiling. The utility chase would need to be as deep as your recessed can lights.

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