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Venting and Insulating a Mono-Slope Roof

DanShow | Posted in Energy Efficiency and Durability on

Hello,
Located in Zone 5 and currently finishing up details on an upcoming build this summer and have some questions regarding our roof assembly. Have been researching and reading heavily trying to educate myself for this project. We are leaning heavily towards a Pretty Good House level of build, trying to be as green and energy efficient as our budget allows.
Advice, experience, and feedback is very much appreciated.

Exterior house plans are attached.

Summary: Essentially we have two mono slope roofs that come together to form our roof line. Lower roof is 3/12. Upper roof is 6/12. As you can see there are 4 windows in the upper wall section. Have been planning on running an unvented assembly, but am open to options if I am missing something. I have a few options on potential roof assemblies and not sure which would work best in our situation. (All from inside out)

Option 1: (after much reading, not sure polyiso directly against roof sheathing is what we want)
1-Drywall
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- 1.5”/2” Polyiso Foam
5- 1.5”/2” EPS or GPS Foam (taped seams, seams staggered and opposite of first foam layer)
6- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
7-Standing Seam Metal Panels/Legacy Panels

Option 2: (polyiso free + smart barrier)
1-Drywall
1.5 – Intello Plus (taped)
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- 1.5”/2” EPS or GPS Foam
5- 1.5”/2” EPS or GPS Foam (taped seams, seams staggered and opposite of first foam layer)
6- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
7-Standing Seam Metal Panels/Legacy Panels

Option 3: (single foam, not sure this will get us to the Rvalue we need on the exterior side, unless we went with a thicker single layer of foam)
1-Drywall
1.5 – Intello Plus (taped)
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- 1.5”/2” EPS or GPS Foam (taped seams)
5- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
6-Standing Seam Metal Panels/Legacy Panels

Option 4: (double wool + foam free)
1-Drywall
1.5 – Intello Plus (taped)
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- 1.5” Rockwool Comfortboard
5- 1.5”  Rockwool Comfortboard
6- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
7-Standing Seam Metal Panels/Legacy Panels

Option 5: (single wool, not sure this would keep roof sheathing warm enough)
1-Drywall
1.5 – Intello Plus (taped)
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- 1.5” Rockwool Comfortboard
5- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
6-Standing Seam Metal Panels/Legacy Panels

Option 6: (no exterior insulation, not sure if this makes sense since we are running exterior insulation on walls)
1-Drywall
1.5 – Intello Plus (taped)
2- Rafter Insulation (Rockwool batts or blown in cellulose)
3- Zip Roof panels (this is my WRB, taped and sealed)
4- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
5-Standing Seam Metal Panels/Legacy Panels

Option 7: (no exterior insulation, “interior Mooney Wall”, this could potentially be similar to our interior wall assembly)
1-Drywall
1.5 – Intello Plus (taped)
2- 2x4s running horizontal (Rockwool batts)
3- Rafter Insulation (Rockwool batts)
4- Zip Roof panels (this is my WRB, taped and sealed)
5- combination of 1×4 or 2×4 (unsure yet which to use) screwed down at 45 degrees and cora-a-vent at top and bottom (this would be my venting portion directly under the metal panels)
6-Standing Seam Metal Panels/Legacy Panels

Have also considered running the Pro Clima Solitex Um membrane directly under our metal panels.

Thank you and please ask questions if I am missing something.

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Replies

  1. Expert Member
    Akos | | #1

    With a simple mono slope roof without any dormers, you can easily vent the entire assembly. Venting should be bellow the roof deck not above it. You don't need to vent under metal roofing panels, they work perfectly well installed directly over sheathing or strapping.

    If you are looking for a high R value, low cost roof, your best bet is to go with I-joists (size sets your R value, 14" is pretty good), staple house-wrap / fiberboard / insulweb to the underside of the top flange to form the vent channel and dense pack the roof. If you can reduce your slope to bellow 4/12 on both sides, you can even loose fill saving some installation cost.

    I guess what I'm saying, is that I would pick option 8.

    If the clerestory windows are serving the same space, it is simpler to go with wider windows instead of having to trim around two closely spaced ones. The header can be raised to be in-between the rafters (ie. rim board header here: https://www.finehomebuilding.com/project-guides/framing/maximize-insulation-in-minimum-headers). This would let you reduce the height of the roof step plus you won't need siding above and bellow the window.

    One item to watch with south facing clerestory is where the sun will fall throughout the year. I didn't fully consider this and I have a period in the year where there is just too much glare in the kitchen.

    P.S. Having a way to open some of the clerestory windows really helps with natural ventilation, I use it all the time.

    1. DanShow | | #3

      Akos,
      Thank you for the reply and for the link. Couple questions I have after reading your response.
      1. Running a vented assembly as you describe makes sense to me, my question is at the ridge of each mono slope roof, how do I vent that portion?

      2. I had not previously considered making the clerestory windows 2 larger units rather than 4 smaller units, but I think I might like that better. Not only from a detailing and siding perspective, but also aesthetically. Also thank you for mentioning the glare issue from your south facing clerestory windows, I think our position will be okay, but gives me something to think about.

      3. When you open your clerestory windows, do you have to use a ladder or do you have a platform built up under them? I am trying to think about the interior design to make operable windows work in that location.

      4. If I went the vented assembly route as you described, could I use mineral wool batts or would blown in be my best choice?

      5. Would you still have exterior insulation in this assembly? If so, foam or mineral wool in your opinion?

      1. Expert Member
        MALCOLM TAYLOR | | #6

        Dan,

        You vent the peak much like you would if it was the ridge of a pitched roof. The flashing at the peak (a bit like half a ridge-cap) sits on the ribs of the roof panels. In between are vented closure strips that allow air-movement, which your metal roofing supplier provides.

        1. DanShow | | #8

          Malcolm,
          Thank you for the reply and information you provided, it is much appreciated.

      2. Expert Member
        Akos | | #7

        1. For the upper section you can also use soffit vents as exhaust vent. For the lower section use the detail Jon linked to. In a pinch I have used regular ridge vent material (stuff that looks like a furnace filter) cut into strips between the roof standing seams as a vent covered with a regular flat ridge cap.

        3. I used to use a ladder but eventually moved the operable window over to the loft so I can access it from the stairs. You can also get a curb mount skylight which come with either a crank or electric remote to open. I've never done it but can't see why you couldn't mount a curb mount skylight on a vertical wall.

        4. You can use any fluffy insulation you want. Just make sure to order batts for metal studs as these are wider and fit snugly between the I-joist web. Blown loose fill cellulose tends to be the cheapest if you can get your roof slope down to 4 in 12.

        5. Since it is a vented assembly, you don't need any rigid insulation. You do need a solid warm side air barrier ( ie. drywall + taped poly installed with care), make sure you figure out how to have this continuous out to your wall air barrier and across the step for the clerestory.

        1. DanShow | | #9

          Akos,

          1. My first thought when trying to vent this assembly was to do what you are describing in #1. Basically run soffit venting at both the top and the bottom with adequate venting channels running up the rafters. Was thinking at least 2''?

          3. This is good to know, thank you.

          4. Not sure I understand this part, "Blown loose fill cellulose tends to be the cheapest if you can get your roof slope down to 4 in 12." Are you saying that from the perspective that my 6/12 roof is going to take more cellulose, therefore it might not be the cheapest option?

          5. So you would advise not rigid foam in a vented assembly? Would I be putting myself at a disadvantage by putting exterior insulation on the walls and then not putting any exterior insulation on the roof?
          My thought of drywall plus Intello Plus (taped and sealed) would cover the warm side air barrier is that correct?

          1. Expert Member
            Akos | | #11

            The min went gap depends on your local code. On a pitched roof generally anything above 1" works well enough, 1.5" is a common gap.

            A low slope roof can be insulated with loose fill as it will settle evenly and won't create voids. Steeper cathedral ceilings need to be insulated with batts or dense packed.

            Loose fill tends to be the cheapest insulation option for R60+ roofs, if you can make it work it saves a bit on the cost. Batts also work great and not too expensive, so I wouldn't go way out of my way to loose fill.

            There are many high performance houses that have no rigid insulation in the roof, provided you have the correct R value and you have an air tight ceiling, it will work great.

  2. Jon_R | | #2

    +1 on a vented design and making sure that moisture can easily make it into the vent (no low perm, cold Zip). Review below for the minimum (more is better) foam you need for an unvented design.

    https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies#t01

    1. DanShow | | #4

      Jon R,
      Thank you for the reply and for the link as well. Couple questions I have after reading your response.

      1. Can you explain your comment "(no low perm, cold Zip)"? I am not following.

      2. Wanted to be sure I am reading the chart in Table 2 correctly that you linked to, for my zone (5) I want at least R20 on the roof if I am going to be at R49 for my assembly? If we are wanting to be at a total of say R6O or R65 or even R80 for the roof, we would need to be at 40% of our total assembly on the exterior side of the sheathing to control condensation is that correct?

      3. Table 2 calls for CC Spray Foam, but reading the information from Figure 4 in the article, it sounds like I can use "anything"?
      "In Figure 4 the rigid insulation can be “anything” as in Figure 1. The main stipulation is the thermal resistance of this rigid insulation based on Table 1. The “air permeable insulation” can be pretty much anything as well…just as long as the thermal resistance of this layer does not violate the “ratio”."

      1. Jon_R | | #5

        #1 - just make sure there isn't Zip (or anything low perm) between the Winter moisture source (the interior) and the moisture exit (the vent channel). Zip above the vent channel is fine.
        #2 - yes, and check your local code.
        #3 - yes, or perhaps IRC R806.5 is clearer. IMO, cellulose is better than batts.

        Should be useful:

        https://www.ascprofiles.com/vented-peak-metal-flashing/

        1. DanShow | | #10

          Jon R,
          1. So if I run Intello Plus under my drywall followed by either mineral wool batts or blown in cellulose, then my vent channel, directly under my roof sheathing that should be good? If I do site built vent channels from foam board, is that an issue in this set up? And just to be clear, don't run foam board (low perm) directly under my roof sheathing?

          2. Sounds good, thank you.

          3. This is more clear, here's a couple questions.
          "4. In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation." "5.1.1. Where only air-impermeable insulation is provided, it shall be applied in direct contact with the underside of the structural roof sheathing."- Does this basically mean only CC Spray foam works directly under the roof sheathing?

          5.1.2. Where air-permeable insulation is provided inside the building thermal envelope, it shall be installed in accordance with Section 5.1. In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing in accordance with the R-values in Table R806.5 for condensation control. - So my plan to go EPS or GPS foam above the roof and mineral wool or blown in cellulose below the roof fits into this category? What about if I wanted to use mineral wool both below the roof and above it?

          5.1.3. Where both air-impermeable and air-permeable insulation are provided, the air-impermeable insulation shall be applied in direct contact with the underside of the structural roof sheathing in accordance with Item 5.1.1 and shall be in accordance with the R-values in Table R806.5 for condensation control. The air-permeable insulation shall be installed directly under the air-impermeable insulation. -Again is this basically only CC Spray Foam?

          5.1.4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). For calculation purposes, an interior air temperature of 68°F (20°C) is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months. - Think I have a handle on this part, need at least R20 if going with an R49 roof assembly (or 40% of my total roof r value) on the exterior of my roof to keep away condensation issues?

          5.2. Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer. - Is this meaning seams are staggered and taped if using multiple layers of rigid foam above the roof sheathing or below the roof sheathing?

          Long response and more questions I apologize, but I would rather ask lots of questions, it is much cheaper than the alternative.
          Thank you for the link to the vented flashing, have you used this product before?

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