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Community and Q&A

Reference to your article “Smart Retarders for Walls and Roofs,” July 29, 2016

southernbuilder123 | Posted in General Questions on

I am in the process of installing Roxul R30 batts in a catedral ceiling. The roof is metal with some type of rubberised barrier under the metal roof. The roof sits on 2 x4 perlins. The Roxul batts will leave a 1 and 3/4 inch space between the batts and the roof

I will then install a nominal 6″ wide v groove white pine ceiling. Do I need a vapor retarder under the pine ceiling? If so what would be best in my climate in central Georgia?

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  1. GBA Editor
    Martin Holladay | | #1

    Southern Builder,
    Your planned assembly raises many questions. (For an overview of the issues raised by your question, see this article: How to Build an Insulated Cathedral Ceiling.)

    1. Are you sure the R-30 batts will meet minimum code requirements? In your climate zone (Zone 3), I would imagine that the building code calls for a minimum of R-38 roof insulation.

    2. You mention that there will be a 1 3/4 inch space between the top of the batts and the underside of the roof sheathing. But will you be including the other required elements of a ventilation gap? You will also need (a) ventilation baffles to provide a top-side air barrier and to reduce wind-washing, (b) soffit vents, and (c) a ridge vent. For more information on these components, see Site-Built Ventilation Baffles for Roofs.

    3. The type of roof assembly that you are describing definitely needs an air barrier on the interior side of the insulation. Pine boards are not an air barrier -- they are extremely leaky. If you build the assembly as you propose, interior air will rush through the cracks in your ceiling, escaping through the ridge vent. You'll have very high energy bills and (eventually) sheathing rot due to condensation on your cold roof sheathing.

    The best air barrier in your situation is taped drywall. Once you have installed a drywall ceiling -- detailed with attention to airtightness -- you can install boards on the interior side of the drywall if you want.

    Now that we have addressed these three issues, we can answer your question:

    Q. "Do I need a vapor retarder under the pine ceiling?"

    A. No.

  2. southernbuilder123 | | #2

    Thank Martin, Here is more details I left out in my inquiry.
    The metal roof has a synthetic underlayment "Grace Mfg." and is taped at all seams. The 1 3/4 space mentioned above the rockwool insulation is clear from soffit to the ridge vent. There is 28 square inches of vent opening between each rafter bay.
    The area to be heated and cooled is 672 sq ft. This space is above an detached garage and will only be used for storage and part time for other purposes. The inspector has approved the R-30 ceiling insulation since this will not be considered a residence.


    1. Since the rockwool batts fit the bay very tightly, do I still need ventilation baffles?

    2. I thought I could use 'Intello Plus" to create an effective Perm rating, I am assuming from your comment concerning drywall that the rating is too low without the drywall?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Since the rockwool batts fit the bay very tightly, do I still need ventilation baffles?"

    A. My standard advice is: Yes, you need the ventilation baffles, because rock wool insulation is air-permeable. There will be wind-washing as the air moves through the ventilation gap. Wind washing lowers the thermal performance of the insulation. The reason for the baffle is to provide a topside air barrier. You need this topside air barrier to reduce wind-washing.

    That said, the effect of wind-washing depends on the density of the mineral wool insulation. Studies have shown that very dense products can withstand wind-washing without a big thermal penalty. (For more information on this topic, see Windwashing in Exterior Mineral Wool.)

    Before you apply the lessons of that article to your case, remember that the type of mineral wool panel used on the exterior side of wall sheathing is much denser than the type of batt you intend to use between rafters. Because you are using a product with a lower density, wind-washing is more of a concern than with panel-type mineral wool products.

    Q. "I thought I could use Intello Plus to create an effective perm rating, I am assuming from your comment concerning drywall that the rating is too low without the drywall?"

    A. You are still confusing vapor barriers with air barriers. In your climate zone, you don't have to worry about vapor diffusion or vapor barriers. The perm rating is irrelevant. But you do have to worry about air leakage. For more information on these issues, see these articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

    So you don't need a vapor barrier. But you do need an air barrier.

    Can Intello Plus act as an air barrier? Yes, as long as it never gets ripped and it isn't compromised by fastener penetrations. Because you will be putting hundreds of holes in the Intello Plus when you attach your board ceiling, there are worries about whether the membrane is a durable air barrier.

    Experience shows that drywall works in this type of assembly. Drywall is durable and less fragile than Intello Plus. I would use the drywall if I were you.

  4. GreyWolf92 | | #4

    Martin - Is a soffit vent absolutely essential for roxul insulated roof, if I have a 2" gap on the interior and a 1" gap on the exterior (airflow above the WPB) for a shed roof?

    Because of width restrictions, I literally only have 1.5" eave on each side. I'm trying to figure out if I can get away with a 1/2" soffit vent made from insect screen. If you don't think this will work, do you have any alternative suggestions?

  5. GBA Editor
    Martin Holladay | | #5

    There are lots of vent products available, including fascia vents and vents that introduce air at the roof slope. Google "fascia vent" to see some of them.

    As long as you introduce air at the bottom of the ventilation channel, it doesn't matter where the air comes from -- it can come from the soffit, from the fascia, or from slots near your dripedge.

  6. GreyWolf92 | | #6

    Hi Martin,

    Is this what you mean by a vent introduced at the slope?

    When you say introduced at the bottom, do you mean the lower side of the sloped roof? I am planning on having vent on the top (taller side) of the shed roof as well.

    I will have a 2" air gap above the roxul insulation with a fascia vent as you suggested. And there will also be a 1" ventilation gap on top of the WPB (under metal roof). Do you think this is sufficient? In your response to SOUTHERNBUILDER123, you mentioned a ridge vent but I assume that is not needed for my shed roof?

  7. GBA Editor
    Martin Holladay | | #7

    What you are calling the "taller side of the shed roof" is the ridge. That's where the ventilation air exits. If you have some type of vent up there, as well as one that allows air to enter near the eave, you are all set.

  8. GreyWolf92 | | #8

    Excuse my construction vocabulary, learning as I go!

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