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Proper Insulation Metal Siding & Roof

HeidiH | Posted in General Questions on

We are building in climate zone 2a (39470) Poplarville, Ms.  La.  It is slab on grade with footings. We are slightly raised-maybe 3 steps above grade. We are doing a metal roof , mainly metal siding with 2ft brick skirt except at garage which will be fully bricked. Builder is proposing open cell foam for an unvented conditioned attic as we have to put our HVAC in attic. We will have a dehumidifier.  I prefer to use Rockwool in the walls.  Siding is OSB with Tyvek housewrap & then R panel metal exposed fastener siding.  Rockwool for exterior insulation & we have 2×6 walls. Roof: hip/standing seam/no exposed fastener with synthetic underlayment & plywood then open cell foam.  Manufacturer install instructions for roof say apply directly to sheathing.  Does this wall/roof assembly pose a risk for condensation & rot anywhere?

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Replies

  1. Malcolm_Taylor | | #1

    HeidiH,

    Attics with open-cell foam can have moisture accumulation at the ridge, which can led to rot. Keeping the humidity low helps mitigate the risks, but it isn't as safe as using closed-cell foam. If you use open-cell, include a diffusion vent at the ridge. Using a permeable underlayment also provides a little drying.

    Depending on the profile of your metal siding it may form its own rain-screen. Safer is to mount the panels on furring. In each case you should vent the bottom. This is most easily done by using perforated stock for the L-flashing at the base.

  2. walta100 | | #2

    The way I see it you have chosen a sow’s ear of a building and are asking how the make it into a silk purse of a high-performance building.

    I think every dollar and more you “saved” by chousing the “low cost” metal building will be spent in any effort to make it a tight well insulated building.

    Open cell foam applied to the steel is a very risky game. As water vapor can pass thru the open cells get to the cold steel and condense into liquid water. Rust and mold seem likely.

    You may find this article of interest.

    https://www.greenbuildingadvisor.com/article/insulating-a-metal-building#:~:text=For%20insulation%2C%20use%20MBI&text=Basically%2C%20it's%20fiberglass%20insulation%20adhered,the%20insulation%20to%20the%20purlins.

    Walta

    1. Malcolm_Taylor | | #3

      Walta,

      It isn't a metal building, and no foam will be in contact with metal. They are just using metal roofing and siding on a conventional framed and sheathed house.

      1. HeidiH | | #6

        Correct - we were looking for as low maintenance as possible :)

  3. matthew25 | | #4

    My family is in Purvis. I know how humid it gets there. Closed-cell foam is definitely the right decision here, do not entertain open-cell even for a minute. And make sure your dehumidifier has return/supply ducts in the conditioned attic space since humid air is more buoyant it will rise to the highest part of your house. Wouldn't be a bad idea to put an air conditioner supply and return register up there too to make sure you have adequate air circulation.

    1. HeidiH | | #7

      Do you have closed cell foam? I inquired with the insulation company. He says they rarely use closed cell foam. He mentioned the open cell changing colors if for some reason there would be a leak. Again there are so many varying opinions on the spray foam in general but then also between using open cell vs closed. Both the roofer and the insulation company said that what is used the most in the south is open cell for roof & exterior walls. We ARE putting all HVAC equipment in the attic which is why we are not venting it so are you suggesting adding something additional?

  4. HeidiH | | #5

    Good Afternoon - sorry I just noticed all of the replies. I appreciate everyone who piped in. Since my question was posed a couple of things have changed. We don't have the option of using Rockwool anymore due to the order lead times due to shortage. My husband spoke with HVAC company & they are definitely on board with Spray foam in attic & exterior walls & have priced out according to these specs. Builder is on board with it. Insulation guy doesn't see any issues nor does the roofer so the building has proceeded with this in mind. My husband is getting frustrated with me constantly questioning & he wants to move forward as is. He is putting his trust in the team. There ARE a gazillion varying opinions on this matter out there & people either seem to hate or love spray foam. FYI - we are going with R-Panel siding now because it was too difficult to get the metal board & batten we wanted & have all trim/soffit etc match accordingly. I am limited with options where we are building. As I understand it the R-panel metal will be installed directly to the OSB. We will have a an approximate 2 ft brick skirt. Regarding a Diffusion Vent - is that something that can be done after roof has been decked & the underlayment is on? I asked about this & was told it was not necessary with the foam & conditioned attic. Also in my research I found varying consensus on the necessity of using one in the south with a conditioned attic and using spray foam so I am still utterly confused. I wish there was not so many varying opinions on this.

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