Replies

1. 
   kbentley57 | Mar 20, 2023 01:42pm | #1

   David,

   Mineral wool typically hasn't been installed on sloped roofs, and it's unlikely you'll find many people willing to do it, short of doing it yourself.

   We'll need a little more detail - what dept rafters under the roof? What are your ground snow loads? Wind loads? metal roof type (standing seam or ag panel).

   If you're wanting a roof that is well insulated, and fire resistant, I'd be looking at an unvented roof with polyiso between two layers of decking sans ventilation gap. There are fire rated assemblies available for this type of setup that are very safe, lighter, and much less squishy than putting mineral wool on a slope.

2. 
   DavidM | Mar 20, 2023 02:04pm | #2

   Kyle,

   Thanks for your reply. Heavy snow loads will be designed for by my structural engineer, wind loads are not that much of an issue. Probably trusses. The vented (fire protected with screens) metal roof will be a panel system. I wasn’t planning on the mineral wool taking any load as the metal roof will be on battens (options for supporting battens tbd). I’m not so sure about polyiso, R loss over time, shrinkage, lower R in cold weather. I’m not sure why mineral wool would not work on this slope if it is not taking any load and is pinned by batten screws?
   Your opinion much appreciated - specially if you can convert me to polyiso!

   David

1. 
   kbentley57 | Mar 20, 2023 02:51pm | #3

   David,

   Maybe I was quick to reply without knowing all the details. If your metal roof is setting on some sort of z girt where the load is transferred directly to the sheathing/trusses, then having mineral wool there doesn't impose any problems, other than the additional weight.

   You're right to be curious about it, but there's a lot of research out there on shrinkage, temperature vs r value, etc. The shrinkage can me combated with multiple layers, staggering seams, tape, and all that. While mineral wool doesn't shrink, it's not a perfect material by any means, and you're destined to install it with a gap every now and then - don't think it'll for a continuous plane, while polyiso will not.

   One thing is for sure, there's no temperature so cold that the R value of polyiso will ever be less than mineral wool, at about 1/4 of the weight. When it's warmer, it'll be nearly 1.5x the R value.

   On the other hand, I'm not sure how to fit the mineral wool well in the web of the truss, without a lot of tedium with cutting it to fit well. Did you plan on blown in, or do your trusses have a top chord that's deep enough for a regular sized batt?

   Not trying to sell you either way, but to me MW has several strengths that make it suitable in some locations, and weaknesses in others. Same with polyiso. But for roofs, polyiso is the winner to me.

3. 
   DavidM | Mar 21, 2023 09:01pm | #4

   Kyle,

   Thanks for your reply. I was considering mineral wool for insulating the roof exterior in this case.
   I think encapsulating the polyiso with roof sheathing on top per the standard details would be what you recommend. It would also allow for more flexible connection points thru to the trusses specially where more are needed at solar panel locations to prevent wind uplift. Metal roof would be on battens for ventilation on top of this sheathing. (Vents in eaves and ridge fire screened even though this location not a very high fire risk landscape).
   What would you recommend for lower cost insulation on the roof interior. A simple rectangle with trusses 2’ O.C.? This is a conditioned attic space with overall insulation R 65 approx total for the roof.

   Thanks,
   David

1. 
   kbentley57 | Mar 22, 2023 11:32am | #7

   David,

   I think a good detail for a roof like this is something like as follows, from inside to outside, assuming 2x8 rafters or a 2x8 top chord on a truss:

   5/8" drywall
   R25 fiberglass batt (8" thick)
   19/32 sheathing (following the fortified roof quide lines for installation)
   synthetic underlayment
   6" (3x2") foil faced polyiso, staggered
   7/16" osb with radiant barrier / flame retardant - attached through with pancake head screws (headlok, spax, etc.)
   metal roof

   That will give you something like an R55 effective with the U value analysis, when air films, framing factors, and others are considered. It has a much higher balance of insulation on the exterior, keeping the interior 19/32 sheathing from condensing. It's very light, easy to construct, and cheaper than anything mineral wool would come close to.

   Don't be put off by fiberglass. R value is R value, and while it melts at a lower temperature than rockwool, by the time it mattered it wouldn't matter.

1. 
   DavidM | Mar 26, 2023 06:51pm | #12

   I have a zoning mandated height limit on the house so overall thickness of insulation matters. However, I’m going to create as much space as I can to allow for all options. Then I will factor labor and material costs along with everything else. So the answer is…procrastination!

4. Expert Member
   Michael Maines | Mar 22, 2023 10:17am | #5

   I have used mineral wool on the exterior of one roof, a renovation. We over-framed the existing roof with 2x6s on 2x4 horizontal sleepers and used 6" (~R-23) Rockwool batts, leaving space for ventilation. Over that we installed plywood and an asphalt shingle roof but a metal roof would work just as well. It has been in place for a few years now with no apparent issues.

1. 
   DavidM | Mar 26, 2023 06:47pm | #11

   Thanks!

5. 
   xbcornwellco | Mar 22, 2023 10:23am | #6

   Membrane.
   2 layers of mineral wool. (any thickness)
   2x4 furring strips over top insulation (creates vent)
   Plywood nailed to 2x4's (can be fire retardant)
   Membrane.
   Roofing.

1. 
   DavidM | Mar 26, 2023 06:46pm | #9

   Thanks!

6. Expert Member
   MALCOLM TAYLOR | Mar 22, 2023 11:48am | #8

   DavidM,

   R64 in the PNW seems very high by any metric I can think of.

1. 
   DavidM | Mar 26, 2023 06:47pm | #10

   I’m designing a long life house so that means for a hotter climate.

1. Expert Member
   MALCOLM TAYLOR | Mar 26, 2023 06:59pm | #13

   DavidM,

   Good point.