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Community and Q&A

Insulating Metal-Framed Building

Jeffb105 | Posted in General Questions on


I have a metal framed building that I will be using as a shop and am in the process of figuring out my insulation. Right now it’s just framed and not sheeted yet. The roof will be metal panels along with the walls. Eventually I would like to frame in a portion of it for some office space/living space. I live in climate zone 5a. My heat source is radiant heat flooring with an insulated and vapor barried slab.

If you can let me know if my idea sounds good or not that would be great. My plan is to sheet the roof and walls with 1″ foiled faced polyiso board. The metal panels would be screwed through the  boards into the metal purlins.

Would the 1 1/4in ribs of the metal panel be enough for an airspace or would I be better off facing the foil towards the inside of the building? With me framing this in eventually there will be a small portion of attic that is created. All taped and sealed as well.

I also planned with a large amount of left over of 2in eps board to cut and fit them into the metal beams/purlins to create a barrier to help with possible condensation of the cooler metal. Or would the polyiso create enough of a thermal bridge to the interior metal?
Eventually with part of it being framed and heated I would like to add additional insulation, most likely blown in. Would it better to have the insulation blown into the void between the metal purlins/rafters or to blow it against the ceiling joist?

Any help would be greatly appreciated or possible changes to my plan.

Let me know if you have any other questions.


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  1. matthew25 | | #1

    A radiant barrier is not really needed in your climate, but even so I vote for putting the foil facing outwards because it is a better WRB than the backside which may not repel water as well.

    For the loose fill, I’m assuming your roof assembly is sealed and not vented? Are you conditioning the “attic” space or only the living area beneath the ceiling joists? If the whole space then put the insulation at the roofline. If just the living area of the building then I guess it doesn’t matter much and you can stick to just the ceiling joists. I think putting it at the roofline would be best practices though and reduce any condensation potential.

    1. Jeffb105 | | #2

      I planned on soffit and ridge vents but wasn't sure if putting the insulation on the roofline made a difference with venting or not.

  2. walta100 | | #3

    Putting insulation between the steel framing is a waste of time and materials as the steel conducts the heat around the insulation rendering it useless.

    Generally, this type of building leaks air badly bypassing any insulation.

    If you want a tight well insulated space inside your barn you will end up building a building inside the barn.

    You will find a lot of better advice about this type of building on the Garage Journal forums.

    This site doesn’t see this style building as high performance in terms of how much energy is required to heat and cool the space. While the Journal looks at it in terms of construction costs per cubic foot.


  3. Expert Member
    DCcontrarian | | #4

    We had a long discussion recently on some of the issues with a heated slab, in this thread:

    Agree with Walta that insulation needs to go completely inboard of the metal framing, the thermal bridging of the metal negates any benefit to insulation alongside the framing.

    Note also that in climate zone 5 for an inhabited building you should have R20 in the walls and R49 in the roof. That's basically 5.5 inches of insulation in the walls and 12+ in the roof. So as Walta says you'll be looking at a building within a building to get those levels. The easiest way to get the roof levels is to close in the ceiling and pile up 12+ inches of either batts or loose insulation above the ceiling. Then ventilate the space above.

    Metal siding is tough because it will be a vapor barrier so your walls will have to dry to the interior. This means you have to have sufficient impermeable insulation against the metal to prevent condensation.

    1. Jeffb105 | | #5

      Would the 1" polyiso board be enough for the vapor barrier against the metal?
      I'll have plenty of space up top to put the 12in but only 5.5 in walls about leave me a pretty sizeable air gap in-between the insulation and the metal siding. Would that be fine?

      Per code I'm supposed to have a vapor barrier on the warm side of my insulation. Would I have a problem with the polyiso board being a vapor barrier for the metal roofing and a vapor barrier against my ceiling as long as I had proper ventilation in the attic?

      1. Expert Member
        DCcontrarian | | #6

        A gap between the siding and the insulation is fine. You want to seal it up because rodents will love to nest in there. You may also be required to have fire-blocking to prevent the spread of fire.

        Vapor barrier on the warm side is only for assemblies that can dry to the exterior. With metal siding I don't think that's possible, so what you have to do is build an assembly that won't allow condensation on the cold side and can dry to the warm side. Continuous foam on the cold side and then unfaced batt insulation should work. The code allows for R5 foam and R13 batt which you can do with a 2x4 wall and 1 inch of polyiso.

  4. Jeffb105 | | #7

    Alright I think that answers all the questions I needed.
    When I go to frame in the living spaces I will just sheet the wall with the polyiso before I stand the wall up.
    Would it ever be beneficial to fill the dead space in between the metal siding and polyiso? Or just leave that as cold air and the venting can take of the rest?

    I appreciate all the help!

  5. walta100 | | #8

    "When I go to frame in the living spaces I will just sheet the wall with the polyiso before I stand the wall up."

    It is a bad idea to leave polyiso exposed as it is a fire hazard Most location would not allow it.

    Even if there is no inspection the risk is simply too great should the tiniest fire get to the foam its smoke could make escape impossible.


    1. Jeffb105 | | #10

      Well the polyiso would be on the outside of the wood framing covered by batt insulation and then drywall/paint. Then it would no longer be exposed, correct?

      I was thinking to add a little more r-value would it be smart to add an inch or two of ridgid foam to the outside of the wood framing then polyiso over that with taped seams?

      Cold siding
      Air space
      Polyiso as vapor barrier
      Ridgid foam
      2x4 framing
      R13 unfaced batt

      1. walta100 | | #12

        The foam can not be exposed from the outside of the wall. Needs to be cover with something rated to give you time to escape.


  6. Expert Member
    Akos | | #9
  7. freyr_design | | #11

    They make insulated metal panels, many of which have clip systems that reduce thermal bridging. I’m not sure how you building is designed but I’m not sure why this would not be a good option, perhaps someone else can note the cons.

    I suppose a lot does depend on where you shear is coming from, but I believe most metal building use x bracing at end walls, again someone more familiar feel free to correct this.

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