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Community and Q&A

How much interior insulation would be safe

smokey059 | Posted in General Questions on

I have a building 2000sqft that I inherited in zone 6a that was built with 2x6walls 16″oc, and exterior 5/8″ osb sheathing. It has house wrap(tyveck) over the sheathing then steel siding. The building is only a few yrs old but has been sitting unfinished in the inside. I want to build a 800 sqft living space into the building. The rest of the space will be a heated shop. The slab is insulated 4″ xps with vapor barrier and it has foundation edge insulation 2″ on the outside.

I would like to airseal the wall as much as possible from the inside since I can’t get to the outside and then fill the stud bays with either blown in dense packed fiberglass or stuff them full of mineralwool. I would then like to put some polyiso 2″ then frame a 2×4 interior wall with osb attached to the exterior of this wall for an interior air barrier. I would then use this interior 2×4 wall for a service cavity and then fill it with mineralwool or batts and finally the drywall.

My question is will this be to much insulation for this wall since there is no external insulation on the exterior osb sheathing and no furring strip rainsreen. I cant take all the siding off and start with the exterior so need to know what would be an appropriate amount of insulation I could use without causing moisture problems in the wall assembly.

I guess I’m looking for some suggestions on how to insulate a building like this without problems. I read (not this site)that r19-r22 is all that can reasonably be done on a wall like this. I didn’t build this my dad left it to me with 20 acres

All buildings in this area are just filled with fiberglass batts or closed foam sprayed too 3″ then interior vapor barrier then drywall. I wont do either of those though. Also no blown cellulose installers around here just one guy I found for blown fiberglass to 2.2cf

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  1. Expert Member
    Dana Dorsett | | #1

    There is usually air space/channels in the steel siding, which allows the OSB to dry toward the exterior.

    In zone 6A it will need an interior side vapor barrier on the insulated 2x6 wall, but a layer of polyiso is extremely vapor retardent. Even fiber face polyiso is a Class-II vapor retarder, but foil faced is a true vapor barrier (Class-I vapor retardency.)

    Air sealing the polyiso with the appropriate tapes on the seams and can-foam would be important. The interior side facer is the first condensing surface, but with R12 polyiso on the exterior of the 2x4 wall you already have plenty of dew point control on any fiber insulation you put in those stud bays. The additional thermal performance of the insulated 2x6 framing exterior to the 2" polyiso layer gives it a large dew point margin- the fiber in the 2x4 wall cavities will remain dry.

    Dense-packing the 2x6 walls with 2.2lb fiberglass isn't going to buy you much performance over 1 lb fiberglass, as long as the sheathing is caulked to the framing and any seams are taped with the appropriate tapes. A 1lb fiberglass isn't very air retardent, but if it's between two reasonably detailed air barriers (the exterior OSB, and the taped polyiso) it doesn't much matter. The higher density yields a ~15% higher center-cavity R on the 2x6 layer but only 5% on that stack layer after factoring in the thermal bridging of the 2x6 framing, and only a couple percent in total wall performance.

  2. smokey059 | | #2

    Thanks for your reply. So the foil faced polyiso would work as the appropriate vapor barrier.
    Thanks for the advice on the fiber glass densities. I would have never caught that.
    Would there be much difference between the 1 lb fiberglass blown and fillng the stud bays with rouxl(mineralwool). The mineralwool I could do myself always preferable for me

  3. Expert Member


    The drying potential of the OSB to the exterior is very dependant on what type of steel siding you are contemplating. Corrugated profiles provide a large enough cavity that they meet the requirements of rain-screens set out in our code here in coastal BC. Some other profiles really need strapping.

  4. smokey059 | | #4

    The steel siding is from Midwest Manufacturing called residential double 4 lap siding. The siding is already on the building. I did not know that it could be considered a rainscreen. It definitely has an air space behind it when installed. As far as code goes I don't worry about that where I live. No one comes out to inspect anything. When you call them up (I have more than once)they just say build it to code. When I asked what code they couldn't even give me a code name. Finally I just asked would the Irc code be appropriate they said yea like they had no idea. It's a poor county all rural.
    Now insurance is another story. If not code and something fails they'll try and stiff you. Most folks just have the electric and mechanical done up to code and do what they want on the rest.

  5. Expert Member
    Dana Dorsett | | #5

    R23 rock wool is probably cheaper than 2.2 lbs blown fiberglass, and might be comparable with 1lb blown fiberglass. R23 rock wool is as air retardent as 2.2lbs fiberglass, if fitted perfectly.

    If you're a perfectionist who likes to DIY things taking the time to do it right, there's a bit of craft to it, but there are no rocket scientists wasting their careers installing batts. This is totally DIY-able. It's worth buying purpose made batt knife even for small projects (or risk raising hell at home doing it with a bread knife.)

    Both the blue & orange box stores are carrying rock wool now. If the local store doesn't stock it, it can be ordered online an drop-shipped to your local store. Building supply distributors catering to building contractors might be able to beat them on price, but won't bend over backwards to order small quantities of something they don't do volume sales in.

  6. smokey059 | | #6

    Thanks Dana. I have worked with roxul before it's not bad to work with. I'll be taking my time as I'm retiring. I'll still price out the blown fiberglass and see how it compares financially to the roxul. What a great site this is. To bad the contractors around here don't learn from it.

  7. Expert Member

    That profile looks like it has a lot of drying potential.

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