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Community and Q&A

Rigid Foam vs. Rockwool on the exterior

nynick | Posted in Energy Efficiency and Durability on

We’re still in the planning stages of our old house renovation. Since we’re replacing the siding, we’ll most likely be blowing in Dense Packed cellulose into the wall cavities so we don’t have to tear down the interior walls.

On the exterior, our energy consultant has suggested either 2″ of Rigid Foam or maybe Rockwool before the Hardi board siding. Are there advantages to either or cost savings as well?

Zone 5 coastal CT.

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  1. Expert Member

    The difference between them comes down to weight, rigidity, r value, and the amount of water vapor that each can pass at a given vapor pressure difference between the two faces.

    Rock wool is nice, because it's allows water vapor to pass through almost freely. It's fireproof, sound absorbant, and bulk water resistant. It's also heavy, expensive, and relatively hard to source.

    Rigid foam is nice because it's R value is a tad higher per inch, but nearly the same for all practical purposes, it's light weight, and has a lesser tendency to allow water vapor to pass through it. Depending on your climate this could be good or not as good. It's not fire resistant, and doesn't absorb sound as well, but it is significantly cheaper and easier to work with.

    You can use either successfully, and it's probably a good idea to add while you have the chance. If you use rockwool, you'll need to use furring strips to attach over the rockwool, into the studs, and then attach the hardiboard to that. It's far too soft to attach siding over directly. While I think you could probably attach hardiboard over rigid foam of that thickness, It would benefit from the furring strips as well.

  2. nynick | | #2

    Thank you. Based on your price estimates, you make a good case for rigid foam. We'd use furring strips either way.

  3. Expert Member
    Akos | | #3

    Rigid mineral wool is only used around me for commercial construction. Never mind the cost, you will have a very hard time fiding contractor that can work with it. Not easy to use as it is still somewhat squishy so it is hard to get the rain screen strapping flat.

    I would go for polyiso for your walls. In zone 5 you only need R7.5 for condensation control on a 2x6 wall, which is about the LTTR of 1.25" polyiso. That is thin enough that you can still nail up the rain screen strapping and saves the time and money of dealing with long screws. You can also look at permeable polyiso such as EnerAir which can add a bit of additional drying capacity to your wall.

    1. nynick | | #5

      Thanks. We have nominal 2x4 walls (old house), so I'll get about R-12-14 of blown cellulose in there. With the whole house covered in Blueskin WRB, you think 1.25" of Polyiso for a total of about R-20 will do it? Town code is R20 or R13+5. I'm trying to get it as tight as possible.

      1. Expert Member
        Akos | | #7

        Lot of this depends on cost. The reality is that a well sealed and insulated 2x6 wall is good enough (~R18 assembly). Adding rigid to does bump the R value by a fair bit but your incremental energy savings going from 1" to 2" is probably less than $10/year.

        A 2x4 wall does have much better ROI for extra rigid, but even there the 2x4 densepacked with 1.25" polyiso (~R20 assembly) is good enough.

        As always, the biggest savings and improvement in comfort come from air sealing, so I would focus on getting that sorted instead of extra rigid. Full peel and stick is good but only needed if your old walls have lumber sheathing. Taped seams on CDX/OSB sheathing work just as well.

  4. bongo30 | | #4

    If you are in termite area, I would go with Rockwool. That was a huge factor in our decision to use Rockwool for our exterior insulation.

  5. kyeser | | #6

    I would suggest two layers staggered of 1" polyiso. Polyiso is so easy to work with and you can use up all your cuts around windows and doors so there is virtually no waste. Also its super easy to source, and if you have extra sheets, you can return them.

    Find a good tape. Some wider aluminum tapes stick pretty good, others not so good. Zip tape sticks like crazy to foil face polyiso. Its a little bit more expensive, but I have seen plenty of aluminum tapes not adhere well after removing siding years later. Go with the zip tape, wipe the joints with a rag to remove any dust, roll it, and you have a great water resistive-air barrier.

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