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foil with airspace or insulation board?

Post in 'The Green Room' started by 4acrefarm, Mar 3, 2013.

  1. 4acrefarm

    4acrefarm Member

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    I'm in the very early stages of a house design and planing wall insulation
    2x6" on 24 center with 2" closed cell foam an fill with cellulose. I am thinking of Roxul 1.5" on outside. on the inside I am considering 2 options.
    1, foil then horizontal 2x2s then Sheetrock.

    2, rockwool 1.5" then Sheetrock.

    any thoughts?

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  2. EatenByLimestone

    EatenByLimestone Minister of Fire

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    I'd just go with sprayed foam. If you want to save cash, the sprayed foam and then the blown cellulose. Adding complexity really won't make it insulate any better. I like radiant barriers, but I don't think they would give you as much in a wall as in the ceiling where it is easier to build in the air space to make them work to their potential. The sprayed foam will make sure you have a tight structure. I'm going through the hassle of airsealing a 1946 house now. If they had foam back then my job would be much easier.

    Matt
  3. 4acrefarm

    4acrefarm Member

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    I currently have an 1850s farmhouse and have been working on it for years so, I here you.
  4. woodgeek

    woodgeek Minister of Fire

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    If I understand...you want to do a 5.5" cavity with a little closed cell and the rest cellulose. You get the benefits of closed cell (airsealing) but the cost is closer to cellulose. This is a pretty conventional thing so far.

    Your assembly R-value is still low because of thermal bridging...so you are putting sheet insulation on the outside. Never heard of using Roxul on the outside...I would go with something like poly-iso or EPS foam. If cost is an issue...you can buy 'used' polyiso torn off other projects by the truckload very cheap. http://www.insulationdepot.com/

    The 24" OC, that's a little weird. Why? To reduce thermal bridging....the exterior sheets take care of that and a lot of the bridging is the headers and footers, etc..

    Now, you want radiant barrier on the inside....no. Don't see it. To get the extra R-1 from the airspace + foil, you could just add 1/4" on the exterior sheet. Roxul outside and in? Huh? Why?

    So, you've clearly given this a lot of thought...but there are a number of advanced wall designs out there that you can cost. There seem to be two main approaches (1) Like you, a 2x6 framed cavity with OSB on the outside and drywall on the inside, and taped foam sheets (usually 2") on the outside to airseal and avoid thermal bridging. and (2) double stud walls basically a 10" cavity with (2) parallel 2x4s on 16" centers. The OSB is on the outer, the drywall on the inner. The whole cavity gets filled with cellulose (the cheapest insulation for the $$) and there are no thermal bridges (b/c of the gap b/w the studs is filled with cellulose. Each window and door hole gets boxed in with plywood for a clean opening. Both approaches get you R-27 or R-30 walls (!!), respectively, and are cheaper than your stack. With (1), the problem is getting good mech support for the siding furring through 2" of foam....usually requires fancy long screws. With (2) you have to find a builder that can do double wall framing.

    Go look at the drawings at BSC....the devil is in the details...and they have the drawings. http://www.buildingscience.com/index_html
    PapaDave likes this.
  5. DickRussell

    DickRussell Member

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    Woodgeek has good advice. To his I would add that in western MA you at least ought to avoid that foil layer inside, as well as polyethylene or other impermeable sheet goods. There are times when a wall cavity ought to be able to dry to the interior, and foil or poly will prevent that. We're talking molecular diffusion here, not convective transport of moisture via air flow (leakage). Worry about air sealing (air barrier). To control moisture transport by diffusion, all code requires is a one-perm vapor retarder (not barrier). That can be provided by vapor retarder latex primer on the sheetrock. Even just a couple of coats of latex paint will give you enough reduction in permeability to prevent moisture problems by diffusion, although the BI may insist on the VR primer at least. If he insists on a sheet of poly, he's ignorant; wake him up, show him good references.

    My own house has a double exterior wall, filled with dense-packed cellulose. Double walls aren't rocket science. If you go that route, any competent framer ought to be able to execute your wall construction plan. The trick is finding a framer who has done it before (less likely) or at least is willing to work with you to build what you want (more likely). The latter is what I did. If a proposed builder doesn't want to build what you want and says he's been building for 30 years, don't tell him how to build a house, tell him to go build for someone else, that you don't want a house built the way it was done 30 years ago. For my house, I provided the wall design, the engineer provided the stamped drawings showing the construction for the BI to approve, and the builder implemented it. I left matters of interior framing and other things not really related to the exterior shell to the framers; I'm not a carpenter. Overall, the project went smoothly, and I got what I wanted.
    woodgeek likes this.
  6. 4acrefarm

    4acrefarm Member

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    thanks for the responses.
    Woodgeek ,yes the 24 oc for thermo bridging The roxul comfortboard is is vapor permeable." http://www.roxul.com/residential/products/roxul comfortboard is" I like the idea of using recycled foam-board, thanks for the link.
    Dick, The foil I was thinking of is perforated, but regardless it appears to be unnecessary.
    Thanks, Gary
  7. DickRussell

    DickRussell Member

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    Gary, even if the foil is perforated, it's overall permeability for molecular diffusion of water vapor likely will be insufficient for drying. Diffusion is an area-based thing, and if even 10% of the surface area is hole area, then the foil material still is 90% impermeable. It's good that you regard it as unnecessary.
  8. seige101

    seige101 Minister of Fire

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    Where abouts are you building in western ma? I am in Palmer my self.
  9. 4acrefarm

    4acrefarm Member

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    Pittsfield, but it is years away. I am still working on my old house.
  10. DBoon

    DBoon Minister of Fire

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    buildingscience.com is a great resource - they have lots of standard wall plans and comparisons. One thing that is clear in reading through their materials is that foam on the outside of the building is really the way to go as opposed to double wall construction, especially in a cold climate.
  11. DHill

    DHill New Member

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    I agree, our house was built about three years ago. I am very happy with how the walls turned out. My builder agreed to pour the foundation to allow enough space between the wall sheathing and brick to fit inch and a half blue board.The R9 foam board and R15 wet cellulose blown in to 2x4 wall cavity together total R24. If I remember right total cost was $4500 for board and another $1500 the framers charged to attach it after they put on the tyvex wrap. This was cheaper than double wall according to the builder.

    DH
  12. DBoon

    DBoon Minister of Fire

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    Hi DHill, what treatment did your builder use on the outside of the walls over the 1-1/2" foam to attach the siding to?
  13. DHill

    DHill New Member

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    My house is brick, so I am not certain about siding. But I believe with siding that furring strips with longer nails was used to attached siding.

    Also just to clarify my house walls are standard 2x4 stick built frame with half inch wood sheathing then tyvex wrap. Then inch and half foam board then half inch space between foam and brick.
    DH
  14. Chain

    Chain Feeling the Heat

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    I have a bit of an off topic question about insulation. My primary water pump is located in my basement directly under my bedroom. This past summer I purchased a nice well water treatment system that automatically completes its "recharge" process every Monday morning from 2:00 AM until about 3:45 AM. Because this requires substantial water from my dug well (which the aforementioned pump is used for), the constant running of the pump wakes me and the Mrs. up nearly every week.

    I've considered either reprogramming the control for the recharge to a different time of day, or simply insulating the floor of my bedroom to help with noise reduction. I'm seriously considering using Roxul as it's much less of a pain to install compared with fiberglass insulation. It's also apparently better at insulating for noise. Has anyone used Roxul? Not the foam board they also make that was mentioned above, but their regular wool product that comes in precut batts? If so, have you found it to be a good noise insulator?
  15. semipro

    semipro Minister of Fire

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  16. DickRussell

    DickRussell Member

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    Chain, if you have plumbing fastened to the floor joists, that could be a good part of the noise transmission issue. I ran into that; the plumbers had fastened copper hangars directly to the I-joists, and the hissing sound of water rushing through the pipes was quite loud above that area. I detatched the hangars and wrapped them around rubber stoppers screwed into the joists. The rubber damped that sound frequency quite well. I also wrapped the pipes in that area in closed cell insulation, and that helped, too.
    woodgeek likes this.
  17. Billy123

    Billy123 Member

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    Sips with ICF foundation.
  18. Chain

    Chain Feeling the Heat

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    Thanks for the info., gang.....In my case, DickRussell, the noise isn't from piping, but specifically from the motor on the pump kicking on when the pressure switch on the pressure tank does it's thing. The problem is this pump is just below my bedroom and the only thing between it and the above living space is a sub floor, carpet pad, and carpet. I've never bothered to insulate this space before as the pump wouldn't typically run given I'm not usually using water at that time of day (again 2:00 AM Monday night). But now I'm thinking I should insulate and not re-program the timer on the treatment system. An added benefit is that I would also cut down on the noise from my boiler that occasionally kicks on late at night (not very often as I use a pellet stove for my primary heat). Again, thanks for the info......
  19. Bret Chase

    Bret Chase Minister of Fire

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    I don't think you'll be able to get away with 2x6 @ 24OC...

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