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Exterior Foam Insulation

| Posted in Energy, Heating & Insulation on August 20, 2002 02:28am

Hey gang. I live in Northwestern Ontario and have a 25 year old home with 2×4 stud walls, unfaced fibreglass batt insulation and a poly vapour barrier on the inside (between studs and dywall). I am residing with hardie plank and would like to put 1 1/2 ” or 2″ of 3M blue styrofoam insulation board over the exterior sheathing and underneath the siding, as with our winters the house is expensive to heat. I am also replacing all the windows and adding another R 20 to the attic. The question is, will the rigid insulation board cause conflict with the poly vapor barrier and in effect give me 2 vapour barriers and an in wall condensation problem? 

 Thanks for any and all input. Jason.

Reply

Replies

  1. NormKerr | Aug 20, 2002 07:51pm | #1

    there have been tons of discussions on this board about putting vapor barriers on both sides of the walls (and why they should be avoided), I recommend you to search the archives under insulation, vapor barrier, moisture, cellulouse, foam insulation and whatever else you can think of.

    Very useful information that many spent much time to provide.

    Andy (web moderator), any hope of a "frequently asked questions" section on this site? This one would definitely qualify.

    1. JasonKehl | Aug 21, 2002 05:12am | #2

      Sorry Norm, did not mean to inconvenience but read the forum here regularily and do not recall this specific situation, perhaps I missed it or have forgotten, will check and see if I can dig it up somewhere.

         I thought the extra R Value offered by the foam would offset the vapour barrier aspect of it as compared to just putting poly on both sides of the cavity as I know that would be wrong. A carpenter I work with thinks the foam would be alright  because of where the dewpoint would be, but readily admits that a second vapour barrier would be a bad idea, just looking for a verification or refutation of this concept.

      Jason.

      1. Gabe | Aug 21, 2002 08:19pm | #3

        In theory....it wouldn't be adviseable to add a second VB.

        In reality....given the age of the house and your climate...you will be fine to add the sm on the outside as described and all will be fine.

        Gabe

        1. JasonKehl | Aug 22, 2002 01:26am | #4

          Thanks a bunch Gabe.  Seems to me like it would be alright, but don't want to mess up my house either.

             Jason.

          1. Gabe | Aug 22, 2002 03:37am | #5

            The long of it Jas is that in a new home that may contain an unusual amount of moisture in the wood from rain and such, we can trap some moisture. But trapping moisture is not an easy sport. I've seen it done but it takes a lot of planning and hard work.

            Most houses, even the R-2000 homes, do leak air to some degree. Sealing the inside is not always easy to do during construction. We cover the walls, ceilings with poly and caulk the seams. Put plastic behind all the electrical boxes and such..........then we drive 4,000 screws through the plastic installing drywall. Freespinning screws do quiet a number on the plastic.

            So even with all the care in the world, there will always be breaches to the system.

            Air moves from the basement to the attic through all the walls via electrical and plumbing wires and pipes holes. Ducts will add to this condition to an incredible level.

            Essentually, your house has dried for 25 years in a fairly dry/cold climante and it would be next to impossible for you to do anything on the outside with sheets of sm that would cause you any concerns. Even with shiplapped sm the all will still not be able to trap moisture to any degree.

            Quick lesson to everyone on wall systems.

            For water to leak into your home or building 3 things must all be present.

            The first is water. ( the enemy)This could be rain, snow or even moisture in the air.

            The second is an openning for water or air to enter(opportunity).

            The third is a difference in pressure.(motive) This can be caused by something as simple as temperature differences inside wall of the envelope, or even wind on one side of the house causing a vacuum on the other side or an over active exhaust fan causing a negative pressure inside the house..

            Eliminate any one of these three and it's impossible for water to enter through a wall system or roof.

            Class over

            Gabe

          2. JasonKehl | Aug 22, 2002 04:04am | #6

            Thanks again Gabe. I appreciate the time and effort to help me out.

               Jas.

          3. Gabe | Aug 22, 2002 04:10am | #7

            You're most welcome.

            Gabe

          4. MartinHolladay | Aug 22, 2002 12:52pm | #8

            Gabe,

            It is possible for water to enter a home even if your third factor (a difference in pressure) is absent.  As any roofer knows, water can enter a roof assembly or a wall assembly by gravity, even in the absence of a pressure difference. 

          5. Gabe | Aug 22, 2002 02:36pm | #9

            Sorry but the formula is fact.

            The effects of gravity on a drop of water would be the same as a pressure differential.

            Gabe

  2. User avater
    rjw | Aug 22, 2002 03:05pm | #10

    Jas,

    What town and neighborhood; I might have some suggestions, I look at a lot of house in the Toledo area and some developments have some hidden, er, issues, especially from that era, because of all of the changes taking place in construction theory and techniques

    Bob Walker

    Paradigm Inspections

    1. JasonKehl | Aug 23, 2002 04:56am | #11

      Hi Bob. I live in Kenora Ont, about a 2 hour drive east of Winnipeg. What kind of issues are you refering to? My home is pretty basic, concrete footing to bedrock, block wall full basement, 2x4 framing on main level, simple truss roof with a 4/12 slope on 16"centres. My dad built it, was a truck driver not a carpenter but seems to have done a good job of constructing it, just not very energy efficient by todays standards. Once again, any input would be appreciated.

         Jas.

      1. User avater
        rjw | Aug 26, 2002 03:42am | #16

        Must have been a senior moment - I thought you'd said NW Ohio now I see you said Ontario.  Never mind {G}

    2. User avater
      Patrickjd9 | Aug 23, 2002 06:05am | #12

      What town and neighborhood; I might have some suggestions, I look at a lot of house in the Toledo area and some developments have some hidden, er, issues, especially from that era [1970s-25 years old], because of all of the changes taking place in construction theory and techniques. 

      Slightly off-topic but, we bought a house late last year after looking at over 50 of them, and kept returning to houses built in the 1960's.  A lot of the 1970's houses we looked at hadn't aged well.  Hardboard siding failing, windows almost flush with the siding, small or no roof overhangs. 

      Edited 8/22/2002 11:10:03 PM ET by Patrickjd9

      1. Tomasaur | Aug 24, 2002 04:13am | #13

        A few minor side questions here:

        Do you put tar paper or tyvek under the rigid foam?

        Do you nail furring strips to the foam so that you can then create a secondary plane for infiltrated water to leak down?

        Thanks,

        Tom

        1. Tomasaur | Aug 24, 2002 07:07am | #14

          A quick clarification:

          What I meant to say was do you put tar paper or tyvek under the foam as opposed to OVER the foam.

          Seems like the tar paper vs. tyvek issue has been thrashed about enough...

          Tom

          1. Gabe | Aug 26, 2002 03:28am | #15

            In most applications the addition of a second vb is not required. The sm is nail or glued directly to the concrete and the seams are shiplapped.

            The strapping is only to attach the drywall directly against the sm without any space loss.

            SM is a vb if joints are taped.

            Gabe

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