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Ridgid foam on the outside of the house..vapour barrier??

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  • Ridgid foam on the outside of the house..vapour barrier??

    I am considering removing the vinyl siding and the wannabe sheathing made of asphalt impregnated fiberboard (used to call it Muskoka siding), and replace with rigid closed cell foam board. I have however been told that it is a vapour barrier if it is to thick..do not want that on the outside. Any advice on the maximum thickness that could be safely installed and not cause a moisture problem on the exterior of the studs.

    Bill
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  • #2

    Re: Ridgid foam on the outside of the house..vapour barrier??

    Re: Ridgid foam on the outside of the house..vapour barrier??

    Why? What's wrong with your so called Muskoka siding and what are you putting on for siding?
    "Do it Right!"

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    • #3

      Re: Ridgid foam on the outside of the house..vapour barrier??

      Re: Ridgid foam on the outside of the house..vapour barrier??

      Originally posted by billyt View Post
      I am considering removing the vinyl siding and the wannabe sheathing made of asphalt impregnated fiberboard (used to call it Muskoka siding), and replace with rigid closed cell foam board. I have however been told that it is a vapour barrier if it is to thick..do not want that on the outside. Any advice on the maximum thickness that could be safely installed and not cause a moisture problem on the exterior of the studs.

      Bill
      The typical offerings in Extruded Polystyrene (XPS) from DOW (blue colored) or Corning (pink colored) will qualify as a 'vapor retarder' under most (all?) Canadian building codes if it is 2" thick. It comes down to a rating called the 'perm' rating on how much moisture can migrate from one side of the product to the other in a given time period.

      Most exterior cladding type solutions are either made from Extruded Polystyrene (EPS or the white crumbly foam) or are limited to 1/2" to 1" thick XPS for this reason.

      You are definitely on the right track however. Ensuring that your walls do not become a moisture trap is key when thinking about augmenting your insulation with exterior foam.

      Even with only 1/2" of XPS applied to the exterior under your siding, or about R-2.5, you substantially reduce thermal bridging that occurs through your wall studs, sill stud, and top plate studs. A 2x4 wall with R-12 fiberglass is NOT an R-12 wall. A 2x4 has an R value on edge of about R-2 or R-3. I wont do the math here, but its fairly easy to calculate the square footage occupied by studs at the R-2 or R-3 value, and then blend that area with the R-12 fiberglass cavity area for an overall R value. I think it usually averages out to about R-10 or so. My personal biggest concern for XPS on the exterior of a home is the fact that it is highly flammable. Foam faced polyiso is much less flammable, but is much more costly.

      I'm not a pro, but have done a TON of research on this topic. I used 2" thick XPS against my basement walls before studding, insulating the stud cavities with roxul and drywalling. City of Calgary building inspector passed my 2" XPS as a 'vapor retarder/barrier' with no issues. The inspector just examined a few areas to ensure I had a fire/flame barrier over all installed XPS (either roxul or roxul + drywall) since it was *inside* the building envelope. Exterior wise I think you are fine without a specific fire barrier but call your local inspector.

      Also, check the install instructions for your siding product before you start. Heavier products like HardiPlank will only certify installs over styrofoam with certain limitations.
      Last edited by calgaryguy2; 06-24-2014, 07:56 PM.

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      • #4

        Re: Ridgid foam on the outside of the house..vapour barrier??

        Re: Ridgid foam on the outside of the house..vapour barrier??

        Just to clarify the nomenclature. Muskoka siding is a name that civilized homebuilders used to disparage a product also called tentest. It was the beloved sheathing of Ontario tract builders of the not too distant past. I'm sure anyone over fifty is familiar with the product.

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        • #5

          Re: Ridgid foam on the outside of the house..vapour barrier??

          Re: Ridgid foam on the outside of the house..vapour barrier??

          Well, I,m over 50 an don't relate to those descriptions, Is that X90 siding you are referring to?

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          • #6

            Re: Ridgid foam on the outside of the house..vapour barrier??

            Re: Ridgid foam on the outside of the house..vapour barrier??

            When I replaced the pine shiplap on the east side of my house due to rot I added 1 or 1.5'' blue xps under it The wood started rotting almost imeadiatly and I wonder if the foam is responsible at least in part.
            Rob

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            • #7

              Re: Ridgid foam on the outside of the house..vapour barrier??

              Re: Ridgid foam on the outside of the house..vapour barrier??

              Originally posted by iamtooler View Post
              When I replaced the pine shiplap on the east side of my house due to rot I added 1 or 1.5'' blue xps under it The wood started rotting almost imeadiatly and I wonder if the foam is responsible at least in part.
              Rob
              I cant see how. XPS doesnt really hold moisture of any noticeable content, but it definitely wouldnt breathe as good as the wood siding over sheeting. Building Sciences Corp has some suggestions for strapping over top of your foamboard exterior and then anchoring your siding to the strapping to add a breathing space behind the siding. You definitely need to do your homework before adding EPS, XPS, or PolyISO board to the exterior of your home to ensure you are accounting for all the variables.

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              • #8

                Re: Ridgid foam on the outside of the house..vapour barrier??

                Re: Ridgid foam on the outside of the house..vapour barrier??

                Originally posted by calgaryguy2 View Post
                I cant see how. XPS doesnt really hold moisture of any noticeable content, but it definitely wouldnt breathe as good as the wood siding over sheeting. Building Sciences Corp has some suggestions for strapping over top of your foamboard exterior and then anchoring your siding to the strapping to add a breathing space behind the siding. You definitely need to do your homework before adding EPS, XPS, or PolyISO board to the exterior of your home to ensure you are accounting for all the variables.
                Excellent advice. One of those variables is rotting INSIDE the structural wall.
                "Do it Right!"

                Comment


                • #9

                  Re: Ridgid foam on the outside of the house..vapour barrier??

                  Re: Ridgid foam on the outside of the house..vapour barrier??

                  Originally posted by calgaryguy2 View Post
                  I cant see how. XPS doesnt really hold moisture of any noticeable content, but it definitely wouldnt breathe as good as the wood siding over sheeting. Building Sciences Corp has some suggestions for strapping over top of your foamboard exterior and then anchoring your siding to the strapping to add a breathing space behind the siding. You definitely need to do your homework before adding EPS, XPS, or PolyISO board to the exterior of your home to ensure you are accounting for all the variables.
                  This IS my homework! The original shiplap was nailed to the wall through a sheet of tarpaper in 1920 and the back of the siding was untreated. Lasted 60 years. I made the new siding myself from KD pine and soaked each piece in Pentox before applying, using end dip for every cut. It was primed and painted shortly after and painted again 5 years later.
                  After 20 years I could poke a finger through at many places. Foam looks great underneath!
                  Rob

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                  • #10

                    Re: Ridgid foam on the outside of the house..vapour barrier??

                    Re: Ridgid foam on the outside of the house..vapour barrier??

                    Interesting discussion that I am following with interest as I have a house built in 1912 that has nect to nothing in the walls for insulation and is clad in cedar shingles and it is a constant struggle to keep up with peeling paint (from moisture passage I assume).

                    I have considered opening up the walls from the outside and using spray foam for a sealant/insulation between the studs and then wrapping the house in styrofoam and cladding with either plywood and vinyl or stucco over the foam.

                    No hurry to do either one, but I gather that an air gap between foam and wood sheathing is important to prevent rot where there is no interior vapor barrier.
                    Erik

                    Canada's Island Paradise - Prince Edward Island

                    Proud member of the Wadkin Blockhead Club

                    Comment


                    • #11

                      Re: Ridgid foam on the outside of the house..vapour barrier??

                      Re: Ridgid foam on the outside of the house..vapour barrier??

                      Originally posted by ErikM View Post
                      Interesting discussion that I am following with interest as I have a house built in 1912 that has nect to nothing in the walls for insulation and is clad in cedar shingles and it is a constant struggle to keep up with peeling paint (from moisture passage I assume).

                      I have considered opening up the walls from the outside and using spray foam for a sealant/insulation between the studs and then wrapping the house in styrofoam and cladding with either plywood and vinyl or stucco over the foam.

                      No hurry to do either one, but I gather that an air gap between foam and wood sheathing is important to prevent rot where there is no interior vapor barrier.
                      Erik be very careful. Cedar shingles should never be painted. As Rob mentions above and you mention here, Paint and or Pentox could be the problem issues. Although I'm not totally familiar with Pentox, [having never used it] I can GUARANTEE painted wood will most definitely rot. Paint is a vapour barrier and if allowed to trap water under it's surface the result will be rot! An interior painted wall forms a VB as does an exterior painted wall. Two VB's are unwanted in a wall structure. It is admittedly a bit of a stretch here but I'm just offering a caution.
                      "Do it Right!"

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                      • #12

                        Re: Ridgid foam on the outside of the house..vapour barrier??

                        Re: Ridgid foam on the outside of the house..vapour barrier??

                        Cedar shingles should never be painted.
                        Rusty,

                        I agree with your comment, however, the paint is already there and unless I want to spend all year with the heat gun on three buildings to strip it, I figure it is easier to just keep the paint in good repair. The two newer building have a vaport barrier inside so not much moisture getting out through the wall.

                        I have one wall of a newer garage where shingles only six years old are rotten because water leaked from two poorly installed windows behind the shingles and the resulting water vapor rotted out the shingles under the paint. That whole wall is getting stripped and resealed and clad this summer as the problem windows have been replaced. Don't know how the PO missed this but I will fix it.
                        Erik

                        Canada's Island Paradise - Prince Edward Island

                        Proud member of the Wadkin Blockhead Club

                        Comment


                        • #13

                          Re: Ridgid foam on the outside of the house..vapour barrier??

                          Re: Ridgid foam on the outside of the house..vapour barrier??

                          Originally posted by iamtooler View Post
                          This IS my homework! The original shiplap was nailed to the wall through a sheet of tarpaper in 1920 and the back of the siding was untreated. Lasted 60 years. I made the new siding myself from KD pine and soaked each piece in Pentox before applying, using end dip for every cut. It was primed and painted shortly after and painted again 5 years later.
                          After 20 years I could poke a finger through at many places. Foam looks great underneath!
                          Rob

                          The most obvious answer was that the siding has not able to breathe sufficiently from its backside. Why? Not sure.

                          Comment


                          • #14

                            Re: Ridgid foam on the outside of the house..vapour barrier??

                            Re: Ridgid foam on the outside of the house..vapour barrier??

                            It's not a type of siding at all. It's a type of sheathing. It's low density fibreboard soaked in asphalt. The Muskoka siding name arose because people would build cabins and sheath them in the stuff and then never get around to covering it with proper siding.

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                            • #15

                              Re: Ridgid foam on the outside of the house..vapour barrier??

                              Re: Ridgid foam on the outside of the house..vapour barrier??

                              Chris if painted wood cracks open in even a very small crack and water gets under the paint it has no way out so the paint becomes a water trap and the result is rotten wood.
                              "Do it Right!"

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