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Air Barrier vs. Insulation

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  • #91
    Re: Air Barrier vs. Insulation

    Condensing surface is a better descriptor. When I was taught, our exercises were to find where the dew point occurred (condensation) in the assembly, hence why I phrase it like that.. :)
    “Racism is man's gravest threat to man - the maximum of hatred for a minimum of reason.”
    Abraham J. Heschel (Jewish theologian and philosopher, 1907-1972)


    • #92
      Re: Air Barrier vs. Insulation

      William Rose has a great section on this topic in his always-worth-checking-out book, Water in Buildings.


      • #93
        Re: Air Barrier vs. Insulation

        Mr Worthy thank you for the articles reference i awoke way to early today reading them sure came at the right time- lowering 10' ceilings to 8' and superinsulating, the insight about air reduction at knee walls is just the infor at a time i needed it.


        • #94
          Re: Air Barrier vs. Insulation


          Why not a hybrid approach with spray foam and cellulose? Most of our projects have stick framed, vaulted roofs...but our last new house used a conventional vented attic space above the second floor. I prefer simple and foolproof (even if somewhat more expensive) because I know it will work and is easy to test.

          After the second floor ceiling was rocked, I had my spf guys come in...flanged, plastic domes were put over all cans, fan bodies, etc. Then they sprayed about 5" of Icynene over the entire ceiling assembly, completely encasing all of the domes and sealing every possible penetration, plate line, truss edge, etc. Then we blew about 14" of loose cellulose over the top of that.

          The end result was a mid range cost roof assembly that relied on only 1 subcontractor to execute. Nobody else had to do anything special for it to be bombproof. The beauty is that it is very flexible, you can spray up and over trays, cover soffits with some cardboard and spray over them, spray kneewalls from the backside, whatever. Some very complicated assemblies may require some spray work before the sheet rock goes up...but you get my point.

          When I tested the whole house with a blower door after the insulating was done, we ended up some where between .6 and .7 ACH 50...we were guessing a little because the company doing the blower door test didn't have a small enough ring for his blower to get the house to stabilize.

          All things being equal, I like open cell foam better for something like this because it is softer and is less likely to crack away from a truss member when they flex under load.
          Attached Files


          • #95
            Re: Air Barrier vs. Insulation

            I just spent a good 1/2 hour responding to Allan's wall assembly.

            Clicked "submit reply" and the website logged me out.

            That JLC, for lack of a better word, sucks.


            • #96
              Re: Air Barrier vs. Insulation


              This has happened to me as well.

              I open a word document and type up my reply, do a copy and paste, that way I have my work saved in case this happens.
              This is a hard learned lesson and I still at times find myself relearning it.
              Last edited by Mark Parlee; 06-19-2011, 08:53 AM.
              Mark Parlee
              BESI(building envelope science institute) Envelope Inspector
              EDI Certified EIFS Inspector/Moisture Analyst/Quality Control/Building Envelope II
              EDI Seminar Instructor
              Level one thermographer (Snell)
              You build to code, code is the minimum to pass this test. Congratulations your grade is a D-


              • #97
                Re: Air Barrier vs. Insulation

                when you see open cell foam in a trade show you see a very consistant fine grain foam. when you see open cell on a job you frequently see cell structures with holes at identifiable diameters from 1/4 inch down. you occasionally see bigger holes. you occasionally see holes bigger than baseballs. if youare spraying walls and you are shaving them, then the bigger holes becaome obvious to the installers and they fix them (?). But, if you are spraying thick passes of open cell on a surface where shaving is not necessary it is very easy to get voids that will detrimentally impact airtightness you don't necessarily see.

                Open cell grows so magnificently that people overestimate hiow well it will seal. 'How can there be a void there?' was my most frequent lament when I used open cell.

                That is why I favor closed cell.

                sheetrock is a good air barrier. sealing seams in sheetrock, and penetrations in partitions with closed cell foam was my preference for years. then we had some occasions where tray ceilings and soffits that intersected exterior walls did not get sealed well enough. Due to technique, I believe these flaws would have been there even if we had sprayed open cell everywhere. Now we fill all hollow partitions around soffits before wew airseal over them. that gives clients an added benefit in sound attenuation and it si CHEAP insurance. The only problem with that is the occasional speaker ina wall or duct in a wall. in those places we still cap that individual wll cavity, then fill the soffit above with celluloes, lay in a membrane, airseal that, and blow oveer the whole mess with cellulose.

                cellulose over an airsealed shetrock deck works great in my central NY climate.

                on one occasion. using these methods did let me down sort of. after we finished, they were doing plaster in February and had fired a radiant floor for the first time. there was significantly more water in the air than there ever would be in a normal residence. the builder called complaing that he had condensation dripping out of CENTRAL VACUUM lines. yes, vacuum lines, not plumbing, ducts, or framing cavities.

                We ended up insulating the vacuum lines too.

                BTW, the house was pretty tight too.