Insulation in exterior walls

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Author Message David L. Heuring, AIA, CCCA Senior Member Username: daveh Post Number: 13 Registered: 04-2020 Posted on Tuesday, October 11, 2022 - 01:37 pm:    Interested in comments: Pro's and Con's of using 3 1/2" batts in 2 x 6 studs. I realized there are a myriad of scenarios, but we have R-5 CI on the exterior, 2 x 6 studs at 24" o.c. for structural reasons, and only need R20 or R-13+5 (CI) per code. If the 3 1/2" unfaced friction batt insulation is installed tight to the exterior sheathing (with straps to keep in place where necessary) and there is a gap between the insulation and interior gyp bd, then what comments would you have as a designer and/or specifier? Specs indicate "Install batts to fill entire stud cavity, with no gaps, voids, or areas of compression" so this would need to be coordinated with a wall type/section on the drawings. David L. Heuring, AIA, CCCA, LEED AP, NCARB Wayne Yancey Senior Member Username: wayne_yancey Post Number: 951 Registered: 01-2008 Posted on Tuesday, October 11, 2022 - 01:58 pm:    Your assembly is perfectly acceptable. The only comment is use mineral wool batts versus fiberglass. Mineral wool batts are heavier/denser than fiberglass. More rigid during install. For thermal and acoustical control, mineral wool has slightly better performance than fiberglass. While, both unfaced fiberglass and unfaced mineral wool are noncombustible and help delay the spread of fire, mineral (stone) wool does have a much higher melting point than fiberglass, and as such it is often considered the more fire-resistant material. I think one of the fiberglass batt manufacturers is now producing a denser product to mitigate the slumping of traditional fiberglass batts in the stud cavity. No personal experience if this is true. John Bunzick Senior Member Username: bunzick Post Number: 1897 Registered: 03-2002 Posted on Tuesday, October 11, 2022 - 02:43 pm:    My instinct is that the insulation should be tight to the sheathing so that there is no air space between the two layers of insulation. This would prevent air circulating within each stud cavity that would reduce the effectiveness of the assembly. I'm not sure if the building science supports this approach. Also, if using breathable, paper faced fiberglass batts with the stapling flange, that could be easily used to support the insulation. A question to consider, though, is why not use thicker insulation? What's the cost differential between 3-1/2 inch and six? Wouldn't there be additional energy savings? I guess that comes down to why build only to the minimum, unless the project's climate is really such that there is no difference. Or, if the heat/air/moisture dynamic is such that this creates a condensation plane where it is not desired. David L. Heuring, AIA, CCCA Senior Member Username: daveh Post Number: 14 Registered: 04-2020 Posted on Tuesday, October 11, 2022 - 02:44 pm:    I am wondering why it is so difficult to find any industry backup for this 'design'. To minimize risk, it is always better to be able to defend a design with backup vs. stating that 'I just thought it was more cost efficient...' David L. Heuring, AIA, CCCA, LEED AP, NCARB ken hercenberg Senior Member Username: khercenberg Post Number: 1521 Registered: 12-2006 Posted on Tuesday, October 11, 2022 - 05:00 pm:    It's an interesting question, especially if it's true that labor costs are the lion's share of the equation. Obviously it would up to the owner as to whether or not the additional cost was worth it to them. Ever use "bagged" fiberglass insulation like JM ComfortTherm Hi Perm - https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Data%20Sheets/Resi%20and%20Commercial/JM_BI_ComfortTherm_DS_BID_0146.pdf It makes handling and installing a lot easier on the Installers than unfaced fiberglass. To that end I would prefer handling mineral fiber over unfaced fiberglass, all else being about equal (except cost and maybe availability). Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 411 Registered: 08-2004 Posted on Tuesday, October 11, 2022 - 05:59 pm:    David, Try these websites for backup, then run a WUFI analysis: https://www.pacerepresentatives.com/wall-bods/ https://www.buildingscience.com/ https://www.owenscorning.com/en-us/insulation/commercial/enclosure/applications/walls - Greta Eckhardt (Unregistered Guest) Unregistered guest Posted on Tuesday, October 11, 2022 - 08:17 pm:    The issue with thick insulation in the stud space is that the dewpoint may move into the wall where moisture could condense and cause mold, corrosion and other water damage. There is no universal answer for your question because the answer will depend in part on the climate of the building's location, as well as the permeance of the sheathing and other materials in the exterior wall. Consulting the references Lisa suggests would be an excellent first step, followed by hygrothermal analysis such as WUFI. Phil Kabza Senior Member Username: phil_kabza Post Number: 726 Registered: 12-2002 Posted on Wednesday, October 12, 2022 - 03:30 pm:    Place some emphasis on John Bunzick's comment regarding air circulation within the stud space. If the airspace left over after the batt installation is large enough, a convective loop may be induced, somewhat degrading performance. How much? Maybe a building scientist like Joe Lstiburek knows. Back in my prehistoric superinsulated house days, we read that cavities should be full even to the point of slightly compressing batt insulation to reduce potential for air movement. Phil Kabza FCSI CCS AIA SpecGuy Specifications Consultants www.SpecGuy.com phil@specguy.com David L. Heuring, AIA, CCCA Senior Member Username: daveh Post Number: 15 Registered: 04-2020 Posted on Wednesday, October 12, 2022 - 04:55 pm:    Phil (and John)- I am an old school Architect- contractor. I agree with you. I learned at Ball State University- Architecture college and CERES. We calculated Dew points by drawing graphs thru wall sections using u-values of the components, including air spaces. I keep going back to the risk management inherent in design and specifications- bucking a manufacturer's recommendations leaves us standing there alone with the responsibility! Keep it coming- all good opinions!!! David L. Heuring, AIA, CCCA, LEED AP, NCARB Greta Eckhardt (Unregistered Guest) Unregistered guest Posted on Wednesday, October 12, 2022 - 05:01 pm:    If one installs 3-1/2" batt insulation in 6" stud space, as long as it is secured tightly to the exterior sheathing, I don't think there would be much in the way of convection in the air space between the insulation and the interior gypsum board, since it would be at interior room temperature. It seems to me that convection would be a problem only if there were an air space between the insulation and the exterior sheathing. In one sense the best thing would be to pack the 6" stud space with 6" of insulation so there are no air spaces, but at that point one might have moved the dewpoint into the stud space, depending on various factors as I mentioned earlier. Brian E. Trimble, CDT Senior Member Username: brian_e_trimble_cdt Post Number: 133 Registered: 08-2005 Posted on Wednesday, October 12, 2022 - 11:50 pm:    Maybe I'm missing the question, but continuous insulation (CI) would be better than batt AND continuous insulation due to thermal bridging through the studs. With CI you have the same thermal profile anywhere on the wall, while it varies if you mostly have batt insulation between the studs. Of course, we add more wall thickness with just CI. And if you have an air barrier would there be many convective loops? I think it would be minimal, but with batt insulation this could still have an impact and you want to fill it as much as possible. And maybe that's another reason for CI - you don't have to worry above crevices, you just have to make sure the board is continuous. Phil Kabza Senior Member Username: phil_kabza Post Number: 727 Registered: 12-2002 Posted on Thursday, October 13, 2022 - 06:40 am:    Brian: I believe you are correct. I think we fool ourselves with framing cavity insulation, especially with metal studs. There's a wide range of quality control on jobsites with respect to insulation installation because of which the real world performance may or may not achieve the theoretical numbers. CI is easy to inspect; stud cavities are not, especially if you specify faced batts. A modest crack alongside a batt bypasses the entire batt, which is another reason why stud cavities should be filled or over-filled. Phil Kabza FCSI CCS AIA SpecGuy Specifications Consultants www.SpecGuy.com phil@specguy.com John Bunzick Senior Member Username: bunzick Post Number: 1898 Registered: 03-2002 Posted on Thursday, October 13, 2022 - 04:03 pm:    One of the downsides of thicker continuous insulation is that it can complicate the cladding installation. R-5 insulation is probably only about an inch thick, which can be dealt with by long fasteners. As the insulation gets thicker, that no longer works so well. Then you need to have a system of cleats, and they, too, are a thermal bridge.