Cost-Effective Energy-Smart Wall Assembly
Back story. Building my first and personal home. My plan all along has been to flash and batt. Because of Covid and costs our budget like any one else’s is hemorrhaging. Because that was my plan I didn’t air seal from the outside. I will have an unconditioned vented attic.
I live in southern MN. Zone 6. In a pinch I’ve been researching and reading for 2 days straight trying to find a cost effective way to make my house as tight as possible and insulate. My potential plan and questions are below I’m praying someone with have the time for some valuable input on.
Air sealing:
Walls/Floor:
Caulk every stud bay all the way around where the OSB Sheathing meets the framing. Caulk top plates, double studs, bottom plate (slab on grade foundation), headers, studs along top and bottom plate and around the face of the stud to top or bottom plate (Everywhere). Closed cell foam for any small or tight bays I cant get at with caulk. Foam all windows and doors. Spray back of electrical boxes, and caulk the gab between the rock and boxes in the front.
Ceiling:
Spray closed cell foam over framing to sheetrock joints. Spray all can lights, electrical boxes, ect.
Insulate walls with:
Rockwool or OC Thermafiber – Walls – Spending a little more here over fiberglass makes sense to me and is still half the cost of the flash and batt.
20″ cellulose – Attic.
Questions:
Any issues with the proposed route? Any different suggestions?
The infamous do I need a vapor barrier question? But for real should I use something like membrane? It seems from all the reading that I don’t need either if I just create a vapor retarder with my paint and do a good job air sealing. This applies to both the ceiling and walls correct?
I considered the “Airtight Drywall Approach” but I feel like over time and especially after the first couple of years I wont want to address any small cracks that end up popping up.
Any suggestions on caulk? It seems as though I want an acoustical caulk that never sets up? Something like OSI SC175? Green Glue seems spendy? What have you guys used and had success with that doesn’t break the budget?
If I overlooked something or you have any questions obviously ask and I’ll get back to you.
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Replies
Do you have a 2x4 or 2x6 wall? I’m assuming the latter as it’s zone 6, but thought I’d ask in any case.
You are correct 2x6 walls.
Peel & stick house wrap - like Henry Blueskin. Move the air barrier to the sheathing.
+1 on fully adhered WRB. Also consider cellulose in the walls. Be sure to meet the guidelines here:
https://www.appliedbuildingtech.com/system/files/200311_abtg_rr_1701-01_moisture_control_guidelines_figure_update_final.pdf
If you don't have the siding on yet, the easiest and cheapest is to take of the house wrap, tape the seams of your sheathing and re-install the housewrap. Peel and stick over the whole thing is a bit of an overkill but does work.
In cold climate without exterior rigid insulation you need a warm side vapor retarder. Standard poly is the cheapest and works quite well. You can detail this as your secondary air barrier (make sure to get vapor tight electrical boxes) and not have to worry about drywall cracking. Green glue sealant is more expensive but worth the price of not having black gooey mess over your clothes, tools, car, sometimes even makes it to the sofa at home.
The SPF in the attic is the best option. Make sure to get the top of your exterior walls as well to connect your sheathing air barrier to the ceiling.
Hey Akos sorry should of pointed out the siding is on which is why I'm in the predicament that I am.
Would my suggested route of caulking be an appropriate location for the stage I'm at? I was hoping if I'm stopping the air flow through the wall that I could avoid a vapor retarder such as poly or membrain, and just stick with the paint as the vapor retarder.
What is SPF? By it being the best option your saying if I go around with a froth pack and spray all the connections and seal up any other penetrations. I'd be good to blow cellulose at that point. No vapor barrier needed? If so why is it not needed there vs the walls?
I also was thinking about getting the vapor tight electrical boxes but was thinking if I just sprayed the back tight, and caulked around the faces it would serve the same purpose.
Thanks!
Caulking the perimeter is the next best thing. Lot of work though. If you have any horizontal seams, it is best to tape those with a quality sheathing tape.
I would do a quick blower test after the ceiling goes in and before the walls are drywalled to check. Fixing leaks is easy at this stage and worth the money for the test. You DIY test with a box fan in a window if it is colder outside.
SPF is spray foam. The two part kits are definitely worth it for that type of a job. You can also seal up your rim joists at the same time with it.
Code and building science both require a vapor retarder for your wall in your climate. Not something you can skip. An air tight assembly does reduce the amount of moisture that makes it into the wall but doesn't eliminate it, a vapor retarder is still a must.
Definite will be some work. Though I'm pretty committed to it if its saving me money, performing as good of job as flash and batt, and ultimately is greener.
So just to 100% confirm because I don't actually have any inspection or code I have to follow. Based on Martin Holladay's article here: https://www.greenbuildingadvisor.com/article/do-i-need-a-vapor-retarder
He says I dont need one or see if I can get away with a smart retarder or a vapor retarder paint. Dont know if that is just any decent quality paint or something special.
If I do in fact need one. I can put poly up. I wont lie I have in the past thinking it was needed until reading many articles here. Do I need it on both the ceiling and the walls or just the walls?
Thanks again for all your help. I think this will get me over my last hump of concerns and get me on my way feeling confident.
Akos is correct, in Zone 6 you need (and want) at least a class 2 vapor retarder. Kraft faced batts or a roll of asphalt-saturated kraft are the cheap route.
"Airtight drywall approach" is also your ticket. Dense packed cellulose or fiberglass can also significantly reduce infiltration. Not saying it would hurt to seal framing to osb, but it would give only marginal improvement on top of well-executed airtight drywall. Too much discontinuity to be a good "barrier" otherwise.
Sealants aside, you could go the drywall gasket route. EPDM gaskets may not be cheaper but certainly would be cleaner and more consistent.
Around me (zone 5/6) poly has been used for a very long time without issues. I like the fact that it acts a secondary air barrier, this seems to be the biggest benefit of including it in colder climates. Smart vapor retarder is a better option, you are climate is too cold for plain drywall+latex. The vapor barrier paint is a special type of paint, which would also work.
You can look a the details here:
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers#Side_02
Ceilings doesn't need a vapor barrier. There is really no drawback in putting one there, and again, it gets you a backup air barrier.
I would strongly recommend the air tight device boxes. For the small extra cost, they seal way better. Trying to air seal device boxes after the fact from the inside it is next to impossible.
@Jason S. and @Akos thank you both so much. My only concern with the Airtight Drywall approach really is just there is no way to blower door test until your done and at that point your kind of committed. Also am I caulking between the framing and poly, or between the poly and sheetrock. If I caulked the OSB and Framing I felt like I could run the blower test and at that point know if I have a good air seal or not and feel comfortable proceeding from there.
Thanks again!