Insulating walls and ceiling for a barn workshop
Hi Folks,
I am in the process of building out my 1350 sq ft barn for use as a sculpture studio (i do mostly wood and metal fab). I use the space year round, and before insulation I can barely keep it in the low 40s with a 35k BTU pellet stove running 24/7.
The barn is sided with 1×12″ eastern pine board and batten (all dims are actual not nominal) The roof and walls are 2×6 rough hemlock or eastern pine. Between the b&b siding and the studs is a very cracked/UV damaged poly barrier.
My goal is to bring the temp up to about 55-60 during the day and let it go back to 40s-50s at night. I’d like some feedback about my plans for insulating both the walls and roof.
Walls:
I’ve spoken with my local code official in western Mass and he seems to think it’s OK to install 5-1/4″ R21 faced fiberglass into the wall bays (6″ deep actual) and the resulting space will be enough to allow sufficient air flow for drying whatever small amount of snow or driving rain that makes it through. He also suggested that I leave the damaged poly barrier intact as it may help to keep moisture away from the batts. Over this I would like to install 7/16″ OSB as an interior wall surface, then possibly 1/2″ sheetrock. The sheetrock in this case would be aesthetic rather than acting as a fire/air barrier, and the ceiling will remain open to the unfinished loft area above.
Roof:
The roof is 3/12 @ 70′ long and has 1″ vents that run the length of both eaves and a full ridge vent. The rafters are also 2×6 rough timber, which is slightly under sized so I can’t hang a ceiling without doing a ton of additional carpentry (which is not in the budget). Above the 2×6 rafters are 2×4 purlins then the roof deck and a 25yr roof. My plan is to install the same 5-1/4″ R21 batts into the rafter cavities.
Given the depth of the purlin (2″ actual) and the remaining 3/4″ space left in the bay is there enough airflow to vent the roof?
I understand that without the walls and ceiling being finished I’m still going to have considerable heat loss through air movement, however as I said, this is a workshop and a finished ceiling is not in the cards at this point.
What are you thoughts on these plans?
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Replies
Benjamin,
I think that with just a little bit of additional work, you can include an exterior air barrier in your walls and ceiling. If you do that, you will significantly reduce your heating bills.
Tomorrow morning, GBA will publish my latest blog, titled "Insulating Walls in an Old House With No Sheathing." Before you make any decisions, I think you should read that article.
Unlike your building inspector, I don't think that it's a good idea to keep the polyethylene that is now installed on the exterior side of your wall studs. I would get a brand new utility knife blade and cut away all of the polyethylene and throw it away.
Thanks, Martin.
The timing couldn't be better, I'll look for it tomorrow.
-b
Benjamin,
Here is the link to the article I was talking about: Insulating Walls in an Old House With No Sheathing.
You are in Climate Zone 5. If you want to take the same approach to create an exterior air barrier in your roof assembly as I recommend for walls, you would want to use rigid foam with an R-value that is equal to 40% of the R-value of the total assembly. (For more information on this topic, see How to Build an Insulated Cathedral Ceiling.)
If your fluffy insulation has an R-value of R-21, the R-value of your entire roof assembly will end up at about R-35, with R-14 of rigid foam and R-21 of fluffy insulation. If you use the cut-and-cobble method, you'll need about 3 inches of rigid foam for your air barrier. (We both realize, I assume, that R-35 isn't enough to meet R-value requirements for roof assemblies in Massachusetts. In your location, you really want R-49.)
Of course, this is a barn / workshop -- so you may not want to go to the trouble of creating an exterior air barrier. The better your air sealing details, however, the lower your energy costs will be.
Whatever you do, remember to remove that exterior polyethylene from your walls.