How can I add insulation to an insulation-board sheathed building?
We recently purchased a pre-built 12 X 24 shed. It has unfinished interior walls 2X4 stud 16″oc, exteriorly sheathed in insulation board foil-faced on the interior, painted wood siding covering the insulation board. The ceiling is sheathed in LP Tech Shield radiant barrier.
We had the previous owner/builder add a simple interior wall to divide the space into two approximately 12X12 spaces. We intend one side for gardening equipment. The other side is to perform as an art studio.
For the studio, the insulation is not adequate to maintain a decently cool interior in summer midday in the piedmont region of North Carolina (Durham, 27701). The inside of the sheathing is very warm to touch wherever walls face full sun and the inside of the building becomes uncomfortably warm even with windows open. We would like to add insulation so that we could minimize, if not eliminate artificial heat/cooling.
I found and read your extensive blog about insulation board and moisture retention but found it treated mostly cold climates farther north (I tried to add this question to the blog but your website blocked me):
[[ QA-spotlightheader image Can Exterior Foam Insulation Cause Mold and Moisture Problems?
A thick layer of rigid foam insulation amounts to a vapor barrier on the outside of a house. Won’t this trap moisture and rot the OSB?
Posted on Jun 23 2010 by Scott Gibson ]]
We had thought we would add fiberglass batting for insulation between studs in walls/floor/ceiling but after reading your article and others on the internet, we worry about creating moisture problems due to the existing sheathing. We can’t find advice for our climate and situation.
We will finish with wall board on the walls and ceiling of the studio room and will leave the garden storage room as is. How can we add additional insulation to the walls before adding the wall board? We rarely have temperatures below 20 F in our region in winter and in summer, mid-90s F are possible but rarely does it go beyond 100 F. The studio will be unoccupied and therefore unheated or cooled in off-hours except on rare occasion. In daytime, hopefully it can be used quite regularly throughout the year.
Thank you for your advice.
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Replies
Barbara,
The safest approaches are either (a) using spray foam insulation between the studs, or (b) the cut-and-cobble approach.
For more information on cut-and-cobble, see Cut-and-Cobble Insulation.
I agree with Martin that spray foam and cut-and-cobble would be low risk. But spray foam is expensive and cut-and-cobble labor intensive. And both can have high environmental impact. (Best choices to avoid that would be polyiso, EPS or (best) reclaimed foam-board insulation for cut-and-cobble, or Lapolla 4G spray foam.)
But even though those are the lowest risk, I think you will be OK with batt insulation, which could be mineral wool or fiberglass. That's particularly true if you would be cooling much more often than heating, and if moisture sources inside are limited. (Some studio activities might generate moisture.) How thick and what type is the foam board?
In the case that it's marginal, a reasonably cheap way to make it more robust against possible problems during a cold spell with the interior warm and humid would be to add "MemBrain" smart vapor retarder over the insulation under the wallboard.
Right up front, I will apologize for my clumsy answer to your question about the type of foam board used in the building. It will certainly reveal my ignorance to the product though I do feel more certain about the board thickness! This is the best I can read from looking at several places around the room where labels are exposed. I hope some of this makes sense to you! Also, a few of these numbers may be incorrect since all of the writing was fuzzy on the boards. Again, sorry!!!:
ICC E 98-L301. ItUD-MR-L318. PR-H12??. APA rated exterior wall siding-sheathing. A 7/16 category. MILL 416; USA 57(??) CA SFMWUI?. Listing 8140-2027:0002. Thickness .375.
The ceiling sheathing is much clearer: Radiant Barrier US Patent 6:251 495. LP Tech Shield. Customer Service 1 800-648-6893.
Thanks so much for your input. Both of you give us hope! It is very nice having access to good information from people who emphasize "green" as in environment (as opposed to accumulating green as in $)!
While we await your response, we will look investigate the suggestions you have given us here so far since some of it is quite new to us.
I would like to add that, when I think of modifying anything in my environment, I try to think into the future about whether my modifications will enhance or deter changes down the line. For instance in the case of this building, if I add insulation for what I think will be a low-moisture situation, will it make it difficult for someone in the future if they (or we) decide to use the space differently and in a way that could be high-moisture? I think your assessment of our usage is quite correct. Most moisture activity within the room will be from glue/paste/paint and of course, breathing! We tried to position the building for solar advantage in the winter and coolness in the summer.