Cut-and-cobble for a three-season cabin
I have a 3 season cabin in (Zone 7) of northern Wisconsin. 2×6 walls 16″ OC. The cabin is sheathed in OSB,
Exterior foam of 3″ is not an option. The cabin is 20×26′ and I plan to cut & cobble 2″ XPS on a table saw and fill the cavity with fiberglass batts. Granted…this is time consuming….but there’s no hurry. I realize this doesn’t address thermal bridging. My concern is longevity with OSB in a northern climate & still being insulated. Yes…I will install a rainscreen if vinyl is not used for siding material.
1st…Are there any major flaws with this plan? moisture along edges, moisture between foam and OSB even though the surfaces touch, drying, etc?
2nd…would a 1/4 gap between the rigid foam and OSB facility drying if any moisture was to migrate in?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Dan,
1. Yes, 2 inches of XPS (R-10) is enough foam to keep you out of trouble in Zone 7, assuming 3.5 inches of fluffy insulation on the interior side of the rigid foam. For more information on this issue, see Cut-and-Cobble Insulation.
2. XPS is avoided by most green builders because it is manufactured with a blowing agent with a high global warming potential. For more information on this issue, see Choosing Rigid Foam.
3. You asked, "Would a 1/4 inch gap between the rigid foam and OSB facility drying if any moisture was to migrate in?" You've got the right idea, but the wrong location. You want a 1/4 inch (or larger) gap, but the best location for this gap is between the siding and the WRB. The gap is called a "rainscreen gap." For more information on this issue, see All About Rainscreens.
Martin - Thank you. I appreciate your input.
There's a good possiblity I'll use vinyl board & batten over cedar. Since vinyl board & batten has a much greater surface area sitting flat against the WRB opposed to a clap board style; I still may install furring strips(rainscreen) to eliminate moisture or water stiction between the two surface areas....or is that just over kill?
If using vinyl siding, you need to follow the manufacturer's instructions. I believe most products cannot be installed over furring strips.