Insulating diagonal hardwood sheathing in climate zone 3 – insulated lap siding + drainwrap?
My wife and I are completely renovating my great grandparents’ house in central Mississippi, and we will soon replace the siding, much of which is in bad condition. As we have looked at options, the insulated Hardie lap siding has been one recommendation, with the idea being that Tyvek drain wrap would go between the back of the siding and the beautiful diagonal sheathing. That sheathing has some separation between the boards in areas, and I’m nervous about how this home is going to react to all of these changes (re-siding with the insulation backing, adding HVAC, insulating wall cavities, enclosing the crawlspace, etc). Is the insulated lap siding + drainwrap going to provide a sufficient air gap? Is that diagonal sheathing going to be able to breathe? Are the spaces between the diagonal boards a problem? Thanks!
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Replies
Joel,
1. In your climate zone, you can add exterior rigid foam of any thickness to your wall without worrying about moisture accumulation in the sheathing.
2. Insulated Hardie lap siding has an R-value of R-3. You can use this product if you want. In general, I'm not a big fan of insulated siding, because insulated siding doesn't allow you to include a rainscreen gap between the siding and the rigid foam. That said, this type of siding can work well if it is properly installed, and if all openings are flashed.
3. Tyvek drain wrap will work fine as a water-resistive barrier (WRB). The main issue with a WRB is conscientious installation -- the brand name of the WRB isn't very important. Careful installation is the key to good performance.
4. There is no need for your sheathing to "breathe." All that matters is that you choose wall assembly details that don't accumulate moisture. The biggest moisture threat comes from rain, so pay attention to flashing details.
5. The spaces between the sheathing boards are not a problem. But if the sheathing really is a hardwood species, nailing the siding on may be more of a challenge than if the sheathing were pine, fir, or spruce.
6. Every wall needs a good air barrier. You haven't explained your plan to reduce air leakage. If this were my house, I would probably install a layer of OSB or plywood sheathing (with taped seams) on the exterior side of the board sheathing before installing new siding. The purpose of this OSB or plywood would be to act as a durable air barrier.
-- Martin Holladay
Martin -
Thank you for the quick response and for correcting a couple of the misconceptions that I had about things like the sheathing being able to breathe.
To follow up on a couple of your points, I looked at the drain wrap specifically because it is touted as a rain screen, but it sounds like you consider it to be just a WRB.
On the air barrier point, I did l not explain my strategy for reducing air leakage because I had not been able to figure out how to combat air leakage. I was worried about that sheathing being able to breathe/dry and was worried about putting something on top of it.
Joel,
Ideally, a rainscreen gap should be installed directly behind the siding and on the exterior side of the rigid foam. When the siding manufacturer glues rigid foam to the back of the siding, this approach is impossible.
The crinkly housewrap you have chosen does allow moisture to drain, and it is probably a good idea with this type of assembly. But it's not the ideal way to proceed. Crinkly housewraps that are sandwiched between rigid foam and sheathing may reduce the thermal performance of the wall slightly by allowing a little bit of exterior air to enter a gap where exterior air isn't desirable.
For more information on these issues, see:
All About Rainscreens
Mind the Gap, Eh!
-- Martin Holladay
DrainWrap DOES provide a bit of capillary break, but it's nowhere near as much of a capillary break as 1/4" or more of vented air space. When you have vertical or diagonal lap siding where wind driven penetration of the siding layer can be frequent it's well worth building out a rainscreen, or at the VERY least, using a crinkled type WRB such as DrainWrap. In light-wind areas a house with deep roof overhangs it may not be absolutely necessary, but it's pretty cheap insurance.