Can Pine Shiplap be used for sheathing?
Emel
| Posted in General Questions on
Our builder uses sustainable, local wood as sheathing rather than plywood or osb. It is 3/4” pine shiplap. Would there be any issue with this as far as anyone can tell? The shiplap overlaps, but would there still be air leakage areas leaving lots of seams that should caulked to stop airflow? I love the idea of using sustainable materials, but I’m unfamiliar with this option.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Board sheathing is my favorite approach. One concern is structural; plywood provides lateral bracing but horizontally installed boards do not, so you'll need to add something else or install the boards on an angle.
As for airtightness, you should have one really good air control layer but it doesn't have to be at the sheathing. If you want it there, any good WRB can be made airtight, and this is a situation where self-adhered WRBs are valuable.
We’re planing on using the stapled Hydrogap product as the house wrap and on the inside dense packed cellulose and intello for the vapor smart barrier. We are in a northern climate (almost Canada border) so I know it’s recommended our air barrier be placed on the inside. If the connection points aren’t sealed somehow with the pine board and batten will it make air leaking a problem? I know some tape all seams when using plywood sheathing but with pine board and batten I don’t think this would be possible. Maybe it’s not a problem because of how it overlaps?
For bracing, I’m actually not sure which way they install it but I do know on the inside of the 2x6 exterior walls they do horizontal strapping to make the wall a little deeper (providing a little more of a thermal break and room for more insulation). They then use the smart membrane followed by plastered walls. I think that would take care of the structure concerns?
Emel,
"We are in a northern climate (almost Canada border) so I know it’s recommended our air barrier be placed on the inside"
I've never seen that recommendation. In northern climates the vapour-barrier or retarder needs to be close to the interior, but the primary air-barrier can be anywhere in the wall, as long as there is a fairly continuous secondary one (drywall) on the warm side of the insulation.
In most places without seismic concerns, running the board sheathing horizontally yields enough shear resistance. Applying it diagonally adds a lot of time and waste.
The air control layer can be anywhere in the assembly; it just needs to be continuous with the rest of the air control layer (at foundation, roof, etc.). It's ok to have more than one air control layer.
There should also be a vapor control layer, and in a heating-dominated climate, that should be close to the interior. Vapor control and air control are completely separate functions, but a vapor control layer can be detailed to also be an air control layer.
You also need a WRB, or water control layer, often called housewrap but that's an imprecise term. That works in conjunction with the cladding to keep stormwater from entering the building enclosure. In a heating-dominated climate, it should be vapor-open, to allow the moisture pushing through the walls all winter to dry to the exterior. The WRB can also be detailed as an air control layer.
In the past, locating the air control layer near the interior was a popular approach. These days, many designers and builders of high-performance homes find that locating the air control layer at the sheathing is convenient and effective. That's what I'm recommending. Self-adhered WRBs make excellent air control layers, as long as you pay attention to any transitions.
As for structure, if you aren't using plywood or OSB, it's critically important to have something oriented diagonally to keep the house from blowing over when the wind blows. That can be done by installing board sheathing on a 45° angle, or by notching the studs and installing diagonal 1x4s in the notches, or by using metal straps in an "X" shape installed on the face of the studs. The IRC provides guidance but it might look a bit overwhelming at first: https://codes.iccsafe.org/content/IRC2018/chapter-6-wall-construction#text-id-11644969.
“Board sheathing is my favorite approach. ”
That is interesting, why is that?
freyr_design,
We often crib up foundation forms with 1"x8"s and once stripped use them for sheathing. My own house is built that way. It's a lot more forgiving that most sheet goods of wetting and less prone to moisture accumulation on the inside face.
Given the choice I do prefer to use plywood for a few reasons:
- Sheathing with boards is slow with lots of angle cuts, as here you need to use them diagonally to get decent shear resistance.
- There is a good chance you will hit the gap between boards when looking for backing for fasteners on the cladding and trim, etc.
- It precludes using the sheathing as an air-barrier.
- Sheet goods make the whole wall a diaphragm, not only resisting shear, but making the walls into box beams, so they add a lot of redundancy to the structure.
- Uniformity of thickness. No cupping of boards at openings, means the wall thickness is all the same, making trim much easier.
You can safely burn, bury or shred the scraps without releasing microplastics; they don't delaminate or swell much when they get wet; on some projects they can be cut from trees on the site; they are more vapor-permeable than other options, in part due to the gaps between boards. They are somewhat slower to install than plywood but if you run the boards long and cut them all at once it's not that much slower. Shiplap joints eliminate the problem of cladding fasteners hitting open air.
On the projects I design, I spec sheet goods, as carpenters are rarely familiar with installing boards sheathing, but on specific projects, I find it to be a good option.