One thing on Drew Goldsack’s spring cleaning punch list this year is removing the vinyl siding from his Alberta home and replacing it with cedar board-and-batten siding.
As he explains in this Q&A post, the plan is to install 1 1/2-inch Comfortboard mineral wool insulation, a rain-screen drainage mat, then horizontal 1×4 furring strips before adding the yellow cedar siding. His initial question concerns the fasteners he should use to make sure the siding stays put.
“I have been unable to find a local building code that specs fastener penetration for cedar board and batten (3/4″ thick boards and approx. 7/8″ thick battens),” Goldsack writes. “However I did come across the Western Red Cedar Lumber Association specs that recommend a minimum 1-1/4″ fastener penetration for installation of board and batten.”
Goldsack had thought that a 3/4-inch penetration of ring-shank nails into the furring would be enough, but now he’s not so sure. Would that be enough? Should he switch to screws instead, or make the furring strips thicker?
Those are the questions that start this Q&A Spotlight.
Consider stiffer furring strips
Russell Miller reports “zero issues” when he’s used 3/4-inch plywood or 3/4-inch pine or fir furring strips. Either should work, he says, unless Goldsack is building in a coastal or tornado-prone area. “Even then,” he says, “tighter nailing of siding is the main issue.”
But GBA Editor Brian Pontolilo raises another issue—the difficulty of keeping mineral wool flat under 1×4 furring.
“Have you specifically used 3/4 inch furring strips over exterior mineral wool?” he asks. “I’ve heard that thicker furring strips, mainly 2x4s, make it easier to keep the furring strips in plane for a flat siding install and that 1x material bends too easily and therefore compresses the insulation.”
In either case, Pontolilo suggests that Goldsack have a look at a Pro Trade Craft video (below) discussing how to use a string line to prep a rainscreen wall for siding. The technique should be equally useful for horizontal furring strips.
No drainage mat is required
John Clark says Goldsack can skip a drainage mat if he uses mineral wool because the insulation naturally repels water. Bulk water, he says, will just bead off the surface.
Goldsack replies that he had planned to use a rainscreen mat to fulfill a requirement for a 9.5 millimeter drained and vented air space that many building codes require.
“Do you know if mineral wool qualifies as a capillary break?” he asks. “If mineral wool satisfies this requirement, that would be a big bonus.”
In addition to the requirement for an air space, Goldsack also had been concerned that horizontal furring might trap any water that gets past the siding.
The mineral wool is indeed a capillary break, Clark says, and it’s also vapor open. That will allow drying to the exterior. He refers Goldsack to recommendations from Hammer and Hand for both vertical and horizontal furring strips as a preferred method for attaching board-and-batten siding (see the illustration below).
Akos adds that the water-resistive barrier (WRB) should be behind the insulation. “Because mineral wool is self draining, it doesn’t need to be textured,” Akos adds, “regular house wrap is fine. The drainable WRB on the outside is not doing much.”s
Akos does not consider the risk of trapped water on horizontal furring a serious threat. “I can’t see it trapping any water,” Akos says. “You can always bevel the top to slope away from the siding as extra insurance.”
Check your local code requirements
Writing from Vancouver, Malcolm Taylor says code requirements for rainscreens in British Columbia are, as Goldsack says, for drained and vented cavities, not just capillary breaks. There are provisions for proprietary rainscreen alternatives, but he’s never seen mineral wool boards included in that provision.
Practically speaking in Alberta, however, those same rainscreen requirements might not be important. “If your code doesn’t require [rain]screens, I would omit the drainage mat and rely on the mineral wool with WRB behind,” Taylor says.
He adds that after building for a couple of decades on Vancouver Island, he framed a house in Calgary a few years ago and saw common details that would prove to be a disaster on his home turf. “I think the assembly you are suggesting will work well there,” he adds.
Our expert’s opinion
Here’s a word from Peter Yost:
In terms of the acceptability of attaching furring strips with rigid mineral wool, have a look at this blog by Martin Holladay. In short, doing this is not a problem.
For fastening furring through rigid exterior sheathing, refer to Table R703.15 in the 2018 International Residential Code: “Furring Minimum Fastening Requirements for Application Over Foam Plastic Sheathing to Support Cladding Weight.” (Use the link above, then scroll down until you come to the table.)
When you jump to 2x furring, Footnote C to the table states:
“Where the required cladding fastener penetration into wood material exceeds ¾-inch and is not more than 1 ½-inches, a minimum 2 by wood furring or an approved design shall be used.”
The fastener type and schedule is not surprisingly based on the type of cladding. Admittedly, this table is for exterior foam sheathing and Footnote D to the table states that rigid mineral wool designed for exterior sheathing more than meets the compressive strength requirement:
“Foam sheathing shall have a minimum compressive strength of 15 psi in accordance with ASTM C578 or ASTM C1289.”
Rockwool’s compressive strength is only about 3 psi, but Rockwool’s “Test Results & Design Tables for Strapping Attachment on Walls with Long Screws Through Rockwool Rigid Insulation” demonstrates acceptable structural performance as well as a Design Table covering Exterior Insulation Thickness, Vertical Screw Spacing, Minimum Screw Diameter, Minimum Screw Embedment, and Minimum Strapping Size.
In terms of the moisture performance of Rockwool rigid insulation, under Reaction to Moisture, the Technical Data Sheet reports: “Moisture Sorption–0.05%” and a perm rating (dry cup) of 31 perms. But the moisture sorption is based on ASTM C104, which is “Standard Test Method for Determining the Water Vapor [emphasis added] Sorption of Unfaced Mineral Fiber Insulation.”
When I was at Building Science Corporation working on the Building Materials Property Table, it was important to John Straube to include Footnote 2 to the Table:
“Although manufacturers often report this property per ASTM C209 and as a percent by weight, this only gives information about the material’s POROSITY (overall quantity of water absorbed over an indefinite—long, often 24 hours—time period) and what is much more useful is the material’s water absorption coefficient, a measure of WICKING (initial rate of capillary transport). There is unfortunately NO relationship between the two, no ASTM standard for the water absorption coefficient, and few manufactures in North America have measured or reported water absorption coefficients. When researching a building product, strongly encourage manufacturers to PROVIDE the water absorption coefficient.”
So, that is what I did: I called Rockwool’s Building Science Manager, Antoine Habellion, and we talked through the difference between vapor and water absorption and the difference between water submersion and exposure to wicking. Habellion was right on top of this and sent me the following notes:
- Rockwool Technical Bulletin “Water uptake of Rockwool stone wool insulation in exterior wall applications.” In this test, rigid mineral wool samples were placed in direct surface contact with water for 24 hours (Note that this is the very wicking approach that John Straube suggested in his footnote). The water uptake was between 2% and 4%, and that was with the insulation samples held in contact with water on the side rather than the end as it would be in a wall application. And all samples dried within hours of the test.
- Rockwool Research Summary: “Drainage Balance Testing and Wall Comparison.” From the Research Summary Conclusions: “The overall moisture storage in the wall assemblies were below 1%. Minimal differences in storage levels were noted between the Rockwool test assemblies and the XPS test assemblies; with an average difference below 270g (10 oz). Both assemblies also experienced similar drying trends, with an average difference of 67g (2.3oz) after a 5 hour drying period.”
Since there is no ASTM test for water absorption, these third-party tests certainly seem like adequate evidence of the moisture performance of rigid mineral wool exterior insulation.
One final point on the moisture performance of rigid mineral wool insulation in ventilated rainscreen cladding systems: Rockwool requires a 10-mm (3/8-inch) space between the cladding and the rigid insulation.
Finally, although not brought up in this Q&A exchange, another question about rigid mineral wool performance as exterior insulation is its air permeance and susceptibility to wind washing erosion of its R-value. RDH Building Science Labs conducted a study entitled “Summary Report of Air Permeance and Wind Washing.” Testing conducted for this study indicates a 5%-8% reduction in R-value for low-density fiberglass batt with the erosion in R-value for stone wool products close to the range of error in the equipment: around R-0.2.
All of the reports, technical sheets, and studies cited above are either available on the Rockwool website or upon request.
To sum up:
- The moisture performance of rigid mineral wool as exterior insulation (the way it handles both vapor and liquid water) is just as good as, if not better than, any other kind of rigid insulation.
- For most light- and mid-weight wall claddings, 1×3 strapping through just about any thickness of rigid mineral wool insulation works just fine, so long as you follow the fastener type, size, and schedule per the mineral wool manufacturer’s research/recommendations.
- Don’t forget to follow the fastener recommendations/requirements for your cladding.
- Despite the adequate if not superrior moisture performance of mineral as exterior wall insulation, install a mimimum 3/8-inch free-draining space between the backside of your cladding and the face of the exterior insulation.
Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. “Our Expert” is Peter Yost, GBA’s technical director and founder of a consulting company in Brattleboro, Vt. called Building-Wright.