This post first appeared on the Green Building Advisor website. Residential Post-and-Frame Construction, Part 5: Insulation and Air-Sealing – GreenBuildingAdvisor
I was brought on the barndominium construction team after many decisions were already in place. The design was, for the most part, finished. Many of the assemblies had been designed, such as the decision to use laminated posts six foot on center with the horizontal wall girts both inside and out. The original plan for the insulation was to use up to seven inches of closed cell spray foam in the walls. I was able to change the insulation strategy with a plan for a more “forgiving” assembly. We chose to go with Rockwool’s 7.25-inch ComfortBatt, which has an insulation value of R-30.
There are a lot of advantages to using a mineral wool type insulation, it’s hydrophobic in nature, because it is made from rocks (basalt) and steel slag, a by product of the steel industry, it doesn’t care if it’s wet. It doesn’t burn, rodents and insects don’t particularly like it, mineral wool is vapor open, moisture can pass through the material, and it is a much denser product than other fibrous batt type insulations. This density is the biggest reason I chose Rockwool, we were able to stack 24-inch by 48-inch batts inside a six-foot wide by 18-foot-tall wall cavity without having the batts fall out of place. We found the product very easy to cut and work with.
During a meeting with Rockwool while we were planning the insulation details, they suggested we place a board around the mid-point of the wall to eliminate any settling potential, which could create a space at the top of the wall where a gap could form. We installed two batts high, almost 8 feet, then place a 2 x 8 horizontal board inside the wall cavity to support the batts that were eventually placed higher in the wall.
Something unique to this build is we have no bottom or top plates. The posts are anchored to the concrete using a cast in place steel bracket. Because of the hydrophobic nature of mineral wool insulation, this is the only fibrous insulation product I would use in this application. The rafters for the build are notched into the posts eliminating the need for the top plate. The interior and exterior horizontal wall girts keep the posts supported and the rafters in place. Because there is no top plate, we were able to extend the insulation into the attic space, which will eliminate any concerns of fire spread through the wall and into the attic. This lack of a top plate, which complicates any connection to an exterior air control layer, forced us to move our air control to the interior of the structure.
Air (and Vapor) Control
First, a quick review. Of the four control layers, water, air, vapor and thermal, the control layers that manage water are the most important. The water control layer was addressed using Siga’s Majvest, a mechanically attached WRB (Residential Post and Frame Construction, Part 3-Installing a WRB). Air and vapor are above thermal because they also can contribute to moisture related problems in our assemblies. Air control can also affect comfort, indoor air quality and the operating cost of the structure. This project is located in climate zone 7. Even though the thermal control layer is last in importance, we still need to have a good strategy on how to address the temperature differences between inside and outside of more than 100°F during the heating season.
The traditional location of the air/vapor control layer in my very cold climate is on the interior and is usually polyethylene sheeting, a vapor closed product which prevents any drying potential of the wall cavity inward. Some builders in my area are slowly moving away from this assembly, but it’s still very common in my market. I typically try to have the main air control on the exterior of the projects I am involved in, the design of this structure would not allow for an effective exterior air control, so we chose to move it to the inside. Siga’s Majrex was used for a few reasons. Majrex is a “smart vapor retarder”, meaning it has the ability to change its permeance to allow any moisture inside the wall cavity to dry inwards but still remain airtight. Another reason we chose Majrex, it’s a durable product, not easily torn. Majrex comes in roughly five-foot rolls, with 18-foot sidewalls on this project, we needed four courses to get to the ceiling. We used Siga’s Rissan tape, which is an air sealing tape designed to be used on the interior, to seal all the seams and any penetrations through the product. We also used Siga’s Fentrim tape to seal the Majrex to the concrete floor. Fentrim is one of the most impressive tapes I’ve used. It was sticking to the concrete even when applied in temperatures below 0°F.
We were able to install the interior air control before any of the mechanical systems. After the Majrex was installed, 2×4 wall girts were installed over the Majrex, this provides the interior finish attachment and as a service cavity for electrical and some of the other needed mechanical systems.
Once the wall air sealing was complete, we air sealed the ceiling, this time using Polyethylene sheeting taped to the wall air control. In my climate, I’m not overly concerned with drying potential in a well vented and insulated attic. Most moisture that accumulates in this type of roof system comes from either an exterior roof leak or interior air leak. Hopefully the roofers can keep the roof from leaking. Air leaks from the interior are caused by holes in the ceiling’s air control product. We were able to eliminate most holes through the ceiling by installing a ceiling service cavity on the warm side of the polyethylene sheet. 2 x 4’s were attached to the trusses after the poly was installed. All the ceiling electrical wires and boxes were able to stay inside the conditioned space of the building.
There are, of course, some holes that need to penetrate the air control layer. They were addressed by using a gasket system. I chose the Pro-Flash product which consists of a gasket that can be modified for different hole sizes affixed to a plastic membrane that can be taped to an exterior or interior surface. The photo shows how we addressed a gas line that needed to penetrate the Majrex air control layer on the interior of the building, the Pro-Flash created an effective air seal. We could have simply taped around the gas line, but the line might need to be pushed into or out of the wall cavity. The gasket still allowed movement without compromising the air seal.
Another Layer of Insulation
The 7.25-inch cavity insulation got us to an R-30 wall. Because of the needed 2 x 4 horizontal wall girts which were installed 24 inches on center on the interior side of the wall assembly, we had an opportunity to add another R-6 of insulation to further increase the wall’s resistance to heat flow, kind of an interior continuous insulation. Rockwool’s ComfortBoard 80, which we were able to source in 4 x 6 x 1.5-inch sheets were attach to our six foot on center posts. Each piece of ComfortBoard needed to be ripped to 24 inches in width to fit between the wall girts. We cut the rigid insulation around the electrical boxes that were installed in the service cavity.
The last piece of the insulation strategy is the attic blow-in (which at the time of this writing still has not been completed). We are planning to use a blown fiberglass product, mostly because of the reduction in weight over blown cellulose. With having six foot on center roof trusses in the shop area, I felt a lighter insulation when blown to a depth of nearly 24 inches was the better choice to reduce any potential ceiling finish issues. We are shooting for an R-70-80 for this space.
I had a conversation with the blow-in insulation contactor about using our Rockwool batt scraps for the attic insulation. He said he’s run that product through his truck-mounted insulation blower several times without any issues. A nice way to eliminate the accumulation of the insulation scraps.
The installation of both the Rockwool ComfortBatt and ComfortBoard went well. The size of the structure, 10,000 square feet with 18 foot sidewall took some time to insulate, but we were happy with how the products worked. We were equally happy with the Siga Majrex. This is the first full project I’ve had the opportunity to use this “smart” membrane. Durability and ease of install made it a pleasure (if there is such a thing when installing a membrane) to work with. We didn’t have any issues with the interior Siga tapes, even when installing in well below freezing temperatures.
We were able to conduct the mid-build blower door test on the 3,000 square foot living space portion of the barndominium. Our goal was less than 1 air change per hour at 50 Pascals, we achieved .76 AHC50, very happy with the results.
I’ve learned a lot on this project, some good lessons, some not so good. The wall insulation and interior air sealing is an assembly I would not change.