Barndominium Part 4-Insulation and Air Control

This post first appeared on the Green Building Advisor website.  Residential Post-and-Frame Construction, Part 5: Insulation and Air-Sealing – GreenBuildingAdvisor

Cavity Insulation

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.

Attic Insulation

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.

Final Thoughts

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.

18 Replies to “Barndominium Part 4-Insulation and Air Control”

  1. Randy,

    Some really great workmanship on this project, everything neat clean. Quality is hard to come by these days, I know it when I see it but not very often.

    1. Thanks Doug, it’s been fun and a challenge. Pulling blower door test for the whole building this week, the living quarters was good, we’ll see how the rest of the building with the overhead doors came out.

  2. 1) If budget were an issue in this assembly, could the comfort board 80 have been skipped? Would the air gap /uninsulated service cavity downrate the r30 comfort batt?

    2) The wall batts are sitting directly on the slab?

    Appreciate following these detailed posts – planning to build a pole structure in Oregon and insulating only a small portion for living quarters.

    1. Hi Kevin,
      Yes, the Comfort Board 80 could have been skipped without any issues.

      The wall batts are sitting directly on the slab. The framing contractor designed the structure without a bottom plate, which, if installed, would have been placed between the wall girts. I think that decision would be different if they had to do it again, and it would have made our job a little easier. This build happened over the winter, we had an accumulation of snow on the outside of the building which melted and ran under the exterior wall in the spring. Once the bulk water dried up, we cut the tape on the exterior WRB to inspect the insulation. It was saturated up about a foot. It took about a week for the moisture to drain and dry out of the insulation. Rockwool is great in that situation because of its hydrophobic properties, no damage to the product. The structural posts are all treated, and the assembly is completely vapor open and able to dry, no damage. If the bottom plate would have been in place, there would have been less water that moved from outside to inside, though I doubt it would have been completely stopped. With the metal post brackets in the wall, it would have been hard to completely water seal this location. The bottom plate would have also made it easier to tape both the exterior WRB tape and interior air barrier tape, something solid to roll the tape against and help keep the tape straight during installation.

      Hope this helps and good luck with your build.

      Randy

      1. Thanks for sharing the snow story, definitely an issue here as well. If a bottom plate was included from the design outset, it would be segmented (not continuous) and butted between each post, and attached to the slab with some sill seal underneath?

        1. You are correct. A picture of the post bracket is on the Barndominium part 2 blog, second photo. Hard to completely seal a bottom plate when butting up to those metal post brackets. There are other ways to build a post and frame structure, I prefer the foundation and framing is constructed in the way this one was, though with a few modifications. I will be discussing the changes I would make in hindsight in the final blog post on this project. Will probably still be a couple months before we are complete with this build.

  3. Hi Randy,

    I’m considering a similar build of my own, although not nearly as big. The building would be in climate zone 7 and I’ve been leaning towards utilizing Zip-R sheathing with taped joints, which would create an air seal on the outside. What type of sheathing was utilized in your project? And is it the roof trusses that would prevent an adequate air seal from the outside?

    1. Hi Jaron,

      The main wall air barrier on this project is on the interior, the Siga Majrex with taped seams and taped to the ceiling poly, which is the ceiling air barrier. The exterior has a mechanically attached WRB, Siga’s Majvest, simply there for water control. There is no sheathing. The structural support comes from the 2x wall girts that are fastened horizontally to the poles, which are 6 foot on center. I’ve seen sheathing attached to the horizontal wall girts, Kyle Stumpenhorst is building one using that assembly now. You can catch his build on his YouTube channel, RR Buildings. His interior air sealing and insulation details are almost exactly what I did.

      One problem with using an exterior air sealing product such as Zip Sheathing on a post and frame structure is connecting the sheathing to the interior ceiling. If those are not made continuous, there will be substantial air leakage at wall to roof connection. The trusses are fastened to the side of the pole, which extends into the attic. The poles make the connection from exterior wall to interior ceiling a complicated connection. If I had used sheathing, it would have been a secondary air seal.

      I don’t think I would use Zip-R in this case. These buildings already have a much lower thermal bridge than standard construction. Save that cost and invest it in a thicker wall, maybe an 8-inch post which can then be insulated to an R-30. Rockwool would be my suggestion for insulation. Standard fiberglass isn’t dense enough to stay in the wall cavity with such wide centers on each of the posts.

      Hope this helps,
      Randy

  4. Hello Randy,
    In the planing stages on our large Bardominium project here on the front range of Colorado.

    My design vision is to have some exposed vaulted ceilings with exposed rafters. With insulating the roof at the pitched roof line, I have not seen a detail to use Rock Wool/Siga and probably lends the only good option at closed cell spray foam but I have seen a detail to use Rock Wool in the walls and spray foam in the roof.

    Thoughts?

    1. Hi Stacy,

      If I understand your assembly correctly, you want to insulate under the roof sheathing using a fibrous insulation (Rockwool). Colorado has climate zones 4-7. How you choose to insulate the roof will depend on which climate the building is located. Building Science Corportation has a really good article about insulating at the roof slope. https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
      Which way you choose to insulate will depend on if the roof is ventilated or not, the type of insulation chosen (air permeable or impermeable), and the climate zone.

      In the barndominium article, the roof was insulated, and air sealed at the flat ceiling. There was no roof sheathing, the roof has 2x roof purlins, which was a mistake on the post and frame builder’s part. The roof should have been fully sheathed. (We had a weather-related ice dam in one of the valleys where water backed up under the steel and ended up inside, had the roof been sheathed, that risk would have been reduced.)

      Randy

  5. Looks great! I’m planning a build with similar details in NY state. However, I have heard critics say that the service cavity does not allow enough room for the electrical install. According to code, wiring must be held back at least 1-1/4″ from the edge of a framing member to prevent being punctured by a drywall screw.

    Did you have any issues with inspections on this job?

    Thank you!

    1. Hi Yi-Boe,

      I have had a few people bring up the 1 1/4″ code requirement, including the electrician I’ve been working with. We have not had any issues with code officials. I’ve used this detail for ceiling in non-barndo projects many times, also without problems. That being said, I would discuss the detail with your electrical inspector just to make sure.

      Randy

  6. I have a very similar situation here. I am adding on to my post frame building. The original building was spray foamed and it pushed out all the steel on the outside even with doing 2 passes. I was very disappointed in the look in the end. Right now the addition has steel, Tyvek then the wood girts on the outside of the poles. I am looking into the rockwool with filling in-between the wood outside girts against the Tyvek with the 80 board 1.5″ then filling the cavity with 7.25″ rockwool batts. Then probably the sigga. I definitely want a board on the concrete the insulation off the floor. We have 14′ sidewalls Iam in mid WI. Most my electrical is in conduit inside the walls. With the spray foam it had pushed some wires out to the sheeting and when they had to be removed from hail damage, it was a pain in the butt to fix them and get the foam off that had pushed out. They wouldn’t put the steel back on unless it was flat. Do you think this will perform better then the closed cell foam? We have blown in R-50 cellulose in the existing building, looking to do the same or more in this addition. Also the building has to have 5/8s drywall all over.

    1. Hi Garen,

      This is an assembly I would use if I were building a post and frame structure for myself. It will perform similar to the spray foam (depending on how much spray foam was installed), 6″ of CCSF would be about the same as 7.25″ ComfortBatt and 1.5″ of ComfortBoard. The trick will be the air sealing, but the Siga Majrex is an excellent air barrier when installed correctly and using their tapes as part of the assembly.

      Good luck on your project.
      Randy

    1. Hi Ryan,

      Good question. Closed cell spray foam (ccsf) is typically installed to provide the requirements for three of the four control layers: air, vapor and thermal. An added benefit, it adds some level of rigidity to the structure. There are a few drawbacks, I have now been on several building investigations where small movement of the structure caused by shrinking or twisting of framing members, or when the product is installed outside the manufacturer’s required temperature/humidity ranges or mix ratios, small gaps form between the product and material it is sprayed against. These small gaps allow moisture to flow through the space, especially in cold and very cold climates. This small amount of moisture will accumulate over the winter, creating a wetting event that degrades moisture sensitive building materials. This does not happen fast; it’s 20 or 30 years in the making, well after any warranty period. Insurance carriers are just starting to realize this risk, several providers in Great Britain have stopped insuring roofs insulated with closed cell spray foam because of an increase of claims and the high cost of replacement. Insurance companies in the US probably aren’t too far behind.

      Here’s an example of what can happen, I was working on a project a couple years ago, a new home with a cost of just over 2 million dollars. The entire home’s building envelope was insulated with closed cell spray foam. The work I was performing was after interior wall finish was in place. I was back to the home later that year to perform a blower door test. During the return visit, the contractor had removed a small section of exterior wall finish to add a wire that was forgotten. The spray foam had pulled away from the framing in that area. I questioned the workers who had been involved with the project from the beginning, they said that condition was in several places around the home before the interior finish was installed, no one said anything to the insulation contractor, and I don’t think the production supervisor understood the risks of that condition. I don’t have a crystal ball, but I’d bet that home will have structural issues and possibly mold and IAQ issues down the road, all because of the small gaps between the framing and ccsf.

      This sounds like I’m anti-spray foam, I’m not. It has its uses and can be an effective insulation strategy. I often use it in rim joists and raised energy heals. On occasion, I’ll choose ccsf as the primary insulation, air sealing and vapor retarder strategy for an entire home, but only if there are no other options or the homeowner is insistent on its use. In those instances, I’m performing a thorough inspection of the ccsf and blower door testing before interior finishing begins.

      Will we start seeing more and more issues with closed cell spray foam in the future? Time will tell. We have other options that have been around a lot longer that work and are usually less costly. I prefer to use those options whenever possible.

      Randy

  7. Thanks for the very thorough and helpful response, Randy. Structural issues down the road? No thank you!

    Do you think that EPS Ridgid foam, carefully cut with less than a 1/4″ gap and the use of elastomeric caulk like Sashco Big Stretch to air seal would be more forgiving than spray foam?

    1. Hey Ryan,

      The method of installing rigid foam in between roof rafters or trusses is called the cut and cobble method. Much like spray foam, it can work if executed properly, but there is still the potential for an error that creates an air leak and the same problems that we see with CCSF.

      I choose something different for my home. The insulation for my roof follows the roof pitch, what I did was install a ventilation chute from eave to ridge and add an R-15 batt of mineral wool. I then installed an air barrier/vapor retarder fastened to the rafter and sealed at every intersection, then added 2″ of rigid mineral wool to the bottom of the rafter, a continuous interior insulation. I’m only at R-23 or so, but much better than the unvented roof insulated to R-11 that was originally in the home. After the insulation is in place, I strapped the ceiling with 2×4’s, providing a space for electrical to remain inside the air barrier and providing an attachment surface for drywall. The worst part of the job was taping the ceiling air/vapor membrane around the attic knee wall framing. It took some time, and a lot of tape, but it’s a major improvement over what was originally present. I did lose a little headroom, but I was willing to give that up for the reduction in ice dams. I haven’t eliminated them, but they aren’t as bad as they were. A higher R-value of CI under the rafters would help even more, but I only had so much headroom to give up.

      I’ll be writing a future article on this method sometime this year.

      Randy

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