It’s been a while since we posted on any technical aspects of the Passive House we are trying to build in the Passive Project. We recently tested our air sealing with a blower door test prior to drywall at the homes and I figured it would be a good time to bring up some thoughts on our current air sealing methods along with future ideas we plan to implement.
Last time we spoke about air sealing was when we looked at our foundation and slab insulation and under slab air sealing. Since then we have obviously finished framing, sheathing and cladding the homes. Let’s recap our basic air sealing strategy since these posts on the foundation.
Above Grade Air Sealing Techniques on the Passive Project
- SIPs walls and roof – Review SIPs Construction Details and notice how much sealant they spec in the assembly of a SIPs building. This along with the SIPs themselves, makes for a much tighter home than an average stick built home. We improved our sealing details from experience gained on the 100K House installation.
- The poly air barrier from beneath our slab was wrapped up inside the house and continuously taped to the interior of the SIPs walls to eliminate air leakage at the critical junction between slab, foundation wall and first floor framing.
- A Tyvec air and water barrier was installed on the exterior of the walls. All seams were taped and all windows and doors were taped, caulked and foamed to the Tyvec barrier. Normally people don’t tape the seams. We made sure each seam was taped.
- Tyvec was all used in between the homes at the party wall as seen in the image above. All seams were again taped, and this layer was wrapped around to the facades and to the roofing membrane to form a continuous barrier.
- The typical rubber roof acts as our air barrier on the roof.
- Each penetration through the slab or roof was taped and foamed to its surrounding air barrier with a detail similar to the one shown below.
- A wireless electrical switch in combination with floor outlets along the interior facades of the homes, eliminates electrical penetrations through the SIPs walls.
- All LVLs were designed not to penetrate any exterior walls to maintain the airtight SIPs walls.
- All joists and rim boards were hung internally to make for a simpler and more airtight exterior wall construction.
- All drywall is glued to the interior of SIPs and framing.
This is a long list and if I had to pick the most critical out of this, I’d say it is the SIPs, the under slab poly wrapping up to the SIPs and the envelope penetrations as the key details that are significantly different from normal construction. The Tyvec layer inbetween the two homes was also not the easiest sell to the construction crew.
We implemented all these items to the best of our ability, but mistakes were made along the way. It’s hard to tell where sealant or tape may not have been applied as needed. These omissions have resulted in a lower than desired tightness at our pre-drywall inspection. We’re not exactly sure of the actual result due to some HVAC and electrical openings that were not completely sealed at the time of the test, but we think we are somewhere in the range of 3-5 air changes per house at 50 Pascals of pressure. Our ultimate goal in the Passive House standard is 0.6 ACH @ 50Pa, so needless to say, we are doing our best to patch and caulk any and all air leaks that we found during the test.
The biggest lesson I’ve learned from both building the Passive Project and attending the recent Passive House Conference in Urbana, Il is that this 0.6 ACH @ 50Pa target is by far the hardest spec to achieve in the Passive House standard. I would say it’s even harder for a production builder to achieve in an environment that does not allow extra time and budget to fix some errors along the way. An owner/builder can decide to add another month or three and a couple grand to their budget to make air sealing changes on the fly. A production builder needs to have their plan bulletproof from the beginning and execute it to the T or the entire construction schedule and budget will be thrown out the window trying to recover.
Key Airtight Construction Lessons Learned
- Design multiple layers of air sealing. There should be one primary line of defense against air infiltration in your envelope assembly (A red line drawing), but you should not stop there. Take every opportunity to air seal anything you can from the outside to the inside of your walls and roofs. I have Mr. Hayden Robinson to thank for this advice at the Passive House conference. I was pleased to see him present on his latest Passive House design that looks almost identical to the planned construction on the Skinny Project.
- Design simple to implement air sealing details. If a crew is completely unfamiliar with a certain product or method, then sometimes it is best to re-think a detail. If everyone can understand the detail and believes it will be worthwhile to install, there is less chance of corners being cut when someone is not looking on the job site. Sometimes it is also better to spend more money on a better product that will reduce labor costs when time and budget are a concern.
- Design easy to inspect air sealing details. It is kind of common sense, but it makes sense that it is easier to require strict air sealing if it is easy for your forman to inspect the critical details during assembly and preferably after assembly. If a gap can be caught easily at the end of a daily inspection before it is covered up the next day, you’re going to end up with a much tighter house in the end for the least cost.
- Document your critical air sealing details well. This sounds logical, but most air sealing details can get lost in the mix when surrounded by many other structural and material details and callouts. We prefer to have specific diagrams on separate 8.5×11′s that can be made into a book to reference on the jobsite. These details get down to the very basics of the critical air sealing materials and junctions. They can also be blown up much larger than normal when placed on their own page.
OK, now that we’ve covered that, we can move on to what we are planning for the next project in terms of air sealing. As we have heard, we are going away from SIPs on the next project (dang, I need to post on why) so we have our work cut out for us.
Future Air Sealing Construction Strategies
- Document each critical air sealing detail on it’s own 8.5 x 11″ page for easy reference at the construction site as stated above.
- Tape all seams in exterior sheathing with Grace Vycor Tape.
- Install an one way air tight Water Resistive Barrier (Tyvec, VaproShield…) and tape all seams over the sheathing.
- Use Conservation Technology Building Gaskets at all critical wood to wood and wood to concrete connections while framing. I love these gaskets and will write an entire blog post later on why. They rock.
- Apply construction adhesive liberally at all wood to wood connections near the exterior sheathing where gaskets are not used.
- Install Conservation Techonology’s Drywall gaskets on interior framing to provide an airtight seal once drywall is applies without having to install costly poly or similar air sealing barrier.
- Use rigid XPS insulation with taped seams as air barrier on basement walls that connects to sheathing above and underslab insulation and poly below.
- Use specialized double sided butyl tape of window gaskets to seal around windows and doors in addition to spray foam. I haven’t made up my mind on the best method here. Gaskets are great and I think the tape is even better but only found in Europe right now.
- Use Protecto Wrap’s Dual Guard Threshold Tape under all door thresholds. This is a nasty area where metal is usually meeting concrete and just doesn’t seal well.
Well, I think I’m spent on this post. I apologize for the lack of diagrams. They will follow hopefully in future posts. Please add to my lousy lessons and ideas in the comments below and I will try to participate as much as possible.
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