Air sealing question: Assistance requested for 6th side of building envelope
My husband and I are homeowners who are working on a very deep energy retrofit of our 1860’s house, almost a total rebuild. We have learned so much from this website and the Q’s and A’s from everyone’s contributions, so many thanks to all readers and contributors. We had already started the building process when we learned of this site, and are now attempting to make accommodations for deficits in our education and processes during the early stages of our build.
I feel really good about our air barrier and sealing techniques for the 4 walls and the attic/ceiling components of our building envelope. The 6th side, the interface between basement and 1st floor is perplexing and we are asking this community for guidance.
Background details: 1860’s 2 story home (roughly 25′ x 25′ uncomplicated box) in Zone 5A, western MA. Basement is combo of rubble field stone and concrete block, still a work in progress, but we are using guidance of BSI-041/Rubble Foundations for basement renovation and moisture management issues, high water table due to nearby river. We lifted the house (multiple times) and do have a capillary break.
2×6框架与纤维素致密包计划,浸ered with Zip sheathing (taped, sealed meticulously), followed by Homeslicker for drain plane, followed by (2) 2″ layers polyiso (staggered, each layer taped, sealed), followed by 2×4 furring strips, siding (first floor cedar clapboard primed x2 and painted x2; second floor vinyl/polypropylene siding). Zip Sheathing on exterior walls serves as our air barrier component, and is continuous to the capillary break at foundation, and also is continuous to air seal over top plate to interior. We do have 22″ raised heel trusses, for 8/12 vented roof; soffit to ridge airflow framed by taped/sealed luan to minimize air disturbance to loose fill cellulose (not yet in place) expected to be R-80. We used REMOTE wall manual and GBA writings for lots of guidance throughout the building details.
2nd floor ceiling pitch at 5/12 with flat ceiling in middle third). Tyvek (more vapor permeable) used as air barrier component, continuous with the seal over the top plate, over entire 2nd floor ceiling, taped and sealed, attached to the underside of bottom chord of trusses. We have planned 2×4 strapping to underside of the Tyvek (to which electrical services will be attached), and perpendicular to the 2×4 strapping, we will also use 1x material for airtight method of drywall attachment (gasketed). Gable ends with Tyvek on interior from top plate to top of interior gable. No mechanicals in attic space, all are within conditioned space. All penetrations are gasketed, air sealed. So, I think we’ve done a pretty good job with continuous air barrier installation for these 5 sides of the envelope (let me know if you think that we’ve missed something, as we still may be able to make some corrections).
Our challenge is the 6th side, the continuous air barrier between basement and first floor. I wish we would have put Typar down on top of the beams and joists prior to laying the subfloor, but we did not. Our 1st floor subfloor is a bit of a hodge-podge, about 1/3 of it is the original (fir?) flooring that was in good condition, but does have minor air gaps to the eye, but not acceptable now that we know the importance of air sealing. Another 1/3 of the subfloor is newly installed T&G Advantech, although we must replace a small section since one of our worker’s spilled salt (for snow removal) on the floor and did not clean it up. We are concerned about this spot and are likely to replace that section. And the last 1/3 of the subfloor is not yet installed T&G Advantech, as it is over a crawl space, and we are waiting to drill for pex tubing before finishing the subfloor.
目前,我们的思想是把Typar (vapor impermeable) down over the subfloor. We plan to have hardwood floors, perhaps with an uncoupling membrane which depending on advisement here, may help also with air sealing, but am concerned about the vapor permeabilities of such membranes. An acquaintance builder told us that if we use Typar like material over subfloor, our floors will be squeaky. I’m not sure if this is true. Another concern is that when laying the hardwood floor, nails will be puncturing the air membrane significantly, and so this may not yield a good outcome. Alternatively, to air seal from the underside, the 1860’s floor joists are extremely irregular, have been sistered with new joist material, and we have a multitude of new lam beams to bolster the structural integrity of the existing system. It is a very complex underside to navigate relative to air sealing. We will be installing pex for radiant heating, and so that also complicates air sealing with foam.
Can this community offer us any guidance for the best way to air seal this last piece of our envelope, the basement/1st floor interface? Sorry for being long-winded in describing the problem, but I wanted any advisors to see that we are doing our best to be very attentive to our air sealing techniques, and we don’t want our efforts to fail on the last component.
Howard and Cindy
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Replies
Howard,
The best way to proceed would be to include the basement inside your home's thermal envelope. This would require you to insulate the basement walls with closed-cell spray foam.
If instead you prefer to establish your thermal boundary at the basement ceiling, then you'll need both an air barrier and an insulation layer at that location. Were you planning to install insulation at your basement ceiling? If you were, you can install a layer of rigid foam, with taped seams, under the floor joists (assuming that you have framing that allows that approach) -- and the rigid foam will become your air barrier as well as your insulation layer.
The reason that no one installs Tyvek or Typar under subflooring is that T&G subfloor panels (OSB or plywood), which are usually installed with construction adhesive, are already an air barrier.
如果你有一段底层地板由boards, the easiest way to improve airtightness at that location would be with 1/4-inch plywood underlayment.
Finally, your decision to install plastic drainage mat between your Zip sheathing and your layers of rigid polyisocyanurate is unusual. The air gap will encourage convection, and if the air gap is open at the bottom and the top of the wall, you will be introducing cold outdoor air into your insulation sandwich. This will undoubtedly degrade the thermal performance of your wall. Sealing the gap at the bottom and top would reduce this degradation.
I also have a plank only floor and was considering covering with underlayment. If an unfinished basement is insulated and air sealed is there anything to be gained by air sealing between the basement and first floor?
Shawn,
If you've done a good job sealing air leaks and insulating your basement, then it's not critically important to create an air barrier at the basement ceiling.
In some cases, you may worry about the indoor air quality in your basement. In that case, it might make sense to try to seal air leaks between your basement and the house above.
Thank you very much Martin. We were intending to include the basement inside the thermal envelope, with closed cell spray foam on the walls. Perhaps it is overkill, but I think that we still want some air barrier material between basement and 1st floor. I think your idea to use 1/4" plywood underlayment is great, and I think we will go with that. We didn't think of that, as we were too focused on the need for "continuous", reinforcing that continuous really is a system of various methods.
Thank you also for your comments regarding the zip sheathing and polyiso, separated by Homeslicker material. At the time that we were installing, we explored this question, to include the gap or not, through Q&A's posted within the various green building science communities and based our installation decisions on that feedback. We can re-explore our findings relative to those discussions to assist others who may be considering polyiso over zip-sheathed walls, and whether to include a gap or not between these materials. I think it is a really interesting building science question.
Thanks again,
Howard and Cindy
Howard,
One of the main reasons that rigid foam is installed on the exterior side of OSB is to keep the OSB as warm as possible (to reduce the chance of moisture accumulation in the OSB during the winter). Your decision to introduce a significant air gap -- presumably connected to the exterior air -- will undermine that goal, making the OSB colder than it might otherwise be.
Joe Lstiburek sometimes recommends the use of wrinkled housewrap -- a method with a much smaller gap than the gap introduced by HomeSlicker -- but only when the OSB can't dry to the interior (for example, when closed-cell spray foam is installed on the interior side of the OSB). In your case, the studs are filled with cellulose, so the OSB can dry to the interior when necessary -- making your HomeSlicker gap rather odd.