Image Credit: Eric Whetzel The red arrow shows a thermal bridge and gap in the vapor barrier that would have occurred had Delta membrane not been used. A corner of the footing with the Delta membrane “keyed” into the surface. The Rodenhouse fastener proved easy to install: drill a hole with a 5/16-inch bit in a hammer drill, and tap the fastener home with a hammer. Where the outside corner of the front porch meets the foundation wall is one example of an unwanted thermal bridge. Wing walls that will support a porch stoop are wrapped with Roxul to reduce thermal bridging. The outside edge of the plywood window bucks are flush with the outside face of the mineral wool on the foundation wall.
Editor’s note: This post is one of a series by Eric Whetzel about the design and construction of his house in Palatine, Illinois, a suburb of Chicago. The first blog in his series was called An Introduction to a New Passive House Project. For more details, see Eric’s blog, Kimchi & Kraut.
For the top of the footings we used a product from Cosella Dörken called Delta Footing Barrier. Acting as a capillary break, the membrane is supposed to help prevent moisture from wicking up from below the footing, where it could then migrate into the foundation wall and into the basement, or even the wall assembly above (worst-case scenario), causing mold or other moisture-related damage.
It should help make the basement a very livable space, especially when combined with significant amounts of insulation on the exterior walls and under the slab (see Image #2 in the Photo Gallery at the top of the page).
In other words, this junction represents a weak point in our thermal envelope and vapor barrier. Passive House proponents often talk about using a red pen on a construction drawing to follow the air barrier and thermal envelope (the goal: no gaps in air sealing or the layers of insulation). In theory, you should be able to do this all the way around the structure without once lifting your pen. If you can lift your pen (meaning there’s a gap in your air barrier or thermal envelope — which would be the case without the Delta membrane on top of our footing) — then it’s a weak point that should be addressed, if at all possible.
Even with significant insulation on the exterior wall of the foundation (Roxul Comfortboard 80: 2 inches plus 3 inches), along with a sprayed-on waterproofing membrane, as well as a vapor barrier (Stego Wrap) and insulation (Roxul Comfortboard 80: 2 inches plus 2 inches) under the basement slab, this junction where the three elements meet — slab, footing, wall — is a weak point. Although it doesn’t address the weakness in R-value, at least the membrane should keep the moisture at bay.
With a 9-foot basement, we’re hoping the temperatures at this depth are consistently mild enough to avoid any kind of significant energy penalty. I’m confident this will be the case because in our last home, a typical suburban tract house without much insulation, the basement always stayed cool in the summer and warm in the winter, even though the ducts to the basement had been closed off so the space never saw any direct benefit from the HVAC system.
For minimal cost in materials, the Delta membrane seems well worth it for the added peace of mind (see Image #3 in the Photo Gallery at the top of the page). I couldn’t find any local suppliers who carried the Delta membrane, so I ordered online from Spycor Building Products.
Insulating and waterproofing foundation walls
For the walls, first we used a spray-on waterproofing membrane from Tremco. After the waterproofing was complete, we began installing the two layers of Roxul Comfortboard 80, a dense, rigid form of insulation that can be used below grade. It will give the basement foundation walls R-20 of insulation value.
Roxul is showing up in the big box stores here in the Chicago area, so it’s becoming easier to order. When questions came up about how to install Roxul, or which product to use where, their technical help via email was great. In our case, Fiona Schofield gave us a lot of useful information, including information on the long-term performance of Roxul (a.k.a. stone wool, rock wool, or mineral wool) in a below-grade application.
In addition, after finding a video online, in which what looks to be a European version of Roxul is attached to a cinder-block wall with an adhesive or a thinset mortar, I contacted Fiona and learned it was OK to use an adhesive for our first layer (PL Premium, or similar polyurethane adhesive caulk) as long as we used a mechanical fastener for the second layer. In effect, the first layer just needs to stay on long enough for us to get the second layer up and attached with a mechanical fastener. This really saved us some time since the guys didn’t have to drill two full sets of holes.
Liquid Nails worked really well at keeping the Roxul in place, even when the foundation was damp in certain areas. The guys also didn’t seem to mind cutting or otherwise working with the Roxul. We used serrated knives we purchased from Home Depot, made especially for cutting rock wool.
After some experimenting, the guys also began using a small reciprocating saw, and even a table saw, to cut the insulation to size. I had my doubts about the table saw, but Phil said the Roxul cut easily, and it really didn’t seem to kick up a lot of dust (although he did wear a dust mask for protection).
Once the first layer of Roxul (2 inches thick) was in place, we could then install a second, 3-inch layer on the top of it. After a lot of research, and even posting a question on Green Building Advisor, we decided to go with the Rodenhouse fastener (Plasti-Grip PMF) shown in Image #4 in the Photo Gallery at the top of the page.
These really are easy to install. Using a hammer drill with a 5/16-inch masonry bit, the guys drilled a hole to the depth of the fastener, then tapped the PMF fastener home with a hammer. It’s a genuinely straightforward process. Sometimes a fastener wouldn’t sit perfectly, but as long as a majority of the fasteners on each board did, it didn’t seem to be a problem. Based on what I read online, they were much easier to work with than Tapcon or similar concrete screws.
They weren’t cheap, but they were well worth the cost in materials for the savings in labor (and frustration). And Mitch Mahler from Rodenhouse was easy to work with via email in terms of ordering or getting answers to technical questions.
Roxul normally recommends five fasteners per piece (one at each corner and one in the middle), but we found that four on a normal piece, and three for a long, narrow piece of insulation, worked fine — at least for the foundation where the backfill will help to keep the Roxul in place over the long haul.
Thermal bridging in the foundation
Following Passive House science principles, we tried to remove as many points of thermal bridging in the structure as we could. One area where this was addressed in the construction drawings was a 7-inch thermal break between the basement foundation and the attached garage foundation. In other words, there would be no physical connection between the garage and house foundations at all. The only connection would occur above, at the level of framing, where they would be tied structurally together.
The idea was that we could place our two layers of Roxul (5 inches total) in that gap, thus maintaining our layer of Roxul on the exterior of the foundation, uninterrupted (the key point here) around the perimeter of the basement foundation.
On the day the footings were installed, however, our concrete subcontractor expressed serious reservations about the long-term structural stability of the framed house and garage above this gap. He was worried that over time the two foundations might settle and move apart, wreaking havoc with the framed structures above.
So I was back to post another question on Green Building Advisor: How important is a thermal break between a house foundation and an attached garage foundation? (Here is the link to the Q&A thread.) Unfortunately, there just doesn’t seem to be a lot of information available as to how to proceed. In the end, we decided to ignore these connections, hoping that the thermal bridging at these two points (garage-house, garage-front porch-house) won’t be all that significant to our heating and air conditioning costs.
I assumed that Passive House builders would incorporate rigid foam insulation into the concrete forms at these points, but I couldn’t find any pictures or descriptions showing or talking about this in books, magazines, or anywhere online. Either Passive House builders ignore these kind of connections, or I just missed the information somehow.
We did our best to cover these thermal bridges, but clearly it’s imperfect, so all we can do is hope there won’t be a significant energy penalty associated with these connections.
Installing basement windows
As the Roxul was going on the foundation, Phil and Nils installed window bucks for the two basement windows (see Image #7 in the Photo Gallery at the top of the page). The bucks were sized so they meet up flush with the two layers of Roxul. Eventually, a layer of HardieBacker cement board and two coats of Tuff II (the product we’ll be using for the parge coat) will cover the window bucks, and also the transition between the top of the foundation walls and grade around the perimeter of the house.
I initially intended to use the R-Guard line of liquid membranes by Prosoco for air sealing and waterproofing all seams and window/door openings, but cold temperatures made this impossible (they require 40°F and rising, which would be the exception rather than the rule here in Chicago for December and January).
Our Plan B was the series of Pro Clima products sold by 475 High Performance Building Supply. Most of them, including the sealant, can be used down to 14°F without issues. Another option would’ve been the line of Siga tapes, another popular choice used in Europe, available from Small Planet Supply.
So as the window bucks went in, I followed, applying Contega HF sealant to all the seams and gaps. The sealant is acting as our first layer of air blockage. It’s super sticky, so I don’t doubt that it’s permanently flexible. I did a mock-up of our wall assembly months ago, and the HF on the seams is still tacky to the touch. It goes on light green, then slightly darkens as it dries.
Here are some tips for using the sealant:
- I’ve found that completely snipping off the metal clip on the end of the sausage (as opposed to just cutting a couple of small slits around it) prevents it from getting jammed in the front end of the gun.
- If I have a half-finished sausage of HF at the end of the day, I put it in a tightly wrapped plastic bag overnight, which allows me to use it within a day or two without any problems.
- Use a tiny spatula to tool the HF into place rather than your finger, as you normally would with a caulk — it’s just too sticky to work by hand.
- Because the HF is so sticky, I wear nitrile gloves, so when it starts to get everywhere — and it will get everywhere if you let it — I just simply change to a new pair of gloves.
- For clean up, the Citrus Solvent we’ve been using with the tung oil works great.
- The Newborn brand of sausage gun we’re using for the HF sealant, found on Amazon, is a really well-made tool.
Getting window details right
It’s easy to forget the realities of a construction site when planning details, like the use of the Pro Clima tapes. I pictured it being a pretty straightforward process, not a winter day in the 20s, fingers numb, propped up on an unbalanced ladder in the hole, while the other guys are cutting wood and Roxul around me — a case of adapt or die, I guess, and a reminder not to be too overconfident about the products you’ll be using, or the installation process that inevitably goes with them.
It was important that the connection between the window bucks and the concrete of the foundation be air-sealed and made watertight before it was completely covered by the two layers of Roxul.
It was in the 20s and 30s, so the HF sealant took a couple of days to firm up before I could apply the series of Pro Clima tapes. I used a combination of tapes, including the Tescon Vana (the bright blue), Profil (light blue — great for making inside and outside corners), Contega Solido Exo (black, 6 inches wide), and the Extoseal Encors for our sills (475 has a great series of videos showing how to use each tape).
We knew the bucks would be sitting for some time, exposed to the elements, before the windows actually show up, so we decided to completely cover the openings just to be safe. This gave me extra practice using the tapes, which definitely helped, and it meant not stressing out every time the forecast called for rain or snow.
The only tape that gave me fits was the black Contega Solido Exo. It’s thinner than the other tapes, so it wants to stick to itself (wrinkles are more difficult to avoid), and I find it much harder to pull it away from its peel-and-stick backing than the other tapes. I worried that the Extoseal Encors might be difficult to get right, but it — along with the Tescon Vana and Profil tapes — was surprisingly easy to work with.
In lieu of on-site training from someone who’s used a specific product consistently, videos are invaluable when using new products and you want to get the details right. Without them, you’d be in for a frustrating process of trial and error. This one was my Bible for installing Extoseal Encors:
Extoseal Encors window flashing demonstration with Chris Corson of Ecocor
Because the Extoseal Encors can stretch around corners it’s easy to stretch it too much, thereby inadvertently thinning it out. I’ve found that when I get to a corner it’s better to just fold it around the edge rather quickly, without overthinking it too much, which helps to maintain the thickness of the material at and around the corners.
I can’t recommend enough doing a mock wall assembly, or practicing on scraps, to get a feel for using these products, before you find yourself on-site doing it for real.
We’re almost ready to climb out of the hole. It will be exciting to watch the guys start framing so we can see the basic form of the house begin to take shape.