Every man needs a cave. In the modern world, we use the basement to conduct our instinctive cavemen-like activities. This week, we are building a very comfortable underground cave. How do we make it comfortable? The key is in the insulation underneath the floor.
First, a wet or moldy cave is not a healthy of fun place to hang out, so, you want to sort out the drainage issues. This is why we brought in the stone shooters last week to create a 4″ bed of stones that provides capillary drainage. Otherwise, your slab acts more or less like a giant sponge, bring all the moisture up and eventually ruin your carpet or wood floor. Once the stone is in place, we are ready to build the slab.
Leon Home Improvement came back to do this work for us. They start by painstakingly leveling the gravel and temping it down. Because our foundation walls sit right on top of a bed of gravel, they had to be extremely careful to not get too close to the crushed stone footing and undermine the structural integrity of the Superior Wall system. They were able to devise a way to keep the gravel secure and install the specified amount of foam insulation.
For the insulation we are using termite treated EPS foam (Geo Foam). What you see above is the 4 inch foam that we’ve installed in the center of he slab all the way to 12 inches from the edges. This is one of the issues we have to deal with because we used Superior Walls. We could not insulate all the way to the edge because that 12 inches of undisturbed gravel bed is structurally important to the walls. We mitigated this issue by adding a second layer of 2 inch foam on top of the 4 inch and this time placing it closer to the edge by laying in on top of the gravel footing.
Here’s SW’s footing requirements:
This is what the edge looks like in the end.
In the end, we have 6 inches of EPS under most of the slab, 2 inches along the edges. After the concrete pour, we will place an inch of XPS on top of the concrete to give the assembly about R-30 of insulative power.
Some might argue that by insulating this much increasing our electric bill in the summer time. They are correct. At that level (8 ft below grade), the ground temperature will stay around 55 degrees year round. This 55 degree temperature is perfect for cooling the house in the summer. By insulating heavily, we are losing some of that natural air-conditioning. However, in our area, the winter heating needs are still significant higher on a yearly basis, that insulation will save a lot more money and electricity in the winter. There is a point where it really becomes counter-productive to over-insulate. So, there is definitely a trade-off there.
Arlington Passivhaus 
Posted by Len Moskowitz on 2011/08/18 at 12:35 pm
You wrote: “This is one of the issues we have to deal with because we used Superior Walls. We could not insulate all the way to the edge because that 12 inches of undisturbed gravel bed is structurally important to the walls.”
Is that because you used 9-foot walls? If you used 10-foot Superior walls (set deeper), then could you have run the full 6-inches of EPS underslab foam all the way to the wall?
Posted by zeropassiv on 2011/08/18 at 3:49 pm
We actually used 10ft Xi walls here.
I added to the original post a page from SW’s builder’s manual that shows the specs of the crushed stone footing. Basically, you can’t have insulation come within that trapezoid shaped bed of stones under the SWs.
Another limiting factor was that SW requires a 2″ concrete-to-concrete connection at the foot of the walls.
One solution is insulate above the level of the crushed stone footing, connecting the 6″ EPS with the SW foams but then you would need to cut away a 2″ strip of foam all along the SW to expose the concrete and pour concrete to create that 2″ Concrete-to-concrete connection. This just seems crazy.
Posted by Len Moskowitz on 2011/08/18 at 4:26 pm
What floor-to-ceiling height did you design for in the basement?
Posted by zeropassiv on 2011/08/18 at 7:07 pm
We basically will have a 9ft ceiling in the basement.
Posted by Len Moskowitz on 2011/08/18 at 4:41 pm
Superior Wall told us that when the slab is above the bottom of the wall (say with a 10-foot wall and a 9-foot floor-to-ceiling height), the slab can be tied to directly to the studs using their NuTie non-metallic (fiber-reinforced plastic) rebar. That solves the thermal bridging problem and there’s no need for a 2-inch concrete-to-concrete interface.
Posted by zeropassiv on 2011/08/18 at 7:15 pm
That is very interesting. Which Superior Wall was it? The people we worked with was Weaver Precast out of PA. That certainly solves the thermal bridging issue and makes insulating much much easier. When we were working on this. This was not a solution that was presented. Are you planning to use SW on a project?
Posted by Len Moskowitz on 2011/08/18 at 11:05 pm
We got that information from Wayne McAuliff at Northeast Precast (www.northeastprecast.com, formerly known as Superior Wall of South Jersey).
Yes, we’re considering using SW for the foundation of our Passive House in northern NJ. See the “Passive House NJ” group on Facebook for more details.
Posted by Len Moskowitz on 2011/08/18 at 11:08 pm
For the foundation, did you compare the costs of SW, concrete block and poured concrete? If so, what did you find?
Posted by zeropassiv on 2011/08/19 at 12:08 am
We never considered concrete block but we did compare between ICF, poured and SW. ICF was the most expensive. I think poured and SW came in pretty close. We have the tendency to pick things with fewer moving parts or fewer chances of human error. Also, we had used SW in a previous project and were very happy with it.
Today, I still think SW is a great product but I am not sure if I would use it again if I were to build another PH. First, one of the good things about SW was the speed, but having to arrange power outage and getting rained out on our date meant it took another three weeks before we could set. However, our plans already called for SW, changing it out with something else would have probably taken longer to get through plan review.
I think the solution of using non-metalic rebar to eliminate thermal bridge is a good one. I don’t know how you or your structural engineer feel about insulating under the footings but I know it is not really possible to do with SW. Unless you can use some sort of granular foam glass to replace the crushed stone footing. Without footing insulation, you still don’t have that continuous insulation characteristic of PHs. I understand by not having to do that 2″ concrete-to-concrete connection, you have a continuous insulation. However, I still see the concrete studs as thermal bridges. Especially, in corners that requires a number of them (take a look at the pics and videos I have of our SW, there’s a corner that we have 6 of them together). These are essentially 2.25″x7.5″ solid concrete studs wrapped in 1″ EPS. So, even though you may have great insulative power between the studs, you only have about R4.13 where the studs are. We are adding a 2″ layer of EPS on the outside of the concrete shell, we are still going to have only about R-12.5 with 3 inches of EPS.
Of course, the way we look at it is that okay, we are going to lose some heat from the studs and other thermal bridges that exist, we’ll just fill the cavities with insulation to mitigate that heat loss. I am just not sure if this is going to be something that breaks down our energy performance. When we do our first blower door test, I will be sure to walk around with the thermal cam and shoot some shots of the walls. I have a feeling I will see some blue.
Have you considered poured walls but having EPS cast on one side of the concrete or in between two sides of concrete? I think a lot of concrete companies do that. I will seriously consider this option the next time.
Posted by Len Moskowitz on 2011/08/21 at 7:23 pm
We’ve given up on trying to insulate directly under the walls, so we’re concentrating on thermally isolating the entire wall from the rest of the structure.
As you say, we can make up for the low-R stud area by insulating more between the studs. And fortunately, with deeper ground temperatures in the 50’s, the insulation requirement is reduced for much of the wall.
We haven’t considered poured walls with EPS cast on one side. I’ll pass the idea by our architect and PH consultant. Thanks!
Posted by zeropassiv on 2011/08/21 at 9:44 pm
There are a few variations to the concrete wall cast with EPS. You can essentially do it on the inside, outside or the middle (kind of a reverse ICF). The general idea is that the concrete contractor builds the metal forms, they insert the foam on the desire part of the wall. Fasten them onto the forms. Pour concrete. I like this because you have more choices and you can consider the potential of getting some thermal mass effects while having a high R-value. See http://www.thermomass.com/
I like the idea of having foam in the middle and concrete on both sides. It seems to be a good idea because you get some benefits of the thermal mass, you can mitigate potential moisture issues of moving the dew point and you can rest assure that the EPS is not being chewed up by termites or other things. The only thing is whether you can get a thick enough foam between the concrete. You can always outsulate or add more inside though if you need to.
Posted by Christine on 2011/08/24 at 11:56 am
I’m Len’s PH consultant and I was wondering if you might discuss modeling the Superior Wall in PHPP with me? I also left a voicemail in your office.
I’m glad the earthquake didn’t cause any problems on your site!
Posted by zeropassiv on 2011/08/24 at 1:33 pm
Hi Christine,
I am happy to discuss it. This is a difficult problem because of the different components at different layers that make up the SW system. We are still waiting for PHIUS to give us direction.
Recent events may have caused some delays on their part. However, we do have some technical documents from Weaver Precast to show their test results in terms of R-Value. They basically show that the walls are tested to have R-12.5 on a whole. For now, we are basically assuming that R-12.5 is correct and in PHPP just entering the 12.5 for that part of the assembly.
We are waiting to see what PHIUS tells us to do. We know that thermal bridges exist in SW but with insulation of both the outside face and inside face of the walls, we should have mitigated the horizontal ones. With the vertical ones, we hope to mitigate it with that one inch XPS on top of concrete. There will still be bridging through the concrete studs we are thinking that we can just spray cellulose into the 5.5 inch deep cavities between the studs to counteract the heat loss.
I acknowledge this is not the most precise way to deal with the problem and there are a few assumptions involved here. However, I am not sure if there really is a cleaner way to handle this. You might want to talk to John Semmelhack, too. He was the PH consultant on a school building in Charlottesville that used SWs.
Posted by zeropassiv on 2011/08/24 at 1:34 pm
Christine, I hope that was helpful. You can also call me on my cell at 703-839-2255 or email me if you have more questions.
Posted by Christine on 2011/08/24 at 2:44 pm
Thank you so much, I really appreciate the information. I’m sure PHIUS is pretty crazy right now with all that is going on, so it may be hard to get that answer.Representing the SH in the R-values sheet seems anything but straightforward, but as I say when giving talks about Passive House, it ain’t for sissies!
Posted by zeropassiv on 2011/08/24 at 6:47 pm
That’s right! It ain’t for sissies! Are you planning to come down for the D.C. conference?
Posted by Christine on 2011/08/31 at 2:56 pm
Hello, I’m sorry I didn’t see this sooner. I’m hoping to! I haven’t decided for sure as I have wee ones at home… I trust you’ll be there?
Christine
Posted by zeropassiv on 2011/08/31 at 7:24 pm
It’s ok. I am going to be there. Especially, since I am local. I think it will be an interesting conference and may be a good time and place to hear what folks have to say about passivehouse’s future in the U.S.