Construction week 9: ICF walls

Our basement concrete ICF wall sandwich.

Our basement concrete ICF wall sandwich.

Our house has taken shape! This week revolved mostly around setting up our ICF walls, which Mark discussed in his post from earlier this week

On Friday, we filled them with concrete. Our guys from Cornelis Grey leveled and troweled excess as they went. We rented a concrete vibrator to help ensure the concrete made it all the way down to the bottom of the hollow ICF walls. However, we may skip this step next time 'round, seeing as how the concrete had no trouble making it down and the vibrator actually cause our walls to bulge in a couple small sections. (These bulges are easily remedied, however, by shaving off some of the foam prior to gluing the exterior layer of additional foam. Still...the perfectionists in us would prefer no bulging.)

I was ecstatic to finally be able to walk inside our future home, the first floor at least. I can start to visualize where everything will be. Très cool. 

We posted a site sign over the weekend. Fortunately, our site, with all the construction material lying out on the front lawn, didn’t suffer from any Halloween pranking. I was a little paranoid that our port-a-pottie would be tipped or our foam would be toyed with. We should be able to clear up the front yard this upcoming week, or week thereafter, by backfilling the foundation and freeing up the backyard for materials storage and site facilities. And to make room for our neighbors fantastic front yard skating rink. Yep, winter is coming. Fortunately, the forecast looks pretty decent for the week ahead (no snow yet). Great framing weather! Next week will be another big one. Lots of change as we go up another level.

Wood and hammers and nails — oh my! 

Cheeky site sign.

Cheeky site sign.

Foundation Construction Details

Slab and foundation/basement walls with insulation and frost skirt

With the slab poured and the ICF well under way, I thought I would write a quick post with visuals explaining the construction methods and materials being used. I've created some images to go along with all the photos we've been posting to hopefully add some clarity to what you've been looking at so far!

The images above and below show the concrete foundation and foam insulation as it will be once completed. The image below has labels calling out the various layers. 

Components of the foundation

The biggest difference between our foundation and a typical residential foundation is the lack of concrete footings.  A typical foundation would pour strip concrete footings right onto undisturbed soil, then pour the concrete walls, and finally pour the slab inside the walls. In our home, the slab is poured before the walls and will actually support them, which is why it is so much thicker (8" instead of the standard 4") and has so much steel rebar in it. It is also completely contained within the foam insulation tray, eliminating any thermal bridging through the concrete to the ground. The end result of this is a concrete floor that will retain the heat it absorbs from the house above, rather than simply dumping it through into the ground. 

The walls on top of the slab are made up of three layers. First is the ICF (insulated concrete forms) from Nudura. These are like Lego for grow ups. They snap together to form the walls and are held apart by integrated webbing. The cavity is 6" wide and on Friday we will be pouring it full of concrete. Watch for photos this week showing the alignment system that will ensure the walls are straight and true as the concrete is poured. 

Once the concrete is poured and the walls straightened, we will be adding two more layers of foam from Styrorail to the exterior to build up the insulation value of the walls. The first layer has horizontal wood strapping embedded, and the second layer will cover this wood and effectively embed it in the middle of the wall. The foam will be glued in place using PL 300 glue, which is specifically formulated not to deteriorate the foam over time. The horizontal wood strapping gives us something to tie back into when we go to install our siding above grade. 

The slab poured and the first layer of ICF in place. 

Now let's talk about the big white elephant in the room: why so much foam? The amount of insulation is one of the trade offs required to achieve passive house performance on such a challenging site. Because of the limitations of orientation and south-facing window areas, we have to compensate by beefing up the thermal envelope more aggressively than you might find in other passive house projects. The final thickness was determined after several rounds of refinement of the energy model (using PHPP for those keeping track). The really nice thing about this configuration is that all of the concrete is on the warm side of the thermal envelope, where it will hold its warmth, and is protected from expansion and contraction. This alignment becomes especially important when we get to the design of the framed walls above...more on that soon. 

ICF at the end of 1 day's work.  

ICF at the end of 1 day's work.  

Insulated tray slab foundation

This is the insulated tray into which our structural concrete slab will be poured. This is the cosy foundation of our new home!

The last week has flown by as we've happily watched our house start to take shape. Last Wednesday we received our first insulation shipment from StyroRail. The first shipment of foam is for the insulated tray that will hold our structural slab. The foam is called SRP 400 and is a high density expanded polystyrene with a compressive strength of 40 PSI (aka strong enough to hold up our house). StyroRail shipped all of the pieces of the insulation tray cut to size, so aside from cutting holes to pass the plumbing through there was no cutting or waste on site!

Our tray insulation being delivered by StyroRail.

Before I get too far, I should explain what had to happen before the foam went into the hole. After installing the ground loop we backfilled with 6" of stonedust to protect the loop and create good contact with it. It was then time for the underground plumbing and electrical preps, which were very cleanly executed by Ackland Plumbing and Portage Electric. Then 12" of additional fill were laid in two lifts. Each time fill was added it was compacted to provide the bearing strength we need to support the house. The last pass levelled the gravel pad to within about an inch of level all over, simplifying the installation of the foam.

At last it was time for the foam. After laying the first few pieces we noticed that even though the gravel was nearly level there were still voids under some of the pieces. In order to get as close to perfection as possible we decided to use some extra stone dust to fill the voids and truly line up the foam blocks. It worked like a charm, and with only 2 days of work the team from Cornelis Grey had all the pieces in place, secured with straps and foam and gravel ballast.

Now that's a straight edge. Good job team!

Next steps are to install the frost skirt and drain tile around the perimeter of the tray, after which we will backfill over them to provide extra support to hold everything in place. We will then lay the 6 mil poly vapour barrier and rebar into the tray prior to pouring concrete early next week!
This stage has been really exciting to watch as I think it really showcases how smooth and clean the process can be when well planned. The whole team is doing a great job so far. I can already picture the concrete slab nestled nicely into its insulated tray, safe and warm and comfortable. The robustness of our building shell starts with this foundation, and it's off to a great start.