Installing insulated concrete forms (ICFs) can save you time and money in the long run. Once the site has been prepared and the footings are down, the blocks for most basic basements can be installed in a single day. A little more time is required for bracing and then tying them together as well as rebar, but it’s all worth it. You may pay a small premium of 5% to 10% over a traditional plywood form and pour basement with ICFs, but the R-value you will get (R-24 or better with Logix) will save energy costs down the road almost guaranteed.
Here’s a step by step process for installing a basic 24 x 24 foot walk-out basement (8 foot high) using ICFs.
Step #1: The Footings
It’s important to start with a good flat foundation, especially when installing ICFs. You don’t want to start to assemble blocks on a uneven surface, since any unlevel area will become even worse as you build higher and throw your walls out of plumb. Be sure to box out wider footings where your point-loads (bearing points) are, such as beams, posts and sides of large windows or openings – in my case a chimney cubby.
Be sure to make the footings wide enough to accommodate your ICF blocks. I used 6″ width walls, with 2 3/4″ thick Logix ICF blocks on either side, so my total wall thickness was 11 1/2″. My footings needed to be 18″ wide to ensure that the ICF blocks remained in the middle of the footings. Always allow for a little bit of error. The last thing you want is an ICF block hanging off the edge of your footings. You’ll also need to install the appropriate rebar from the footings, as per your local codes. I need 10mm rebar spaced about 10 inches apart vertically. The density of your rebar, and the height of the concrete walls and structure above will determine how much rebar you need. Ask your local building inspector if you are not clear. My Logix rep was able to fill me in with the requirements. I used Logix since they are thicker than cheaper foam block options and have a great fire rating (they are not the cheapest, but worth the extra few pennies per block especially if you get a blow-out with a flimsy competing product).
Step #2: Assemble The ICFs
Once the footings are down, then the fun begins. Your local ICF block rep may likely give you a quick tutorial on how to start building your blocks, but if not, at least be sure to read their manual. Generally, you will start with a corner block, and then build around one row at a time. You’ll have to thread the blocks over the rebar, and then use plastic zip ties to connect the blocks together using the internal plastic webbing in each block. This webbing also serves to hold horizontal rebar to add strength to your walls. Again, check your codes, but we were required to have horizontal 10mm rebar on every row of blocks.
You’ll also have to cut the blocks appropriately as you go for doors and windows. You will need to use pressure-treated wood (brown in our case) to ensure that the window and doors are framed to last, especially when curing concrete will be covering portions of them. Don’t worry about a few small gaps in the foam blocks here and there, you will fill them later with expanding foam to ensure that no concrete leaks out.
Step #3: The Bracing
Now you should have an impressive castle of white blocks in front of you, congratulations, almost done! Well, not quite, since a very important step to avoiding costly and hair-raising concrete blow-outs is bracing. Your ICF block manufacturer should supply you with the appropriate bracing. With Logix, my rep show up with heavy duty metal bracing for the inside which also doubles as scaffolding for the pour.
You can never have too much bracing, and in fact, I recommend you add your own wood back-up braces around the lower blocks. Remember, you are about to pour 8 feet high walls of concrete, which weights a ton. There will be an incredible amount of outward pressure on the bottom 2 or 3 rows of blocks (18″ high for Logix per row). We installed some extra 2×10x along the bottom seam between row 1 and 2 to guard against a blow-out scenario.
Step #4: Rebar and Sealing
You now should insert the final vertical rebar required by your local buidling codes. Again, in my case, it was 10 mm spaced 10 inches or so apart due to 8 foot walls that were 6 inches thick. With 8 inch thick walls, the spacing is wider, but you’d again have to talk to your inspector. Also, code requires damp proofing membrane on the outside of your ICFs. It does seem like a bit of overkill, since you already have a thick and highly water resistant foam barrier around your concrete walls. But, the code is the code and it will help ensure that you are not going to get a leaky basement. Just a note, when you backfill, we use left over plywood against the Blueskin membrane to prevent rocks from tearing or damaging the membrane. The plywood can then mostly be removed afterwards, or just left underground. And finally, you’ll need to go around with your expanding foam to fill in any gaps or cracks in the foam, especially around doors and windows and along the base of the footings.
Step #5: The Concrete Pour
OK, finally we can pour the concrete, wahoo! But don’t get too excited, this is where you can really blow it, literally! The key is to get advice from someone who has done this before to handhold you through it. You want to start pouring slowly into the corners. You also want at least 1 right angle pipe on your concrete pouring spout to slow down the pour. We recommend even 2 right angles to slow down the concrete, and also use not larger than a 4 inch spout. If you get too much volume of concrete into your forms too quickly, they will blow out.
You will also need to vibrate your concrete in each block to get all air bubbles out. Your ICF vendor or local rental shop should be able to get you a motorized concrete vibrating tool. Again, be gentle so you don’t damage the foam blocks.
Now listen up, this is important. You also only want to pour the first 3 feet (2 rows or so) of concrete at a time, and no more! This is key. You must let the first few feet of concrete set up for 1/2 hour or so to ensure that it hardens enough not to want to blow out sideways through the seams. Once you are confident that the first few feet has set up enough, only then start to pour the next few feet. You’ll need to do 3 passes for 8 foot walls.
Once poured, you will have to wait for the walls to set up a good week before you can put any significant weight on them. In a month, the concrete will have set up to 90% of it’s full strength. It can actually take up to an entire year for concrete to cure to it’s final full strength, depending on your climate.