As we continue the process of designing and building our home, we are at the stage where we are faced with making decisions. Our first big choice is critical to the rest of the design and has a big impact on the overall cost. The building envelope, otherwise referred to as the wall construction type, is a high cost item, and can affect aesthetics significantly. One of our goals is to create a design that is sustainable, and for an exterior wall, that usually equates to energy efficiency.
Insulated concrete forms (ICF) offer a high level of energy efficiency compared to traditional ‘stick built’ wood framed walls. ICF construction is essentially a concrete and foam sandwich which is put together like a set of Legos on site. Once the foam blocks are in place, concrete is poured in the center cavity and allowed to set. This forms a sound structure with excellent thermal properties. The disadvantage of this type of construction is the overall wall thickness; ICF walls tend to be very thick, and total wall assemblies (the structure plus interior finish and exterior cladding) can be upwards of 14 inches thick. It is easy to see that choosing ICF means giving up some interior space to the wall.
For our design, space is at a premium. We are on a relatively tight lot, and every square foot counts. During our late night study break, we took a walk and ended up discussing ICF as it relates to cost and square footage requirements. What we discovered was that at high level, ICF demands not only a cost premium, but a space premium. Here is what we worked through:
Using basic averages for our design, the house has about 620 linear feet of walls. If we multiply that out by 12 to convert feet to inches, we get 7,440 linear inches of wall. Next, we looked at the delta between an ICF wall thickness and a SIP (structural insulated panel) wall thickness. SIPs are similar to ICF walls in that they are a sandwich of foam and sheathing; typically a wood sheet is glued to the foam, or some types use steel studs (which is the kind we are considering). Basing our numbers on the products I’ve seen, there is a difference of about 5 inches between the two product types. That means for every linear inch of wall, we give up 5 inches of depth if we choose ICF construction. Length x width reveals that we lose 37,200 square inches to ICF; breaking that back into square feet for easy reference means we give up almost 260 square feet of space if we build with insulated concrete forms. That space is eaten up by the walls, and we’ll never get to enjoy it beyond the energy we save (i.e. dollars not spent on electricity).
The next big item we looked at was the cost of the lost square footage. If you build it, you pay for it, even if you never get to ‘use it’. For us, we multiplied out the cost for an ICF house by the lost 260 square feet and realized we would be spending a whopping $41,600 on lost space! Then we had to also account for the increased cost of ICF compared to our SIP alternative. The product I’m considering boasts that it comes in at half the price of ICF, so the delta between the two wall options means we’d spend an extra $20,000 to go with ICF over SIPs. Add the two numbers together, and we’d pay over $60,000 for lost space and the same R-value (which tells us how energy efficient the wall is). The increase in cost between the two walls types alone isn’t terrible, but once we account for the lost space due to increased wall thickness and look more closely at the two products side by side, we don’t add a lot of value for the increased cost.
Granted, we may lose out on some sound transmission advantages and additional natural disaster resistance, but conceptually, a well made and properly installed SIP wall should perform on par with an ICF wall at a much lower cost – both financially and spatially. When it comes down to energy efficiency, we wouldn’t see any kind of significant savings going with ICF compared to SIP, as the cost broken down over time would take decades or even centuries to pay back.
Finally, as our builder pointed out, we would also need to accommodate the added weight of the ICF structure on the foundation. That means a costlier foundation design that requires more engineering and more material (concrete, re-bar, deeper form work, deeper excavation, etc.). One decision quickly affects a host of other components.
Now that we’ve taken a high level look at the hidden costs of ICF, we can start to see that paying a lot more for lost space doesn’t make much sense for us. The premium we pay for space in the city only compounds this figure. We’ll be taking a closer look at these wall type choices and doing more research on the pros and cons of each before settling on a final choice; maybe we’ll find something that surprises us. Do you have experience with an ICF or SIP home? Let us know in the comments!