ICF or Interior Rigid for Basement
Hey everyone, We are in the middle of planning stages for our new house which we hope to start in the next couple months. We’re just north of Worcester, MA (CZ5) and are aiming for a ~1,500 SF pretty good house (R40 walls(2×6 w/ exterior rigid) , R60 ceiling (i-joist rafters), triple pane windows, ASHPs and an ERV), plus a basement that we will eventually turn into a playroom / fitness room.
The debate I’m currently having is how to best insulate the basement walls, with the plan to condition and utilize the basement as future living space. We will have a vapor barrier and subslab insulation, but I’m curious as to people’s opinions on the tradeoffs between comfort, efficiency and cost for the different options.
Option 1 ICF Foundation – I know some local foundation companies offer this as an option and we will most likely price it out.
Option 2 – 2-3″ Interior rigid + 2×4 walls w/ batt insulation and GWB – I’m strongly considering this as it’s something I can complete while the house is being built, and i have access to some reclaimed rigid for cheap. I may have an insulator sprayfoam the rim joist as well. This option I feel leads to the least thermal bridging, as it connects the slab insulation to the above grade wall insulation best.
Option 3 – 2-3″ exterior rigid – This would tie in well to the exterior rigid I’m planning on for the above grade walls, but it would end up requiring some sort of protection where it’s exposed above grade. It also leaves a thermal bridge at the footer.
I’m interested to hear thoughts on these options, or something else i haven’t mentioned. Thanks in advance!
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Additionally, before anyone comments, we are planning on either reclaimed XPS or EPS for subslab insulation, but will be pricing out Glavel as well.
I'm surprised there's no opinions here. Even if someone has an idea of costs between the different assemblies, that'd be helpful!
I don't do a lot with foundations, but since you've been waiting a few days with no comments, I'll try to help you out. Some similar questions to yours have come up here on GBA in the past, and the usual comments have been along the lines of "ICF is great, but expensive and doesn't offer a huge performance advantage over the traditional foundation with interior (or exterior) rigid foam". ICF's main advantages seem to be that it makes the construction process a bit faster.
Note that there are some advantages to putting your rigid foam on the EXTERIOR too, since it brings the concrete into the "warm" side.
XPS is a better choice for sub slab insulation, and if you're using reclaimed materials anyway, there is no issue with "greeness" (normally XPS is a less-green material due to the blowing agents used in it's manufacture). There was an article about a study done by the Alaskan highway maintenance agency if I remember correctly that found that EPS took on significant amounts of water when used as a subslab insulating material, and this water greatly reduced it's effective insulation value. XPS was much less prone to this issue.
All options mentioned can work fine. Dealer's choice. Cost will largely depend on contractor familiarity. I know carpenter/GCs that will self-perform ICF foundation for cheaper than the other options (faster and one less sub), but more often Option 2 interior EPS+furring is most cost-effective and allows for the least thermal bridging to subslab foam. It also has the advantages of being somewhat DIY friendly and doesn't limit options for waterproofing the way ICF does -- has to be self-adhered membrane for ICF. Exterior rigid allows the concrete to stay within the envelope and can be the 'finished' surface if you can handle that look, and saves a bit of floor area. Big disadvantage is protecting the foam. Usually an ongoing maintenance item. As you said, not pest proof. Everyone ignores the fourth option-- foam in the center of a concrete sandwich. Pretty much always the most expensive and requires proprietary foam inserts, but very robust:
Personally, I like no basement. Otherwise I spec Option 2 save for the rare days Option 1 prices out better.
Appreciate the responses. That kind of lines up with what i figured, and I'm leaning toward option 2 given the inout. It's something I can do myself during construction and I won't have to worry about the exterior foam getting damaged or chewed through by pests.
I really tried hard to get away from a basement, but I think we can really benefit from the large open space. My wife practices yoga daily and I have a young daughter that would absolutely love a playroom (and eventually a hangout area away from her parents for her and her friends). A basement seems to be the most cost effective way of getting a large space like this. Seeing as how we already have to excavate for footings, pour a slab, etc, it seems cheaper to do that than to try and add 900+SF above grade to get that space.
I would look at pricing out some of the wood fiber based insulated concrete forms (ie Nexcem). I haven't tried it yet but it seems to solve most of the issues with exposed insulation while reducing the amount of concrete needed.