Foundation options
I am exploring efficient foundation options for a residential project located in Western Massachusetts. The house will sit on a slope and have a partial walk out basement. The basement will be a conditioned space, with only a small portion requiring finishes.
My options are ICF’s, a standard poured in place foundation or the Thermomass system. The site has a high water table so drainage and waterproofing are mandatory. ICF’s insulate well, but waterproofing is a concern, as is finishing the above grade portion of the wall. Waterstops and spray waterproofing work best with a standard poured foundation, but would require interior basement finishes. The Thermomass sytem – rigid insulation cast into the center of a standard wall seems to work best, but it is new and I have no experience with it. Has anyone used this foundation system and/or have any recommendations?
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Replies
Jon,
If you can afford it, the Thermomass system is the absolutely best way to build a basement -- or even above-grade walls, if you don't expect to do any remodeling for the next 100 years. The system puts the insulation where it belongs, sandwiched between two layers of concrete.
Thermomass walls can be poured on site or manufactured in panels and shipped in. In either case, there are different insulation options available. It's always best to choose thicker insulation rather than thinner.
I use a 8" poured concrete wall with R-10 or 15 EPS foam on the outside (cold side) from top of wall to the top of the footing. Choose the highest quality waterproofing and start a bit above the grade line and cover the wall down to and over the footing. Run a drain tile system on the outside of the footing and the inside of the footing connected to an airtight sump basket that will do double duty as a radon removal system. For this you need some crushed rock beneath the basement slab but you should have this anyway to keep water from wicking through the basement floor. Most radon protection systems call for a quality vapor barrier right under the basement slab that is sealed to the foundation walls and any interuption like load bearing partitions. If you really want some comfort in the lower level, get some foam board under the slab with a thermal break at the footings and foundation walls.
Jon,
The Thermomass system has some advantages, though you would probably need a concrete pumper to fill the forms (as you would with ICFs), and a contractor familiar with the system.
Conventional 8" externally-insulated concrete foundation walls are certainly the least costly and simplest, and they would offer the best thermal mass advantage (though concrete-foam-concrete is a close second).
Since your walkout would be a stepped foundation, parts of it would be wood-framed. With an 8" concrete wall and 4" exterior XPS, you could build up the steps and walkout side with a 12" double-framed or trussed wall with integral thermal break and densepack cellulose (this is what I'm designing at the moment for a lake cottage in VT).
I cover exterior above-grade XPS with hardware cloth and surface-bonding cement (fiberglass reinforced modified cement) for an attractive, protective and water-tight finish. Since you'll be daylighting your perimeter drains to the walkout side, with good granular backfill you should not need additional waterproofing beyond staggered and taped XPS joints and the above-grade surface bonding. Top of XPS is capped with thin copper flashing for insect barrier and capillary break to framing.
Make sure the slab is well-insulated (4" XPS), has a capillary break (min. 4" crushed stone), vapor barrier and radon vent exiting through the roof. I pour all interior footings and pads even with exterior footings, cover all with XPS from wall-to-wall, and place 2" slab edge insulation before pouring slab. Be sure to install slab edge insulation at walkout edge of slab and down a couple feet over frost wall.
The foam I was suggesting is extruded polystyrene, like Robert has mentioned, not to confuse with beadboard.
Thermomass is not a new product. It was developed in 1980. It is used in both residential and industrial projects (industrial freezers for Anheiser Bush, jolly green giant) My son has a foundation business just south of Springfield MA and has installed about 100 thermomass foundations in CT and MA. I am a the regional distributer for thermomass and can answer all your questions. Please reply to me this a simple cost effective system that is used all around the world in all types of construction.
I'm not sure If you see the email address (it didn't show up on the screen. Reply to [email protected] with a phone number and the best time to contact you. Thanks again Dennis
Martin,
I'm interested in learning more about Thermomass vs ICF vs poured concrete w/exterior EPS. Can you expand on your recommendation of Thermomass compared to these other options? If the foam is not in the middle, is it best to have it on the outside for a cold climate? Why does Building Science show interior foam on their cold climate profiles (http://www.buildingscience.com/documents/designs-that-work/information-sheet-chicago-profile/?searchterm=foundation%20walls)? Thanks, Mike
Michael,
Poured concrete foundations can be insulated from the interior or the exterior. Exterior insulation is bettter from a building science perspective, but interior insulation is easier for builders to install.
I like to see rigid exterior foam extend from the concrete footing up to the rafters (or even beyond -- notched around the rafters to fill in the blocking between the rafters). If your plywood or OSB wall sheathing is co-planar with your concrete, you're good to go with rigid foam all the way.
Michael,
It seems Martin sidestepped your question. These are the differences:
ICFs are the most costly, with Thermomass costing little more than an exterior-insulated concrete wall since the latter requires additional labor and protection for the exposed foam. Both ICFs and Thermomass require a concrete pumper because the grouted spaces are narrow.
ICFs put foam on both sides of the foundation, which require protection from physical and UV damage on the outside and from fire on the inside. Some ICFs have built-in screw channels for mounting drywall directly, and the Rasta-type hybrid ICFs can receive a screw anywhere.
Thermomass also creates a capillary break as well as thermal break within the concrete wall, thus reducing the potential for moisture infiltration. EPS ICFs are not a vapor barrier, while the internal XPS of Thermomass is.
Besides the location of the foam board, which effects interior and exterior finishing requirements, the primary difference is in the thermal mass benefit. Thermal mass is more advantageous in hot climates than in very cold climates, and most advantageous in climates which regularly experience diurnal temperature swings both above and below indoor setpoint temperature. In such climates, a heavy mass wall can more than double the effective R-value.
But, for a mass wall to function as thermal mass requires that it be thermally contiguous with the conditioned space and thermally isolated from the outside environment. Exterior-insulated concrete offers the best Dynamic Mass Benefit (DBMS), with mid-position insulation (Thermomass) a close second. ICFs and interior-insulated concrete offers very little DBMS, so the R-value of the foam is about all you get and that's often inadequate for today's IECC insulation standards.