Can I use polyiso foam for interior basement wall insulation?
A recent GBA article (https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/18454/what-best-way-insulate-interior-finished-basement-walls-have-2-ri) discussed options for internal foam insulation on a wall already with external insulation. Another article (https://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation) tells us that polyiso shouldn’t be used for below-grade applications because it can absorb water. Our design includes a poured basement wall with 4″ of internal XPS foam. I am concerned about external foam being mangled during backfill. I’d prefer to use polyiso over XPS because of the higher r-value per inch, but I’m not sure if it will work well for interior basement walls.
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You're correct that polyiso should not be used below grade and it should not be used interior to the concrete foundation since the foundation must be able to dry to the interior.
XPS has very high compressive strength (it can support a house) so backfilling with mixed aggregate should not adversely effect it.
A better approach than exterior foam, however, is the ThermoMass foundation system (which I just used on a walkout basement for a client's lake cottage), which places up to 4" thickness of XPS midline inside the concrete wall. The outer and inner concrete is connected through the foam board by fiberglass tension ties with the strength of ½" rebar in a 12" grid pattern, so the two walls are effectively one. But with an internal thermal and capillary break, the outer wall is cold and the inner wall is warm and dry, and there is no need to protect the foam from insects, UV or physical damage or fire.
I've read about ThermoMass - sort of a reverse ICF. I'll take a look...
I would say that ICF is a reverse foundation system, since it puts the foam both on the outside - where it's vulnerable to UV, physical damage and insects - and on the inside where its vulnerable to fire. And it almost eliminates any dynamic thermal mass advantage.
CIC (concrete/insulation/concrete), not only puts the foam in the most advantageous and protected position, but also offers almost as much dynamic thermal mass advantage as an exterior-insulated 8" concrete wall. And, because the foam requires no attachment or protection, it costs no more than exterior foam, while also eliminating the problem of structural support at the exterior wall line.
The 4/4/4 wall is also perfect for the kind of 12" thick double framed wall that I design and build, and allows both exterior and interior walls to be load-bearing.
Do the ThermoMass connectors act as thermal bridges?
Michael,
Many builders have successfully used polyisocyanurate to insulate the interior of a basement wall. Like closed-cell spray polyurethane foam, which also has negligible permeance, polyiso can work well in this location.
It is illogical and unnecessary to encourage a concrete wall in contact with the damp soil to dry inward.
In some jurisdictions the polyiso can be left uncovered where as extruded polystyrene must have a drywall layer.
"Do the ThermoMass connectors act as thermal bridges?"
No, they are non-conductive fiberglass. The metal form ties, however, will be thermal bridges but they are spaced much more widely and must be sealed on the outside with hydraulic cement which offers a little isolation from the outside environment.
Reformatted for your reading pleasure:
Building Science Digests
BSD-103: Understanding Basements
By Joseph Lstiburek
The traditional approach to basement water control has been to place the barrier and control layers on the outside and then allow drying to the inside. Drainage, damp-proofing or water-proofing and vapor control layers have historically been located on the outside of basement perimeter walls and crushed stone layers and plastic vapor barriers have been located under concrete slabs. The operative principle has been to keep the liquid, vapor, and capillary water out of the structure and locate vapor barriers on the outside – and allow inward drying to the basement space where moisture can be removed by ventilation or dehumidification.
By locating the insulation layer outward of the structure and outward of the water control layers the foundation is kept at a constant temperature and the insulation system does not interfere with the inward drying of the assembly.
Unfortunately, locating insulation layers on the interior often conflicts with the traditional approach of foundation water control – namely inward drying. Constructing frame walls, insulating the resulting cavity and covering with an interior plastic vapor barrier is common and often leads to odor, mold, decay and corrosion problems.
Basement walls should be insulated with non-water sensitive insulation that prevents interior air from contacting cold basement surfaces – the concrete structural elements and the rim joist framing. The best insulations to use are foam based and should allow the foundation wall assembly to dry inwards. The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm). The greater the permeance the greater the inward drying and therefore the lower the risk of excessive moisture accumulation. However, in cold climates or buildings with high interior relative humidity during cold weather, the upper portion of a basement wall may become cold enough that a vapour permeable insulation will allow a damaging amount of outward diffusion during cold weather. A semi-permeable vapour retarder or foam or a supplemental layer exterior insulation can be used in these situations.
In all cases, a capillary break should be installed on the top of the footing between the footing and the perimeter foundation wall to control “rising damp.” No interior vapor barriers should be installed in order to permit inward drying.
Up to two inches of unfaced extruded polystyrene (R-10), four inches of unfaced expanded polystyrene (R-15), three inches of closed cell medium density spray polyurethane foam (R-18) and ten inches of open cell low density spray foam (R-35) meet these permeability requirements.
Per a Dept of Energy white paper (http://www.eere.energy.gov/buildings/building_america/pdfs/db/35017.pdf), I have insulated the interior, above-grade portions of my concrete basement walls with polyiso sheeting faced with what looks like fiberglass cloth. The white paper actually recommended foil-faced polyiso, but I found a very good deal on recycled polyiso with the fiberglass cloth facing. My foundation is a bit odd in that I have quite a bit of concrete above grade. I plan on insulating the below-grade interior portions with polystyrene. My question is: should I put a vapor retarder on the interior of the polyiso and/or do I need to worry about moisture damage to the polyiso? It seems like I don't need to worry since the concrete wall above grade has opportunity to dry to the outside. One other detail: the exterior concrete is painted.
Mike,
You don't need to protect the polyiso with a vapor barrier.
The permeance of polyiso depends on the permeance of its facing. You'll have to ask the manufacturer of the polyiso about the permeance of the product you are using.
One inch of unfaced polyiso has a permeance of 26 perms. One inch of foil-faced polyiso has a permeance of 0.05 perm. It's impossible to guess the permeance of the fiberglass facing on the polyiso you are using.
Mike,
Depending on the type and thickness of the foundation paint, that may inhibit drying to the exterior but also prevent wetting from the exterior.
You don't need a vapor barrier, but you do need an air barrier to prevent interior air from contacting the cold in winter concrete. Sealing all joints and edges of both types of foam board will take care of this. But you also need an ignition barrier over the foam.
Thanks Martin and Robert. I believe the paint is just a standard exterior latex. I plan on sealing all joints and edges. One concern I have is meeting applicable building codes when its time for inspection. I have not been successful in determining exactly what the code says. I live in St. Charles County in Missouri.
I plan on framing and installing drywall in front of the foam.
Mike,
The best way to meet code is to take your plans to the inspector before doing any work. They love being asked for advice, but hate being surprised by a job already done that does not meet code.