Avoiding heat bridging while anchoring furring to interior basement wall through 2″ XPS
I’ve seen references to this issue in the past, but can’t locate in the archives exactly what’s been suggested. Is there such thing as a plastic Tapcon with some structural ability? I’ve seen the plastic anchors made by Hilti and others, designed to hold insulation, but I’d like to hold more than insulation…
For various reasons, I have elected to insulate my concrete foundation wall on the inside. The idea is to go with 2″ XPS, held in place with vertical furring strips 16″ on center anchored through the foam to the concrete. The spaces between the furring will be filled with 3/4″ pieces of XPS, and then drywall is hung in the usual way, screwed to the furring strips.
Conventional wisdom would have me anchor the firring strips to the concrete with 4″ Tapcons. Many, many, very expensive and time-consuming 4″ Tapcons. PLUS, screws are metal, and I fear they will conduct the cold from the foundation wall, and a little bit of condensation will eventually cause trouble at each and every Tapcon head, which is separated from the conditioned space by only 1/2″ of drywall and some latex paint.
Any non-conducting alternatives?
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Replies
Andy,
You really don't have significant "structural" requirements for these fasteners.
If you are patient and conscientious, all of the layers you describe can be attached with construction adhesive. Make sure that you use a foam-compatible adhesive. The best way to apply pressure to the foam while the adhesive sets is to temporarily apply scraps of plywood or OSB against the foam, and wedge the panels in place with long 2x6s that extend to the opposite wall.
Construction adhesive alone won't cut it, since it isn't a recognized tested fire-rated assembly. Some fire-resistant mechanical fasteners need to be used, unless you're using fire-rated Thermax rather than XPS. Putting the TapCons 24" o.c. is fine.
Using polyiso rather than XPS would give you a higher R at any given thickness, and since it's blown with pentane (~7x CO2 global warming potential) rather than HFC134a (~1400x CO2 GWP) it's a much greener product to use in this application. If you paid the upcharge and used fire-rated Thermax you could skip the TapCons, go with construction adhesive only. The caveats with polyiso is that the cut edges need to be kept off the floor (1/4" will do) to prevent it from wicking ground moisture, in the event that your slab doesn't have a ground vapor barrier, or your footing doesn't have a capillary break between it and the foundation wall.
Dana, this wall is not structural in the sense that it holds anything up but itself, and besides, wouldn't the drywall give it a 20 minute fire rating at least?
I'm sure adhesive is a satisfactory attachment for the wall system itself, but In the real world of post-construction living, people are bound to stud-find the furring strips and hang stuff on the walls (shelves, TVs, etc.) that would suddenly require much more of the adhesive bond.
Maybe the best solution is just a few tapcons (i.e. one at top and middle) in addition to adhesive.
The drywall is only guaranteed to be there until the adhesive gives up, which can occur at lower temperature than the fire rated assembly test requirements.
What I would consider doing is hanging the sheetrock from a single horizontal piece of 1x or 2x along the top edge of the sheet, and perhaps another along the bottom. If the top piece is attached to the wood framing, ceiling or wall, then no thermal bridge, the bottom would be. Construction adhesive for the rest of the sheet. There is no reason for 16 inch center strapping except for hanging other stuff on the wall.
Is not foam only required to have an ignition barrier, not a fire rated assembly?
If it's a conditioned and used space, it needs to be a fire rated assembly. If it's a closed crawlspace or attic with no ignition sources and no secondary use (not even storage) the lowered ignition barrier standard is allowed.
If it's an open basement or finished room, from the IRC 2012:
"R316.4 Thermal barrier. Unless otherwise allowed in Section R316.5 or Section R316.6, foam plastic shall be separated from the interior of a building by an approved thermal barrier of minimum 1/2 inch (12.7 mm) gypsum wallboard or an approved finish material equivalent to a thermal barrier material that will limit the average temperature rise of the unexposed surface to no more than 250°F (139°C) after 15 minutes of fire exposure complying with the ASTM E 119 or UL 263 standard time temperature curve. The thermal barrier shall be installed in such a manner that it will remain in place for 15 minutes based on NFPA 286 with the acceptance criteria of Section R302.9.4, FM 4880, UL 1040 or UL 1715."
To guarantee it stays in place for 15 minutes under the test conditoins with just construction adhesive that stackup would have to been tested & verified, and SFAIK it has not. Wood furring & steel fasteners don't have that issue. My gut feel is that no construction adhesive would actually pass muster, but I don't know that. But I also don't know of any construction adhesive rated for even 150F, let alone the 250F max allowable temp. The "low and high temperature" Liquid Nails (acceptable for contact with foam board) has a max service temperature of 140F:
http://www.duspec.com/DuSpec2/product/ProductDocumentSearchController.htm?documentFormat=pdf&systemSetId=70&productCode=LN-902&documentType=datasheet
So what happens to 140F rated adhesive when it hits 180F at 8-10minutes into the test? My guess is that it melts, gravity has it's way with the gypsum, and the foam is exposed to the flame front. But I don't know that either, without testing it.
24" o.c. furring and 24" o.c. fastener spacing works, and is fine for hanging stuff on the wall. I used horizontal furring with 36" spacing and 24" o.c. TapCons in my basement, but if you care about wall flatness hold the line a 24" max between furring.