Fastening 3/4” wood through 4” foam to a masonry wall?
Jay Hoinville| Posted inEnergy Efficiency and Durabilityon
I have attached four inches of foam to my walk-out basement masonry wall using Rodenhouse, Inc, 5 3/8″ Plastic Masonry Fasteners. I now want to attach strips of 3/4 inch thick wood on to the foam. The strips of wood will be used to hold up sheets of drywall. I would use the Rodenhouse fasteners if there was an appropriate size (6 3/8”), but they don’t manufacture any that large. Is there a similar fastener that can used to attach the wood strips? If not, what is the appropriate way to attach the strips? Thank in advance.
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Jay, you need 6" concrete anchors (like Tapcons) to securely hold vertical firring strips through 4" of foam. (In fact, you could have skipped the expensive Rodenhouse fasteners, and just used the firring strips to secure the foam.) But the problem is, ANY fastener won't be able to hold up the weight of the drywall through that thickness of foam--the whole assembly will sag downward. Since you must keep both the firring strips and the drywall off the concrete basement floor, you'll either have to anchor the firring strips to something up in the ceiling, or put a non-wood shim under them, like a little piece of Azek vinyl trim.
Jay,
Andy gave you good advice on the type of fasteners (Tapcons) required for this work.
However, I disagree with Andy's statement that "ANY fastener won't be able to hold up the weight of the drywall through that thickness of foam--the whole assembly will sag downward."
Builders routinely attach furring strips to walls with 4 inches of rigid foam, without suffering the sagging problem that Andy describes. For more information on this topic, seeFastening Furring Strips to a Foam-Sheathed Wall.
Thanks Martin and Andy for your prompt answers. I will install the furring strips with Tapcon screws and a non-wood shim.
Take a look at Windlock
http://www.wind-lock.com/
Martin, thanks for correcting me. I hadn't seen that article, and am just off a basement job using big-box 4" EPS, which must be far less dense than the Type-II EPS in the article. The material was so easily compressible when gingerly cinching up the tapcons that even the homeowner watching over my shoulder said, "uh, maybe that needs some extra support?" While four inch thickness is on the outside edge of what this engineering study looked at, I see in the comments section that the numbers are probably very conservative, and that drywall is approximately equal in weight to the lightest siding listed, vinyl.
Andy,
In some ways supporting exterior siding and interior drywall with no framing behind it aren't analogous. Although it seems like you may be able to easily support the basement drywall on long fasteners, how many homeowners expect their interior walls to be unable to have some additional load added to them, like shelves, pictures or TV mounts? I think your suggestion of making the drywall support a bit more robust is a good idea.
There seems to be a re-occurring theme running through some green building details that once energy efficiency becomes paramount, you can forego a lot of what was considered as necessary robustness in the structure - Advanced Framing, exterior sheathing and some really precarious foundation detailing being the ones that first come to mind. A lot of the fastening through foam and mineral wool don't appear to have much engineering behind them either.
Malcolm,
Thanks for your comments. I share your opinion of advanced framing, as you may know if you read my article on the topic (The Pros and Cons of Advanced Framing). But the reasons for my qualms about advanced framing have nothing to do with engineering. The engineers have studied advanced framing, and the system works.
The same can be said for the recommendations in my article on fastening furring strips through rigid foam. These recommendations are based on engineering calculations, not guesses, and are time-tested.
Martin,
You are as usual right - but perhaps what engineering calculations stop is where my concerns start.
When the Soviet Union fell and the destructive forces of a collective economy were replaced by rapacious capitalism, a group of Canadian builders were asked for advice on what to do with under-used or idle saw mills. They suggested dimensional lumber and built a demonstration house using stick frame construction techniques. Local building inspectors were sceptical, so the Canadians gave them a chainsaw and asked them to cut away what they thought was supporting the house. They cut very other stud and floor joist without apparent reduction in structural integrity. As a result the Canadian Building Code was used as the basis for revised Russian ones.
The real beauty of wood frame construction is it's robustness and redundancy of structure. It can sustain a fair amount of unanticipated loads and damage without too much ill effect. Advanced framing, and some of the other detailing I alluded to, represent the type of lean engineering you see in race cars, where the loads are covered but the level of redundancy is little to none. And, like the more delicate air-sealing strategies that have emerged recently, don't seem to take into account human's natural tendency to want to alter or improve their homes.
Everyone has a different take on how important this is. My own experience working of older houses here in the PNW is that these techniques might cause problems over time.
Malcolm,
Fair enough. Thanks for your comments -- as usual, always worth reading.