Firring strips over 4" rigid foam board

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Old 12-12-08, 12:00 PM
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Firring strips over 4" rigid foam board

Hello,
I need to attach firring strips over 4" of rigid foam board which is glued to a concrete wall. Wood siding will be attached to the firring strips. I would like to get opinions on the best fastener to use - Tapcons? Anchor Wedges? something else?
Thanks for any help!
 
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Old 12-12-08, 01:46 PM
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Welcome to the forums! Exterior siding will have a little weight to it, so whatever you do will have to be structurally sound enough to hold it. Furring strips won't help as the tapcon would have to be 6" long and would prove unstable. Gluing the strips to the insulation would be too weak for the siding. You may have to consider building a wall independent from the external wall and attach your siding to that. Your support would be on the floor and ceiling and could handle the weight.
 
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Old 12-12-08, 06:25 PM
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I will be hanging 3 1/2 of rigid tomorrow, but mine is on the outside of a traditional wall. As chandler mentioned, stability is a primary concern. I'm providing a ledge at the bottom to carry the weight. Some thermal bridging down there isn't a problem for me as that is the rim joist in the basement.

I've never used Tapcons that long, but the ones I have used have all been great. I also use a heavier strap when possible. The lumber supply sent me the wrong ones once and I love them. I think they are used to strap a roof, a full 3/4 by 3 and better wood. The inside ceiling strapping we buy around here if pretty bad.

I assume you are ready for the strapping. If you describe it a bit more perhaps we can find a way to add support.

Take pictures of your stages as I would be interested in seeing what you choose and how it works out.

Bud
 
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Old 12-13-08, 04:52 AM
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More info

Thanks for the responses. We're building a new house - but in an unconventional way. The main floor is poured concrete walls (8") with 4" rigid foam board glued on the outside. That gives us a lot of thermal mass on the inside. The house is bermed into the hill on two sides, with two sides exposed. Lots of big windows on the exposed south side to collect warmth from the sun. The exposed sides have a buried stem wall sinking down about 4' for frost protection. The stem wall has 4" of foam board covering it, too. All that work has already been done. Doors and windows are in, too. We lag bolted pressured treated 2x12's to the window and door bucks put in by the concrete guys. Our bucks extend out to the edge of the 4" foam.

We could "side" it with synthetic stucco, but we would really rather have wood siding. Trying to find out if there's a secure way to apply firring strips. We plan to use ripped down pressure treated decking for firring strips - it would be 1 1/4" thick.
 
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Old 12-13-08, 05:59 AM
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It would be good to create a break between the foam below and above to limit any insects tunneling up through, although they will be hard pressed to find anything to eat. But that break, if below floor level, could act as the ledge to support the wall above.
The other option might be to extend those straps up to the rafters in the soffit. Of course I don't know what your roof looks like, but a 90 degree hurricane bracket on each strap might give you some support.
I will be using wide trim windows so need support 4" out from the opening to nail them to. Also, the corners require some wood for the corner trim. I'm hanging vinyl of course.

One comment on long hardened screws. I will be mixing in 50% spikes as I'm using 5" drywall screws and experience has shown any flexing and they break. I just removed a couple of 1x6" deck boards to get at the door area and had installed them with torque screws so they would come out easy. ALL screws were broken, all. Just the constant walking on the boards took them out in 8 months. I have many, many examples of movement breaking them. Now, Tapcons, I have never used them in any place where they have been exposed to movement. But they are certainly a hardened material.

My 2 cents
GL
Bud
 
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Old 12-13-08, 10:41 AM
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Bud, thanks for idea of attaching them to the rafters. That would work perfectly for the south side. The roof, which is going on this spring, will be a plain gable roof with a 10/12 pitch. We can certainly attach the firring strips to the rafters in the soffit area.

I can't really see any good way for us to make a bottom support. There's nothing to attach it to.

You've gotten me thinking in a different direction. I have an idea for the firring strips on the west wall - no rafters there, of course. The west wall has the gable wall on top of it (yet to be built). Had planned to build the gable wall so the sheathing was flush with the concrete wall and then put 4" of foam on the outside. We could use wider lumber on the gable wall and bump it out so that only 1" of foam board would be needed to make the foam flush with the foam on the first story. Firring strips would securely attach to the studs through that 1" of foam. Could have the firring strips on the first story wall extend up the gable wall as far as possible (depending on how long a firring strip we can get). They would be fixed securely where they attached to the gable will through the 1" of foam, so all the tap cons into the concrete would have to do is keep the strips from pulling out.

I hear you about the insects. We don't want to run the firring strips right to the ground because that would leave a space between the foam and the siding for insects to enter. We're planning to put horizontal pressure treated lumber running right against the group. We'll make this at least 12" high (maybe using 3 decking boards on top of each other) so the siding doesn't extend to the ground. The firring strips will but into this horizontal lumber. We'll flash it to allow any water that gets between the firring strips to escape. The underground foam is covered with a rubberized waterproofing and a tough plastic/wire bubble sheet. I don't think insects will make it through all that.
 
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