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Old 04-01-2012, 05:58 PM   #1
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Properly supporting a basement wall under furnace ducting


I'm finishing my basement, using 2x4 framing over foil-backed fiberglass insulation which is tacked onto the poured concrete foundation walls. On one long wall, the duct work for my furnace is run such that there is nowhere to anchor the top plate of the wall to the floor joists of the ceiling above. (See attached photo)

I'm thinking I can build the wall to fit under the duct work and then tip it up into place. To support the wall, I have a couple of options (as illustrated by the two sets of lines in the attached photo). First, I could run a single horizontal piece of PT 2x4 along the full length of the wall (the blue line in the photo), and nail the wall into that to provide stability. Alternatively, I could mount a few pieces of PT 2x4 vertically (the red lines in the photo). Once the wall is secure, I can box in the duct with soffit like you normally would.

One benefit of the horizontal approach is that it would act as fireblocking for the wall since the studs are on top of the insulation like they are.

Do these sound like reasonable approaches? Does one sound better than the other?

A completely different approach would be to do the horizontal piece of PT, but then install the soffit first and then build the wall in place, mounting it directly to the soffit... but that sounds like much more of a pita.

Thanks for any feedback!
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Last edited by reblace; 04-01-2012 at 08:07 PM.
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Old 04-01-2012, 07:50 PM   #2
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Properly supporting a basement wall under furnace ducting


I take it that you are located in Canada. I would just box frame around the duct work, with the stud wall being lower, so that it is able to be attached to the framing that you place around the duct work. You would have 2x4 members vertical that would go across the duct to the framing along the vertical face. basically it would be a soffit.

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Old 04-01-2012, 08:12 PM   #3
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Properly supporting a basement wall under furnace ducting


So, you recommend building the soffit around the duct first and then framing the wall in place after? Would I just nail the bottom face of the soffit into a piece of PT that i anchor to the wall with concrete nails?

I'm in Maryland, USA actually. Thanks for the response!
-Ryan
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Old 04-01-2012, 08:18 PM   #4
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Properly supporting a basement wall under furnace ducting


Read through the info at buildingscience.com about insulating a basement, before you go any further. As for anchoring, you could, but I would talk to some contractors in your area that have done basement remodeling along with the inspection department if you pulled a permit to do this work, before going that much farther into it. No need to have to rip it out and redo it, if the inspector does not like the framing work.
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Old 04-01-2012, 08:29 PM   #5
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Properly supporting a basement wall under furnace ducting


Does the manufacturer of the insulation recommend installing it this way? Any moisture will collect behind the insulation without an air gap. You could build the wall first attach a 2x to the top of the wall, as part of soffit. after the soffit is completed, stiffen the entire wall and soffit bottom with a 1/4 inch layer of plywood.
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Old 04-01-2012, 08:57 PM   #6
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Properly supporting a basement wall under furnace ducting


The foil backed insulation is actually pretty common around here. It's perforated foil, so it isn't a true vapor barrier, it's mostly to provide a radiant heat barrier. It was installed by the builder when the house was originally constructed last year. In most of the above grade and half/half exterior walls I replaced the stuff with 1" xps before I framed and plan to install unfaced mineral wool batts between studs.

The idea to anchor the wall to the foundation wall using the technique I described came from guides describing how to support/stiffen partial/half walls (which is basically what I'm building - its just a really tall one). Thanks for all the feedback.
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Old 04-01-2012, 09:24 PM   #7
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Properly supporting a basement wall under furnace ducting


I attached a drawing of the original idea I had since I suspect there may have been some confusion over what I was proposing.

The insulation will remain where it is, I would cut away/peel back enough to mount the horizontal 2x4 along the length of the wall, and then I would built it up enough so that I can build the wall in place standing off of the foundation wall enough to leave about 1/2" air gap between the studs and the foil backing of the insulation.

My concern about the overall stiffness of the wall without anchoring it to the foundation is mostly because the floorplan is such that there is a span of about 15' of continuous wall.
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Old 04-01-2012, 09:26 PM   #8
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Properly supporting a basement wall under furnace ducting


Use the Plywood to stiffen as suggested before. As long as you frame the soffit & wall properly, you should not have any problems. The only problem I can see, is pulling the foil faced away to secure the wall. It really should all be removed and XPS secured first per buildingscience.com and then follow proper fire blocking.

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