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Basement insulation questions

user-6258511 | Posted in General Questions on

I’ve read through a ton of posts here on GBA and I want to thank the community in advance.

I’m finishing an unfinished basement in Parker, Colorado and I’m performing the following items firs from an insulation standpoint.

My basement is a structural basement floor which is a steel beams based with OSB on top and there no no slab in the basement. It looks very similar to this.

My question are different because most people are using slabs and I’m not. The basement crawl space does draw air from the vents in the floor and sucks it to the outside. Not sure if I’m making sense but there is a 4″ fan.

I removed the Diaper Insulation and recycled it and installed 9 Feet of 2″ of XPS on Perimeter Walls with Great Stuff Foam for the Top Seal and Bottom Seal and Ownes Corning JointSealR Tape for all Seams.

I also removed all the fiberglass insulation from the Rim Joists and I’m planning on hiring a spray foam contractor to spray the to edge of the XPS and Rim Joiss with 3″ of Closed Cell.

I have one issue, the furnace room does not give me enough clearance to install 2″ of XPS behind the furnance and install a studwall/drywall. What options do you suggest? I was thinking Mineral Wool but I’m thinking I might be able to fit 1″ of XPS.

Looking for any thoughts if my plan is solid or if I should adapt.


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  1. user-6258511 | | #1

    Also, I plan on installing Roxul R15 on top of the XPS in the Stud Bays!

  2. GBA Editor
    Martin Holladay | | #2

    Q. "The furnace room does not give me enough clearance to install 2 inches of XPS behind the furnance and install a studwall/drywall. What options do you suggest?"

    A. There are two possible approaches.

    The best approach is to hire an HVAC contractor to move the furnace a few inches. Unfortunately, this can be expensive, because it usually requires reconfiguring the ducts and the gas line.

    The less expensive approach is simply to slide as much rigid foam behind the furnace as the space allows. Make sure that you include a layer of 5/8-inch drywall for fire safety on the interior side of the rigid foam. The best way to do this would be to glue the drywall to the rigid foam before sliding the sandwich behind the furnace.

    For example, if you have a 2.5-inch gap, that's not enough for a stud wall. But it's enough to install 1 inch of rigid foam and a layer of 5/8" drywall, while still leaving an air gap of 7/8".

  3. user-6258511 | | #3

    Thanks Martin. I will do that method and just glue the drywall is construction adhesive.

    Should there be an airgap between the framed stud walls on the other rigid foam or is directly next to the foam fine?


  4. GBA Editor
    Martin Holladay | | #4

    You should make the drywall+foam sandwich wider than the furnace -- wide enough to leave "flanges" on either side of the furnace so that you can fasten the sandwich to the concrete wall with Rodenhouse fasteners or some similar fasteners.

    Your rigid foam + stud wall approach can butt up to these flanges. Trim the corners with drywall.

  5. user-6258511 | | #5

    Any issues with using plywood in the mechanical room to cove the xps?

  6. GBA Editor
    Martin Holladay | | #6

    According to my understanding, you need a so-called "thermal barrier" to protect your rigid foam (especially near a furnace). Drywall is a thermal barrier, but most code officials do not accept plywood as a thermal barrier. For more information on thermal barriers and ignition barriers, see Thermal Barriers and Ignition Barriers for Spray Foam.

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