Discussion in 'Building a Net Zero Energy Home' started by Anthony Reel, Dec 22, 2016.
What is the best way to insulate a floor above a crawl space?
Insulating a framed floor over an unheated space is straightforward. Fill the entire cavity, making sure that insulation touches the subfloor above. The insulation is generally held in place with special twine. If the floor is framed with 12-inch I-joists, it will take an R-38 batt for fill the cavity. If the floor is framed with 9.5-inch I-joists you could use special high-density R-38 batt insulation. It's possible to pack a bit more insulating value into the floor by using a dense-pack insulation process. Sometimes known by the brand name Blow-in Blanket System or BIBS, this will give you around R-44 with fiberglass insulation.
Thanks for the reply Bruce! I think we will go with the BIBS.
Don't forget to seal the sub-floor.
Good point. A relatively easy way to seal subfloor is to use construction adhesive around the perimeter of each sheet. Most builders use adhesive on the tongue and groove edges, but for air sealing, you'll need to hit the square edges as well.
I want to insulate underside in crawl 2x6 tongue and groove decking sub floor built in 1960. 1/2 the floor has oak flooring which I think is pretty tight but the other parts have carpet. I was going to put insulation under floor by draping every 24 inches with a strip of metal plumbing tape hanging down 9 inches so I have something to put twinge threw so it will hold the insulation tight to floor. I have glued a piece of plywood toe 4 inches of ridge foam and put roofing nails in the plywood to tie twine to keep the insulation against the floor. any ideas if I need to worry about air leaking? Or should I glue 3 pieces of 2 inch polyisio cream insulation with foil both sides to floor instead that will give me an air barrier.
Around here that 2x6 T and G is called car decking. I think it comes from its original use as the flooring for railroad cars. It's extremely leaky and should never be used in a new construction project because of the air leakage. Since you are working with an older home, you need to fix the existing problem. If I understand your approach, I think you're on the right track. Attaching sheets of some rigid material to the bottom of the decking will block the leakage. I suggest you then caulk the perimeter of each sheet to the beam and seal the flat joint between sheets with a high-quality tape, such as 3M All Weather Flashing Tape 8067. If you want to add extra insulating value, then the rigid foam sheets will work nicely as your rigid air barrier material.
The extra R-value will probably come in handy. You don't mention the depth of the beams, but I usually see 4x6 or 4x8 beams. This is a little skimpy for most climates. I would like to see at least R-38 in the floor for most northern areas. Standard fiberglass batts need 12 inches to reach this level, or look for R-38 high-density batts that are only 10 inches thick. By using rigid foam against the floor you can reduce the thickness of fiberglass while obtaining your intended R-value.
Depending on the beam depth and the thickness of your insulation, your idea to suspend the insulation below the lower surface of the beam seems like a good idea. Be sure to use a durable nylon twine, and watch out for the sharp edges of the plumbers tape.
Is there underlayment under the carpet? Even a thin layer of particle board sheet will help to block air flow. If you haven't had blower door test, you may want to do that. It will help identify the leakage sites and allow you to target your efforts.
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