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Old 02-01-2012, 11:09 PM   #1
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soft floors, bad joists and shallow crawl space


Question: Can anyone think of a good reason why I shouldnt take advantage of digging out, leveling and sealing the walls and floor of the crawl space under my old home while I have the main floor stripped back to the joists? I feel this would be a far easier task if your not on your back trying to dig down 18" - 24" from under a closed floor.
This is ofcourse after solidifying the foundation supports and joists.
Furthermore, why cant I insulate the floor from above before installing the 3/4" ply and subsequent floor finishings?
Thanks in advance for your comments.

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Old 02-01-2012, 11:21 PM   #2
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soft floors, bad joists and shallow crawl space


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Question: Can anyone think of a good reason why I shouldnt take advantage of digging out, leveling and sealing the walls and floor of the crawl space under my old home while I have the main floor stripped back to the joists? I feel this would be a far easier task if your not on your back trying to dig down 18" - 24" from under a closed floor.
This is ofcourse after solidifying the foundation supports and joists.
Furthermore, why cant I insulate the floor from above before installing the 3/4" ply and subsequent floor finishings?

Thanks in advance for your comments.
It would be a ton of work, unless you pull out all the floor joist it would be near impossible to dig that all out. Smooth it out the best you can lay 6 mi. plastic and call it a day.
If this is ballon style framing add blocking at the bottom of the walls. Seal any holes you find with expanding foam.
Use Advantec subflooring not plywood with constrution adhesive on the floor joist and ceramic coated decking screws. DO not skim on the screws.
You can install insulation from above, just staple the tabs on the sides.

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Old 02-01-2012, 11:52 PM   #3
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soft floors, bad joists and shallow crawl space


I had about 10" under my 1910 house, good thing I dug it out when I was younger.....LOL. Do what you can from in between the uncovered joists, you'll be glad you did.

"Furthermore, why cant I insulate the floor from above before installing the 3/4" ply and subsequent floor finishings?"------ no, sorry, bad idea. Fiberglass HAS TO be in contact with the sub-floor to work efficiently; http://oikos.com/esb/38/floorinsulation.html

The stapling flanges would lower the insulation 1/8 to 5/8", lowering your R-value of the insulation because of convection and air movement in the gaps: "Making Contact", pp. 2; http://www.advancedinsulationinc.com...Insulation.pdf

You could roll out the batts on the plastic vapor barrier just before the sub-flooring. Later, while down there installing the insulation up with spring rods, install some housewrap on the joist bottoms to prevent wind-washing of the air-permeable fiberglass insulation: The "biggest loser" in fiberglass insulation....

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Old 02-02-2012, 08:19 AM   #4
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soft floors, bad joists and shallow crawl space


Thank you gentlemen for your time thinking and responding to my crawl space problems.
I know how expensive foam insulation can be so I was looking for other ideas on how I might solve the problem of insulating the main floor over a crawl space. I found a youtube of a guy who cut what looked like 2-3" foil clad closed cell foam insulation to fit between joists. He then applied a bead of expanding closed cell foam insulation to seal the perimiter gaps. Has anyone else tried this cheaper alternative to 100% spray foam for insulating between the floor joists over a crawl space?
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Old 02-02-2012, 10:13 AM   #5
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LOL.... http://www.buildingscience.com/docum...n-crawlspaces/

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Old 02-02-2012, 10:25 AM   #6
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soft floors, bad joists and shallow crawl space


GBR how do you figure there would be a gap if it was stapled and not one if you used hangers? Theres no way it would not have a gap using hangers between every hanger.
Hangers also tend to compress the insulation where there installed.
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Old 02-04-2012, 12:11 PM   #7
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LOL, I think I was half asleep then.... Good catch, Joe!

"You could roll out the batts on the plastic vapor barrier just before the sub-flooring. Later, while down there installing the insulation up with spring rods, install some housewrap on the joist bottoms to prevent wind-washing of the air-permeable fiberglass insulation: The "biggest loser" in fiberglass insulation...."--------- The houserap is started on one side of the running joists, insulation installed with spring rods as temp method of holding it up while h.w. is brought over and stapled as you remove each rod. Otherwise the h.w. won't be tight enough to hold the f.g. insulation without sagging. Just have to remember to staple it just beyond the rod to reach in and remove it. I do one bay at a time- f.g.- rods- housewrap- remove rods. Easier yet is the pro's with the string staplers, zig-zaging down the bay, after pushing up the friction-fit. Older houses need the spring rods (just before the h.w.) because of the irregular joist width cavities. I use the rods for pinning the plastic over the earth, after bent to "U" shape.
Main thing is contact with the floor and full joist depth insulation thickness. The paper facing would sag/rip/fall from ripping off the staples especially after wetting as it changes its permeability with moisture; http://www.buildingscience.com/docum...vapor-barriers

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Old 02-04-2012, 01:13 PM   #8
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Once again , I thank you gentlemen for your time.
I am somewhat of a novice therefore I am a little dense at times. If I employ this method, I picture the plastic vapor barrier draped over top of the joists and sagging down in between just enough to allow the insulation to be gently pushed into each bay. After the installation of the sub floor, its then time for the house wrap across the joist bottoms. Would that be bout right?
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Old 02-04-2012, 01:36 PM   #9
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Typically, the faced insulation is facing the warm side, up toward the room in a heating climate, down toward the crawlspace in a cooling one. Be sure to cover the ground with plastic or a rat slab (thin concrete):http://www.energysavers.gov/your_hom.../mytopic=11780

No plastic on the joist tops- no plastic at all.. Fiberglass has to be installed in contact with the bottom of the subfloor, and the joist sides to prevent air movement, and get close to the rated R-value. Fiberglass is rated in a closed, six sided box without air flow.The h.w. is installed to the bottom of the floor joists. Foamboard is way better, even foam strips (1-1-2" wide) on the joist bottoms before the housewrap would help against radiant gain or loss. You get this in attics also: http://www.buildingscience.com/docum...bout-diffusion

1. Where are you located?

2. Is the crawlspace vented?

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Old 02-04-2012, 05:54 PM   #10
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soft floors, bad joists and shallow crawl space


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Originally Posted by GBR in WA View Post
Gary why is that document so funny? It looks pretty serious to me. I am a DIYer not a contractor. Look at figure 7 where they purposely illustrate airflow and the insulation not having contact with the subfloor.

My neighbor has a subfloor and it's basically fiberglass between the joists and then sealed blue foam board (no foil) for the "ceiling" so to speak. This is in NY.

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Use Advantec subflooring not plywood with constrution adhesive on the floor joist and ceramic coated decking screws.
Do you mean "use advantech flooring, not plywood, with construction adhesive" ? You are saying to use the advantech flooring with the adhesive? This is not a rhetorical question, I am a DIYer and I don't know the answer.
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Old 02-04-2012, 11:12 PM   #11
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It was funny because the previous poster talked about foamboard at the joists rather than f.g. and the site I gave said exactly that or close enough. Fig. 7 can be gaped because of the foam air sealing the cavity, without the foam- cavity insulation requires direct contact with the floor above. Fiberglass insulation traps dead air to insulate, any air movement at all- above, next to, or through it will degrade its R-value; http://www.buildingscience.com/docum...ty-insulation/

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Old 02-05-2012, 02:22 AM   #12
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Underfloor insulation doesn't need a vapor barrier (your flooring will be the vapor barrier). Buy unfaced R-30 or whatever, tuck it between the joists from above, lay down your subfloor, get into your new confy crawlspace, and retuck the fiberglass from below so it sits against the subfloor.
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Old 02-05-2012, 11:50 AM   #13
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I am located in south western Ontario Canada, near London
The crawl space at present is vented but from everything Ive read, I'll have it buttoned up by the end of this project.
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Old 02-29-2012, 07:36 AM   #14
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I have an idea of reusing the plywood subfloor that I'll be pulling up and installing it in the crawl space after laying down a 6 mil poly vapour barrier. Has this been done before? Could you point out any downsides to this idea?
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Old 02-29-2012, 11:19 AM   #15
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I have an idea of reusing the plywood subfloor that I'll be pulling up and installing it in the crawl space after laying down a 6 mil poly vapour barrier. Has this been done before? Could you point out any downsides to this idea?
In theory, you shouldn't lay down over the plastic anything that bugs might eat or mold might call home.

Personally, under my house a have a plywood path to the hose bib shut-off valves that I travel twice a year. Easier on my knees.

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