Page 1 of 4 1234 LastLast
Results 1 to 15 of 55

Thread: Question re insulating the floor in my workshop

  1. #1
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789

    Question re insulating the floor in my workshop

    This thread is a spin-off from the thread entitled "Frank's Workshop Construction Project" (http://www.sawmillcreek.org/showthread.php?t=7769).

    The workshop that I am building has 2x8 pressure treated joists on top of beams on top of piers. Thus, it is open below, so insulation becomes important. I had planned to install two floors. The lower one would be 3/4 inch pressure treated plywood nailed onto the joists. On top of that, I was planning to put 3" of rigid styrofoam insulation within structure of nailers. One top of that, I was going to put a plastic vapour barrier then 1/2 plywood.

    Three problems with this are:

    (1) it raises the floor about 2.5" more than I would like,

    (2) the styrofoam insulation is expensive,

    (3) I only get R15.

    It occurred to me a couple of days ago that I could suspend fiberglass batts within the joists before installing the plywood. I could support the batts with 1x3 cross pieces every foot. The batts I have in mind are the same ones that I will use in the walls -Roxul Flexibatt which is moisture resistant and which is rated at R21.5. I would then put plastic on top of the 3/4 " plywood and 1/2 plywood on top of that.

    The advantages of this are:

    (1) the floor would be at about the height that I like

    (2) the cost of material is about 1/3 the original plan

    (3_ I get R21.5 rather than R15.

    I have never seen this technique described anywhere so I don't know if there is some problem with it. I wrote to the manufacturer of Roxul Flexibatt insulation asking them if this is a suitable application but have not heard back from them.

    I would appreciate hearing from anyone who can either tell me that the technique is OK, can tell me that it is OK with modifications, or can tell why it is wrong.

    In case the suspension technique that I described is not obvious, I mocked up a small section and took some pictures.
    Last edited by Frank Pellow; 09-24-2004 at 7:26 AM.

  2. #2
    Join Date
    Jun 2003
    Location
    Westphalia, Michigan
    Posts
    425
    Frank, I would be somewhat concerned about pests and rodents getting into that insulation from the bottom and compromising it's integrity. Unless you could completely seal the outside perimeter, I would think racoons and mice and cats.........Also what will keep the snow from blowing underneath and getting the insulation wet during a thaw? I also am pondering on the up front cost of insulating a structure well. I keep reminding myself that it pays off in the long term. I wish the cost of sprayed foam would come down.

  3. #3
    i insulated my shop similar to how you are describing. i put rolls of r-30 in the floor with the kraft facing up and stapled the paper to the jpoists. i then went under the joists and ran chicken wire accross the bottom of the joists to keep critters out. i probably should have used hardware cloth, but i have yet to see a bunch of loose insulation. although i haven't really checked since i moved the building.
    Scott Stefanoski

  4. #4
    Join Date
    Feb 2003
    Location
    Leesville, TX (San Antonio/Austin)
    Posts
    1,203
    I used chicken wire under the joists to hold the insulation on the house...

    KC

  5. #5
    Join Date
    Dec 2003
    Location
    SF Bay Area, CA
    Posts
    15,332
    My neighbor recently insulated under his house with batts. We have a crawl space under our houses.

    When I redid the downstairs 1/2 bath, we found mice in the walls. They had hopped up on the plumbing and walked right into the wall and continued to follow the plumbing and managed to house themselves in 3 sections of wall. I foamed the hole closed and so far so good.

    I think your idea will work fine, Frank. Chicken wire might keep out some of the larger creatures but it won't stop a mouse. Maybe some cheap plywood or chipper board might work?
    Wood: a fickle medium....

    Did you know SMC is user supported? Please help.

  6. #6
    Join Date
    Feb 2003
    Location
    Southern MD
    Posts
    1,932
    They sell little pointed metal rods made specifically to hold up insulation in ceilings and crawl space. Installation is quick and easy. They hold up fairly well over time. Chicken wire holds up for a while, but it rusted out in my old damp crawl space after <10yrs. The little rods occasionally pop out, but you just jam them back in. By occasionally, I mean that every few years I'd find a couple down in a 1100 sf area.
    Hopefully, you'll have a foundation with vents and won't have to worry about rodents taking up residence in the insulation. I had my door latch break for the crawl space one year and had lots of visitors.

    Jay St. Peter
    Jay St. Peter

  7. #7
    Frank, Although the fiberglass batts are cheaper, you will more than likely have problems. First as already noted are rodents or squirrels, not sure where you are in Toronto but most areas I have been to have lots of squirrels and they love this stuff. The best option is to have the joists sprayed with insulation.

    If you do go with batts, make sure you have a thick vapor barrier on the underside. If this stuff gets any moisture in it and chances are it will, you will one again run into problems.

    Andy

  8. #8
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789
    First of all I would like to thank everyone who let me know that something along that lines what I had contemplated is a farily common technique.

    Paul and Chris, you both make good point about mice and critters. I had not thought of that. Thanks!

    I am going to put lattice right down to ground level and that should keep out most of the snow.

    I will not really have room to crawl underneath so anything I put on will have to be done before I put down the floor. Among other things, this means I will not be able to jam in the rods that you suggest Jay.

    Chris, your idea of cheep plywood sounds good. I think I can get some bottom-of-the-line stuff for about $10 (US) a sheet (including taxes). I guess that I should paint the underside. I figure that I will need about 14 sheets. That still leaves me with a big big saving over using styrofoam

    Jay, I don’t have a foundation, just piers. That means there will be plenty of ventilation.

  9. #9
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789
    Andy, I had not seen your response when I posted the above reply. Do you think that enclosing the bottom with plywood will solve the problem?

    Also, you suggest a vapour barrier on the outside. I guess that I could line the plywodd with plastic before installing it. If I did this, do you think that I also need a vapour barrier above the pressure treated plywood sub-floor?

  10. #10
    Join Date
    Feb 2003
    Location
    Southern MD
    Posts
    1,932
    Frank,
    I'm not an expert, but everyone poopooed my plan to put ply on the underside of my crawl space. Said it was kind of like not having ventilation under your roof. Good chance it will rot. Again, not an expert, but closing in your floor joists just doesn't sound right to me.
    Make sure you put a vapor barrier down (i.e. plastic over the dirt underneath). Damp = bugs that eat wood.

    Jay
    Jay St. Peter

  11. #11
    Join Date
    Dec 2003
    Location
    SF Bay Area, CA
    Posts
    15,332
    Hmmm, I think Jay has a good point. I had thought about enclosing it but thought that since it it close to the ground and not an attic, ventilation might be different some how. Now I think not.

    Frank, do you plan to excavate at all under the shop? You know, dig down several inches, maybe back fill with rock or something...something good for drainage. I guess if you seal the floor, you need to vent it...does make sense. Hmmm....
    Wood: a fickle medium....

    Did you know SMC is user supported? Please help.

  12. #12
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789
    I am going to cover the ground with 6 mil plastic then about 10 cm (4 inches) of chipped stone. You can see that ground cover in my photos at the start of this thread.

    Ventialtion -hmmm I need to think about it. I think the problem will be that mice can probably get through any vents.

    Perhaps, if I used the thinest pressure treated plywood I can find to close in under the insulation, that would prevent rot. That will cost a lot more than the $10 stuff I mentioned above but it would mean I don't need to paint and it still be a lot less expensive than my original plan.

  13. #13
    Join Date
    Feb 2003
    Location
    Springfield, OR
    Posts
    122
    Frank---
    In your climate the ONLY place to put the vapor barrier is between the subfloor and the joists, as close to the heated interior as possible. Otherwise you will have moisture and rot problems as condensation collects in the insulation with nowhere to go. The pest problem is real and should be considered. Rodents will chew through almost anything short of masonry and other than erecting a subwall around the perimeter these pests are difficult to stop. Galvanized wire mesh with openings of about 1/4" is probably your best bet. Hope this helps.

    Ed

  14. #14
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789
    Quote Originally Posted by Ed Weiser
    Frank---
    In your climate the ONLY place to put the vapor barrier is between the subfloor and the joists, as close to the heated interior as possible. Otherwise you will have moisture and rot problems as condensation collects in the insulation with nowhere to go. The pest problem is real and should be considered. Rodents will chew through almost anything short of masonry and other than erecting a subwall around the perimeter these pests are difficult to stop. Galvanized wire mesh with openings of about 1/4" is probably your best bet. Hope this helps.

    Ed
    Thanks Ed but I am confused. You say the vapour barrier should be as close to the heated interiaor as possible but go on to say that this is between the subfloor and the joists. Why not above the subfloor? That is closer to the heated interior. Recall that the subfloor is pressure treated plywood. I would not think that that stuff should be above the vapour barrier.

    I will check to see if I can get galvinized wire mesh with such small openings.

  15. #15
    Join Date
    Mar 2004
    Location
    Toronto, Ontario, Canada
    Posts
    3,789
    I have been thinking more about whether or not ventilation is needed. The suggestion was that it is needed because it is needed in ceilings. On the other hand, ventialtion is not needed in walls. So we have two examples to choose from. My hunch is that, because we do not have rising hot air in floors, the wall eample should apply.

Similar Threads

  1. Lurker peering thru the door with a rail and stile question? (Long)
    By Chris Wilson in forum General Woodworking and Power Tools
    Replies: 10
    Last Post: 03-13-2007, 5:20 PM
  2. Frank's Workshop Construction Project
    By Frank Pellow in forum General Woodworking and Power Tools
    Replies: 329
    Last Post: 09-23-2006, 10:36 PM
  3. Workshop Geometry - Arcs & Radii Question
    By Todd Burch in forum General Woodworking and Power Tools
    Replies: 15
    Last Post: 04-22-2004, 1:24 PM
  4. Insulating a Garage question
    By RJ Schuff in forum Off Topic Forum
    Replies: 5
    Last Post: 11-09-2003, 12:15 AM
  5. Old Rebel Workshop FreeStuff Winner! Oct 31, 2003
    By Keith Outten in forum FreeStuff Drawings!
    Replies: 15
    Last Post: 11-05-2003, 3:25 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •