Cheapest lightest material

Started by Dberry, May 20, 2006, 12:11:40 PM

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Dberry

I'm scratching my head trying to think of what to put underneath my floor joists that will support the weight of cellulose insulation.  1/4 inch plywood would work, but I'm wondering what the cheapest material would be.  Any ideas?  Needs to be permenant so I'm not sure if tyvek would do.


Thanks!
Dan

n74tg

Depending on what's under the floor joists (sealed basement vs open crawlspace) maybe 6mil poly stapled to bottom of floor joists.  If that's not strong enough, how about two layers.



bil2054

Well, Dan, I admit to having the same conudrum.  My thought experiment included a scenario in which a....(insert name of favorite rodent here) came along and started chewing on whatever I used under the joists. What material would put up sufficient resistance to a) Discourage the critter in the first place, or b) Allow sufficient time for the resident anti-critter critter to do its job?
I'm thinking either hardboard if I do the crawlspace, or maybe gypsum board (greenboard?) if I do the full basement.  I think the insurance would be worth the extra cost, vs. something less perforation proof
that would let my cellulose trickle out.

JRR

You must like a challenge.  I don't know how I would apply cellulose "upward" and be secure in that it did not fall away, at least a lilttle, away from the flooring above.  Perhaps poultry fencing stapled to the flooring ... followed by a wet application of cellulose??  I don't know.

I would feel better about a layer or two of foam board .... tight fitting between the joists, glued in place.  If you are in a really cold area, perhaps the foam board would be followed by some fiber glass ... held in place with the usual metal stays.  


Dberry

That's too funny!   I was actually thinking of filling the joists from above before installing the flooring...  but I suppose I could crawl under there and spray it up for amusement.   :)

Hal Nash(Guest)

I nailed/screwed up 4x8 sheets of 3/8" peg board on the underside of my floor joists.  It was a dirty job but it has worked.  The holes provide ventilation and keeps any critters out of the insulation.

JRR

Now I understand a bit better.  You intend to "dry in" the building before applying flooring to the joists.  I like it!

You could compress the cellulose a bit before installing the floor.  Then whatever you use to support the cellulose from below can THEN BE RAISED into place ... just an inch or so .... perhaps by using #9 decking screws .... you might have enough compaction that an air gap will never form below the flooring.

Interesting!

Dberry

#8
Exactly.

I had a thought about the foam insulation.  I'm not sure how sturdy it is but...

I wonder if I could place screws or nails half way into the joists near the bottom...  maybe every foot or so per bay and lay a cut piece of insulation on top of the screws and put the cellulose on top of that.  It only has to support 3 pounds per foot.

That would give me a bit more insulation and solve my "what to keep the insulation" in problem.

What do you guys think?   What sort of rigid insulation would work for that?

Hmmm, I could give my daughter a staple gun and some twine and get her to lace the bottoms to support a piece of rigid foam.  Keep her busy for a few hours doing something productive.  :)


firefox

#9
Just an idea. Why not drill a sequence of 1/8" holes about 1 inch up from the bottom of the
joist on 2 foot centers. Thread some galvanized wire thrugh the holes and secure to some I bolts
at each end of the space.
Tightening up on the eye bolts will pull the wire taught.
Bruce

jraabe

Cellulose (blown) can be used as floor insulation. There was a company that did retrofits in my area using waterproof cardboard stapled to the bottom of the joists. There were punched flaps for the insulation hose to fit into and they would pump the insulation in to a specified density. It worked well and didn't settle. Far less expensive than foam and they were faster than batt installers (for a superior job IMHO).

For batts you can use chicken wire stapled to joists, Tyvek or other breathable housewrap, peg board, or poly if you punch holes in it to allow trapped moisture to vent to the crawlspace. You don't want a vapor barrier on the cold side of the insulation.

bil2054

Good point about the vapor barrier, John.
I wonder if pegboard with a Tyvek "liner" would do the job with cellulose?

builderboy

I'm the same thoughts here in Nova Scotia. I'm wondering why use cellulose rather than fiberglass batts?

Dberry

Cellulose is cheaper here.  No other reason.


bil2054

Cellulose actually has a good reputation for sealing against air infiltration, and also will flow around obstructions like wiring and plumbing.  And it's cheaper...(we need a Smiley with a Tam O'Shanter!)  Of course, if a wee mousie nibbled a hole in the bottom of the works, fiberglass batts won't leak out! [smiley=wink.gif]  That's why I'm after summat substantial to hold 'er in.

jraabe

Cellulose (if it can be blown in to density) will fill voids much better than fiberglass batts. Small voids or missed gaps and corners become a major defect in the effectiveness of well insulated walls, floors and ceilings. The better the insulation, the more important it becomes to fill these since their R-value remains the same (just the surfacing materials).

Yes, moisture venting housewrap and pegboard would be a good sturdy solution for the bottom of the joists.

Amanda_931

#16
Somewhere here there is a picture--someone had nailed or screwed the equivalent of tomato stakes (i.e., roughly 1x1) on each side of the (lower edge) of their floor joists, laid pieces of pegboard on  them, then insulated.  I tried to look for it the other day, with no results.

If my dogs could get under there I might have to add a layer of hardware cloth or heavy duty window screen below on the bottom edge of the joists.

Billy Bob

Thanks, John, I needed that!  I think I have worked on more houses with old cornhusks, or newspaper (shudder) or even no insulation at all... lots of old timers around here.  Usually the insulation was subbed out on these jobs.

I'm thinking a TJI joist system with pegboard cut to fit between the flanges, drop 'em in, and use a couple dabs of construction adhesive to hold Tyvek liners in place.
Bill