Afternoon Folks,
My floors are bouncier than I’d like, and I’m inclined to stiffen them with a layer of (PT or exterior-rated) plywood along the bottom. The floor is OSB sheathing on top of 2×10’s, insulated with white and fluffy batts, over an unconditioned crawl space. With the caveat that my local code official would have the ultimate say. With the caveat that my local official would have the last word, would adding plywood under the joists run afoul of the IBC for any reason?
If code is mum on the topic, does this idea, which I’ve only seen suggested for basements, run afoul of good sense in a way that I’m not imagining, e.g. by trapping moisture in the insulation, or providing a home for rodents?
Thanks,
Denner
Replies
If you already have crossbracing, plywood will do little to reduce the bounce. This would be a very expensive option for minimal return. Not likely to run afoul of the IRC (IBC only if its a multi-family home). It must be less than 3/4" thick to avoid being a second moisture barrier. And it does create a wonderful home for all sorts of critters. Accessing wiring, HVAC or plumbing would be a nightmare as would be penetrations by the same.
Bounce is normally caused by joists not being deep enough for their span. I-beam or engineered joists are notorious for this because of their lack of 'cross section' wood. They will support further than standard lumber but are 'rubberier'. Their manufacturers call for L/480 vs L/360 in calculating stiffer deflection for descent solid feeling floors. In an existing floor installing additional joists often helps as this reduces the weight carried per joist. Sistering them to each other can help but isn't necessary as they still carry the same weight. This 'halfs' the LOAD and makes them 1/2 as bouncy as before. This is difficult/expensive to install if there is HVAC, plumbing and/or wiring running through them. New wood sistered to old wood has problems of its own.
The most effective and least expensive option, provided there is enough room under the joists, is a post and beam install. This 'halfs' the LOAD by 'halfing' the SPAN and makes them 1/4 as bouncy as before (it's a calculus thing that only structural engineers enjoy). Have someone calculate the loads on the beams and posts so your footings are sized right. Interior footings don't have to be sunk into the dirt but it doesn't hurt to do so.
Nail and GLUE a 2x4 flat onto the bottom of each 2x10. Does NOT have to be the full length, within a foot or 2 of the ends is sufficient.
Did I mention the GLUE?? Without the glue there will be minimally less deflection, be sure to GLUE and let set before having a party upstairs.
Too lazy to calculate the change in deflection, but from past experience will be about 1/2 of the present bounce.
Did I mention GLUE!!!! Yes, elmers or other white glue will work, if it is damp down ther, use yellow glue.
Hhmm, this may be practical for most parts of most of the space, and certainly easier to negotiate in the space. Would I risk twisting things around when (presumably ripped from a 2x12) 2x2's dry, or would the moisture of the crawl space reduce that risk?* Where services run but can't be moved, would I lose the benefit over the whole span if one section was not made deeper? Thanks for your help.
*Just spitballing; I know there are too many variables for a hard yes/no.
Still more work than a beam & post, but if the string holding up the insulation isn't in the way of the glue, junkhound's option works well.
Rip the 2x4 in half (2-2x2s) as it's the added depth that matters. Use the money saved on the lumber to upgrade to a good construction grade tube glue. Yes, junkhound hit it here too, it's the GLUE that matters. This joint in the joist will always be in high tension so use a glue that is stronger than the wood. Make sure it fully covers the contacted surface. You are creating a makeshift glulam joist here.
OCDer's tip: Take advantage of the split 2x4 still being longer in the split width for even 'deeper' joists.
Following codes about notched joists, keep the ends of these laminations within 1 foot of the ends of the joists. They won't quite calculate to a 2x12 but will be 73% as effective as doubling the joists. Increased depth improves deflection 4x over the same % in increased width.
This would be a good time to take out any dips in the floor by raising them before nailing the strips onto the joists. The added depth will help here too.
Posts and a beam feels like more work, though I'd love to effectively cut every span in half. There are duct runs that would be moved a bit, but also some plumbing that would be a challenge to shift.Frankly, I'm more confident in my ability to rip some 2x2's and glue and screw them in place than to make a sound beam down there.
Regarding the plywood, would I be correct to say that it wouldn't be a code issue per se, but wouldn't be a great idea?
Thanks for your help on this.