I am tasked with converting a slab on grade space to living area which entails 2x6pt on edge as floor joists (resting on the slab) as this planes out with the existing house’s floor. I’ll be using 3″ polyiso to allow for ventilation space beneath (a mere 2 1/2″). I’d like to explore options other than venting every bay (@ opposite ends), and am considering every 3rd bay with bores in the adjoining joists. Admittedly this provides air movement in theory but very minimal and possibly insufficient IMO.
Any ideas and/ or experience in this regards is much appreciated.
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B-r
Is this slab damp, prone to moisture?
If not, why consider venting it? I can understand the insulation, but am confused by the plan.
thanks.
Yes, I would consider this slab prone to moisture. It was an old garage conversion and doesn't have a VB below (or even likely drain rock). We are 'retrofitting' a perimiter footing to pickup the wall load with a 4x sill and HDs and ABs extending into the new footing. I considered sealing or putting a VB on top of the slab (under the joists). More to the point, perhaps, is the inspector wants to see underfloor ventilation, which of course is 'the word' (IMO, without getting into an extended negotiation in this regard).
btw- I did consider 4'oc girders but don't believe I have the room for 1 1/8" ply, though I could if I rip the girders (not desirable but an option nonetheless).
Thanks for your thoughts.
so,
The idea of boring holes to allow air movement between the joist cavities has been used in trapped rafters. To what successful end I can't say. The theory is there, but it would seem the more free flowing vented space would move the most air.
How do you plan on letting it in and exhausting? Passive from the prevailing winds?
What's to keep your joists from continually delivering moisture from the slab to the bottom of the subfloor?
I agree it's 'in theory' and my expectation is that in practice of limited value (bored holes to trapped bayts I'm referring to). Air in and out is passive (like a standard crawlspace), sure not ideal but I'm not calling all the shots here (there wasn't an option to tear-down and start over). As for wicking/ capillary I believe the expectation is that moisture that is absorbed into the bottom of the joists-on-slab would dissipate into the vented area. Could it pass to the subfloor? Sure, of course, one reason I'm considering a VB or sealer on the slab. I have to say I'm not entirely confident in the long term integrity of PE (other than perhaps 20mil) but again I don't have as much flexibility/ budget to implement perhaps more effective solutions here, I'm working with what I have. Appreciate your questions and am hopeful something productive will develop from them.
Just run 2x4 pt sleepers pependicular to the joists and everything will be vented together.
Good thought, Mike, but that would push up the floor height 1 1/2. I could then either notch the 2x6 (in this case) which seems a PITA, or lobby for going to 2x4 with sleepers 12 or 16OC, but I'm not sure that would fly (for the inspectors and plan change) as I'm led to believe 2x4 is not currently rated for standard floor joist spans (despite this seemingly direct support). Perhaps to clarify I have a 5 1/2 cavity to work with and spec'd 2x6 joists, if that helps.
Personally, I'd ignore the venting. It's ineffective in your case. But if you have to accomodate an intractable inspector, ask him/her what is the minimum and do that. (Don't forget to cover any vent holes with screen to keep out vermin.) Then, put down an 8 mill vapor barrier, fill the 5 1/2" space with xps insulation, sheath it, and call it a floor. Any moisture that gets through all that is going to be lonely.
An aside: I agree that it is a waste of time to argue with some inspectors. In many cases it's easier and cheaper to simply do what they want. For this floor, the time spent drilling and screening a few holes is minimal.
Why not?
Why not just use a good vapor barrier, treated 2x4s laid flat with 1 1/2" foam insulation between, another vapor barrier and then the plywood? You could use treated plywood for that matter.
I wouldn't vent it.
I would lay down a VB material, either 6 mil poly or something like Stego Wrap, if possible a single continuous piece that covers the entire floor without seams. If there have to be seams, tape them.
Then, lay down continuous rigid insulation over the VB, and tape the seams in that.
Then lay sleepers flat on the insulation and pin them down with split anchors or similar.
Then... subfloor... and finished floor.
When will the inspector inspect?
That might change how I'd approach this pointless venting. I'm just sayin'...