Basement insulation options

[ChrisZXWJ]

Hi All,

I know I'm starting another basement insulation thread for a topic that's been well discussed, but I'm hoping I'm coming from a slightly different perspective.

We have a new construction house in Rochester NY (zone 5) built in 2014 and we're ready to finish the basement. The walls are 9' high poured concrete, so we have plenty of headroom to work with. I believe the exterior of the concrete has a Delta-MS type waterproofing on the walls. The top of the poured concrete walls extend about 18" above the exterior grade. I'm doubtful there's any insulation under the slab. The finish flooring will be a variety of materials including carpet, tile and rubber flooring (for a gym space).

My current plan for the floor is concrete subfloor --> Delta FL --> 3/4" XPS --> 22/32" T&G OSB secured by Tapcons through everything into the concrete --> finish flooring

Floor questions -
1. Do I need the Delta FL? My thinking is that it would allow the floor 'breathability' similar to something like DriCore R+.
2. Is XPS the right choice for floor insulation? I've read conflicting opinions on using polyiso on the floor.
3. Should I use T&G plywood instead of OSB? Is tongue and groove necessary if I'm securing with Tapcons?

My current plan for the wall is concrete wall --> 2" XPS --> studwall (possibly filled with more insulation)

Wall questions -
4. Should I use polyiso for the walls?
5. If I do use polyiso, should the foil face the concrete or the stud wall?
6. I currently have crap blanket insulation the builder put in. Is it worth trying to pull the insulation out of the blanket and fill the cavities in the studwall?

This is our forever house, so cost is a secondary concern for the project. We'd prefer to have a quality finish.

Thanks in advance!
Chris

 

[Dana]

Unless your slab is at or under the local water table during the spring run-off there's no point to the additional capillary break of dimple board. Almost all slabs in zone 5 would do just fine with an inch or two of EPS covered by a 6 mil vapor barrier with a t& g plywood or OSB subfloor over it. XPS is at best an OK choice for the floor foam, but comes with a heavy environmental hit from the blowing agents. As it loses it's HFC134a soup over the next handful of decades wreaking it's damage it's performance drops to that of EPS of equal density, so you might as well start out with EPS, which is blown with pentane, which has very low damage even if it escapes, but most of which is captured at the manufacturing plant and burned for process heat.

With most floor coverings R4 would be sufficient for summertime dew point control at the subfloor level, but with a heavy foamed gym mat you might want to bump it to R6.

Rather than 3/4" subfloor Tapconned to the slab, if the slab is decently flat it's often possible to use a double layer of 1/2" OSB/ply glued & stapled/nailed to each other with overlapping seams, floating the whole subfloor. (I've not personally used this method, but I've seen it done, and doen well.) You'll need to leave a 1/4" -1/2" gap at the perimeter to deal with seasonal dimension changes from humidity, but it's not a bad way to go.


Never use polyiso on the floor, since it can become saturated with water if it ever floods, and take the next three centuries to dry out.

You definitely need to rip out the old wall insulation, maybe even the studwall and see what you're looking at for moisture conditions. There are both summertime and wintertime condensation issues that potentially could be growing a mold farm in there, and it's possible to re-build it in ways with very low mold risk.

Foil faced polyiso is jus fine on the walls, as long as the cut edges aren't in contact with potentially humid concrete. If you do a full-floor foam-over with polystyrene and rest the edge of the polyiso on the polystyrene foam it's fine, no matter how moist the slab gets. Foil faced polyiso has foil facers on both sides (I've never seen it any other way, though there are some fire-rated polyiso products with white foil rather than shiny on the fire-rated side.) Tape the seams with a temperature rated foil tape (eg Nashua 324a, found in box stores). To hit IRC 2015 code-min performance you could drop back to 1" of polyiso, and use R13-R15 batts in the stud wall. (R15 rock wool, preferred). The foam offers sufficient R value for wintertime dew point control on the above grade portion, and the "whole-wall-R" after factoring in the thermal bridging of the framing will be slightly above the R15- continuous insulation spelled out in the IRC. It would also be just fine to use 1.5-2" EPS on the walls with a batt-insulated studwall, which would give it a hint more dew-point margin than foil faced polyiso.

The foundation sill and band joist need to be both air-sealed an insulated. This is more easily done with an inch or even 2" of closed cell spray polyurethane over all of it, sealing it off to the top of your rigid wall foam as well. It could also be cut'n' cobbled out of rigid foam, but it's more time consuming. While there's an enviro hit from the HFC245fa used for blowing the closed cell polyurethane, it's still worth it for the quality of the air seal. LaPolla makes a MUCH lower impact HFO1234ze blown 2lb closed cell foam, and Icynene makes some 2lb density R5/inch water blown foams that could be used here, provided you can find an installer of any of those products in your area.

Actually there's a small time regional player out of Albany named Aloha Energy (I know, weird name for an NY company, eh? ) that makes a very good 1.8lb water blown foam worth considering- check with them first. All others currently use HFC245fa, but most will be phasing over to HFO blowing agents in the next 3-5 years. Cost for any of them should be on the order of a buck a square foot per inch of depth (so ~$2/foot if 2").

With the spray foam in place you can fatten out the R with sculpt to fit R15-R23 high density batts (rock wool preferred, for it's fire resistance.)

On the walls, if there isn't at least a foot of above-grade exterior exposure on the concrete and good drainage there is some potential for water to wick up to the foundation sill. With the reported 18" exposure on the exterior I'm not too concerned, especially if you have decent surface grading/drainage and roof overhangs. If the basement walls are dry, with no hint of efflorescence it'll be just fine, but it's good to check. As long as there's a good capillary break between the concrete and the foundation sill (EPDM is best) it doesn't really matter if the foundation is saturated with water, the foundation sill will remain dry, but if it's an older fiberglass type sill gasket or a cheap foamy one it's worth taking a hard look at it. Worst-case you can jack the house up 1/4" a section at a time and slip in some EPDM between the concrete & foundation sill (which isn't as bad as it sounds), but that's almost never really necessary.

 

[Sponsor]

 

[ChrisZXWJ]

Whew, that was a lot to digest. A couple followup questions...

Why the 6 mil vapor barrier between the EPS and the plywood? I don't think I've seen that anywhere, but I guess I've mostly focused on using XPS.

What's the advantage of floating the floor vs Tapcons? I have all of the toys to make short order of Tapcon-ing it down.

Thanks again!

 

[Dana]

The poly between the foam & subfloor is a slip surface, and something to keep any spill-moisture from ending up under the foam. If the slab gets wet it takes a long time to dry through 3/4" of OSB due to the vapor retardency of the foam, but as it does it can ruin some types of finish floor.

Floating the floor means fewer holes in the vapor barrier, but since your house almost surely has a sub-slab vapor retarder that's not much of an issue. If you're happier anchoring it to the slab, that is a more common approach than floating it, and works well. If going with one layer of subfloor rather than two that also mitigates against "potato chipping" curl with lifted corners due to seasonal humidity changes.

 

[ChrisZXWJ]

Great, thanks again. Two last questions...

Any tips on attaching the polyiso to the walls? I had planned on using PL300 with with a couple Plasti-Grip fasteners to hold it in place. Will the foil facing change that plan?

What's your take on OSB vs plywood? I don't mind spending the extra money on plywood if it's actually worth while.

 

[Dana]

For a ~3/4" basement subfloor Huber AdvanTech water tolerant OSB subfloor is probably going to be the safest solution. Generic Brand-X OSB could be a problem if it ever gets wet- more susceptible than plywood, but plywood still isn't as resilient as AdvanTech.