Insulating a basement — minimum thickness of rigid foam?
All,
Thanks in advance for reviewing my question. I am getting ready to finish my walk out basement so I’ve been doing my research on the proper techniques. I’ve never had a water issue in this basement (in the 2 years I’ve been there and previous owner did not for the 5 years he was in it and he was the builder of the house in 2008). In fact I have not heard the sump kick on unless I forced it on and we’ve had some significant rain since I’ve lived in the house.
All that being said I still want to be safe than sorry. So far my research has told me to do the following:
use Drylok on the basement walls
use xps rigid foam adhered to the concrete walls (tape and seal gaps and cracks)
frame your wall against the rigid foam with a thermal break of foam (1/4″?) between the bottom sill and the concrete floor.
optional walls insulate with fiberglass batts against rigid foam.
I’ve also read that nominal insulation for the basement walls should be between 15 and 20R.
If that is the case then can you use 1″ XPS (R5) + Fiberglass batts 3-1/2″ (R13) to get to R18?
I am in climate zone 5.
Thanks in advance!
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Replies
Aaron,
In Climate Zone 5, the 2012 International Residential Code requires that basement walls have continuous insulation with a minimum R-value of R-15. That would be about 2.5 inches of polyiso, 3 inches of XPS, or 4 inches of EPS.
While I think it's always best to avoid the use of fiberglass batts below grade -- I prefer the all-foam solution -- you can use a combination of rigid foam and fiberglass batts if you want.
For more information, see How to Insulate a Basement Wall.
Martin, my copy of 2012 IRC says 15/19 for basement walls in zone 5. A footnote explains this notation as follows: "15/19” means R-15 continuous insulation on the interior or exterior of the home or R-19 cavity insulation at the interior of the basement wall. "15/19” shall be permitted to be met with R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulation on the interior or exterior of the home. The OP's proposal is specifically allowed (which doesn't mean it is good).
The thermal performance of 2x4/R13 + R5 c.i. is higher than 2x6 R19, and R5 provides sufficient wintertime dew point control at the foam/fiber boundary to avoid moisture accumulations without need of interior side vapor retarders (other than standard latex paint on wallboard.)
But use 1" polyiso instead. In 50 years 1" XPS will have dropped in performance to about R4.2, as the HFC134a blowing agent escapes, doing it's climate damage. Fully derated 1" polyiso will come in over R5. Even if it may have performance issues dropping to R4 at the colder above grade portion during the coldest part of a zone 5 winter, it's higher performance as the temps warm up (and at the below grade section) ensures that nothing bad will happen- no mold/rot in the studwall. Foil faced polyiso is also easier to air-seal reliably than XPS, using high quality foil tapes.
But a bit more foam-R is always better. At 1.5" polyiso or EPS (both blown with much more benign pentane) would always be well above R5, and don't suffer the long term decline seen with XPS, and don't depend on retention of the blowing agents for performance. (Polyiso used to, when it was blown with CFCs, starting out at R8-10/inch and eventually dropping to R5.5-R6. Now it's all R6-ish.)
In zone 5, put at least an inch of EPS or XPS between the bottom plate (or any wood subfloor) and the slab, as a thermal & capillary break- a mere 1/4" doesn't cut it. The ground temps are below the summertime dew point temps in much of zone 5A, which would cause wood in contact with the slab to take on some moisture from the room air in summer if ventilated with outdoor air.
I agree with Dana:
* Use EPS or polyiso on the walls.
* Although in theory 1" is enough foam, more is better--gives you less chance of summer Once you are bothering to put in foam, you might as well put in at least 1.5" to reduce the chance of summer humidity condensing on cool studs.
What are you doing on the floor? Is there already insulation under the slab?
Aaron,
If you want, you can drill through a concrete slab with a concrete bit to see what's underneath the slab.
If you want to install carpeting in a basement, you definitely need a layer of rigid foam (either under the slab or over the slab). Without the rigid foam, you'll get moisture and mold between the slab and the carpet. (The colder the slab, the higher the chance of condensation.)
For more information on installing rigid foam above a basement slab, see Fixing a Wet Basement.
Thanks all for your helpful responses! It definitely helps me out! As far as the floor....well that was going to be my next question. The house was built in 2008 but I am the 2nd owner. I do not know if there is insulation under the slab or in the concrete wall. I'm just going on the assumption that there is not (I could contact the original owner/builder and ask if needed). I am in central Indiana.
As far as flooring goes my wife wants to mimic the upstairs as much as possible and that means carpet in bedrooms (there will be 2 in the basement) and hardwood (or something close) in the halls and high traffic areas. I am foggy on what to do for the floors in order to have carpet and wood as flooring.
should the polyiso for the walls and/or floor be faced (i.e. foil) and if so what direction should the faced side be oriented (to the concrete to the interior)?
Thanks again for all the help I'd rather do the job right the first time!
Aaron,
Q. "Should the polyiso for the walls and/or floor be faced (i.e. foil)"?
A. It makes very little difference whether you use foil-faced polyiso or polyiso with a different facing. If you use foil-faced polyiso, and if you are able to have the foil face an air space, you will gain about R-1 or R-2 of extra R-value for the total assembly.
Q. "If so, what direction should the faced side be oriented (to the concrete to the interior)?"
A. If you are installing furring strips or studs without any insulation -- in other words, if you are creating an air space -- then the foil should face the air space. Otherwise, it doesn't matter. (Note that many brands of foil-faced polyiso have a foil facing on both sides.)
Hi Martin,
First, thanks for all of the great info on this site! It has been invaluable for planning my basement finishing project. I currently am focusing on one sticking point. I see a recommendation of 1" of foam board under the bottom plate of a wall for a capillary break. The problem is I cannot find powder-actuated fasteners longer than 3". If I am going through 1" of foam, and 1.5" of lumber, that only leaves .5" actually in the concrete, which does not seem like enough.
What is the idea or detail I am missing?
Thanks,
Adam
Adam- Why isn't 0.5" enough? It's not a structural studwall- it's not holding up the house and has fairly minimal forces acting on it. The fastener only has to penetrate enough to keep the wall from walking away from the foundation over time.
With nothing pushing it out it may not need any fasteners in the bottom plate at all- you might use blobs of foam-board construction adhesive to glue the EPS to the slab & bottom plate, but a few nails or TapCons should keep it in place in anything but a serious earthquake.
If only half-inch penetration bothers you, put thinner EPS (or XPS, but not polyiso) under the bottom plate as well as any cut bottom edges of any wall polyiso to keep the wood or polyiso from wicking moisture up from the slab.
Adam,
Were you planning on putting any foam on the rest of the concrete floor, or just under the walls? If yes, then I'd suggest framing the walls on top of the subfloor and foam. If no, why not use a sill-seal rather than foam?
Another option is to switch from power actuated fasteners to concrete screws. They are slower to install, but yield a lot more holding power and you don't have the problems of bent nails and spalling you often get trying to fasten to old, cured slabs.
I would shy away from powder actuated fasteners for this. I’ve installed thousands of the things, and I’d never try a long one through foam. You’re almost guaranteed to have overpenatration issues, and getting the fastener straight enough to not ricochet off the wall will be very difficult in a material that is both thick and soft like foam.
Go with either tapcons or the Hilti press-in fasteners made for this purpose. I’d use the hilti option myself.
Bill