We have a rec room (basement) from the 60's that we are redoing. Currently on the outside walls, they have 1 x 2 studding then 1/2 inch white styrofoam then vapour barrier. WE are wondering if we can remove the plastic vapour barrier and stud over what is there with 2 x 4 then use Roxul Comfort batts? Wasn't sure if there would be an issue with putting the Roxul over the styrofoam. As this is the old type of styro I doubt if would have much, if any R value but does anyone know for sure
Basement – insulate over 1/2 in. styrofoam
basementinsulation
Related Solutions
According to the Material Safety Data Sheet (MSDS) for TRYMER(TM) 3000 Rigid Polyisocyanurate Insulation, there should be no problems (for this specific product anyway).
I also found the MSDS of an example Expanded Polystyrene Board, which does mentions off-gas issues. Though it only applies to large amounts of "fresh" polystyrene, being stored in a "confined, unventilated area".
fresh polystyrene contains residual pentane and if large amounts of EPS materials are stored in a confined, unventilated area such as a semi-trailer truck, pentane may off-gas and create measurable airborne concentrations.
If you know the specific product you'd like to use, it shouldn't be that difficult to find the MSDS for it on the internet.
"Should I use Tyvek or tar paper on basement walls?"
Neither. No building paper or wrap against the foundation. Just use foam it is not really all that difficult to cut.
The best practice really is to use rigid foam against the foundation wall, then a framed (possibly insulated) wall with drywall and no vapour barrier.
Batt insulation in the framed wall is debatable. Green Building Advisor has articles that say it's not a good idea because it can absorb moisture, but others sources have no problems with it. If you do use it (I would if you are in a cold climate and don't have existing moisture problems in your basement), I personally prefer mineral wool over fiberglass.
Don't forget about the cavity above the wall in the joist spaces.
Trusted Sources (read them!)
- Built Wrong From the Start (Fine Homebuilding) [PDF]
- Basement Insulation for all climates (Building Science Corporation) [PDF]
- How To Insulate a Basement Wall (Green Building Advisor)
Good Luck!
Related Topic
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- Basement – rigid foam board in basement with exterior guard overkill
- How to insulate the workshop
- Insulate room over porch
- Subfloor: styrofoam insulation between sleepers or strapping on top of styrofoam
Best Answer
1/2" Styrofoam will give you about R-2.0 to R-2.5 insulation. The best you can expect from any reasonable 1/2" material is R-3.0 to R-3.5, so it's not too bad for what it is. Obviously you will get much better R-values from much thicker material.
Vapor barriers should only be installed closest to the warm side of any insulating assembly and nowhere else. While Roxul is largely water resistant, you still do not want water condensing inside the insulation, it reduces the insulating value and promotes mold growth. You also do not want to block any moisture that gets this far from being able to escape by having additional vapor barriers farther in from the warm side.
So you must remove the existing vapor barrier. Never install insulation on the warm side of a vapor barrier. Unfortunately, styrofoam also serves as a vapor barrier, so you should remove it as well before installing the Roxul. Better to give up that R-2 than risk moisture being trapped in the Roxul. The one and only vapor barrier should be on the warm side of the Roxul.
Note that this is assuming there is no risk of water ingress through the basement walls. If there is any risk, you should be using an insulation impervious to water such as XPS, and some positive drainage scheme must be in place behind the insulation to allow water to drain to a sump for removal.