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Gooter 08-18-2009 01:31 PM

Finishing basement - insulation problem?
I’m in the middle of finishing our basement and I need your help regarding insulation – mainly, what should I do to properly insulate without having to redo any of the work we have already done. FYI… all of the framing is 100% completed and the electrical conduit/wiring is about 75% completed.

Our basement is approximately 8’ of poured concrete walls and floor. We’ve lived in the house (suburb of Chicago, IL) for 5 years (new construction) and have never had any leaking or other water troubles. Unfortunately before starting my basement project, I didn’t do much research regarding the proper way to insulate. I assumed faced fiberglass batts were acceptable since that is what I heard via word of mouth. Now, after reading some posts here and some of the info at Building Sciences, I see that my assumption was very wrong. If I understand correctly, the proper way to insulate is to install rigid foam insulation boards on the foundation walls, seal seams with the proper tape, then install non-faced fiberglass batts in the stud cavities and then drywall. Is this correct?

I need to know what I should do now since my initial plan did not include using rigid foam insulation and I really don’t want to rip out all of the framework and pipe work that has already been completed to install the rigid foam.

Option 1:
Install the rigid foam in the stud cavities while pushing it behind the studs as best I can. The rigid foam will not be 100% airtight as there will be gaps behind the studs and I won’t be able to tape effectively. I can then install fiberglass batts (faced or non-faced?) on top of the foam.

Option 2:
Proceed as originally planned and install faced fiberglass batts in the stud cavities and take my chances.

I realize the proper way to do it would be to start over but that is not an option for me. I’d rather take my chances. So I guess what I’m really asking… is Option 1 an acceptable albeit not 100% correct solution? Or am I wasting my time with the rigid foam since I won’t be able to properly install it like I would have if I knew what to do from the beginning?


johnnyboy 08-18-2009 10:34 PM

I'm no expert but I'm pretty sure you don't install fiberglass below grade.

The rigid board ins. can be cut anyway you need it to in order for it to fit... it's not, nor ever will be, "airtight".

There's a lot of debate on vapor barriers, I believe most people would recommend putting one up right against the concrete, then install the framing.

Growing up I remember doing a basement with my pops, we just glued the insulation to the wall, put furring strips up and hung the drywall, 20 years later it's still fine.

Gooter 08-21-2009 08:52 AM

Anymore help out there? Thanks

Scuba_Dave 08-21-2009 09:03 AM

There is lots of discussion on this
Growing up the old man put reg wood studs against the wall & faced insulation
These were dry houses - no problems

Now several ways:
Space between stud wall & concrete, then either bats or rigid insulation

Rigid up against the concrete, framing up against the rigid insulation

Myself after reading I'll do the rigid against the concrete then the framing against the rigid
I need to read more about the type of rigid & if/where a vapot barrier will go
My house I consider a damp basement - high water table
Despite the fact I no longer get water anywhere

If you do a search on the board for "basement insulation" you will get different viewpoints

You can cut the rigid & install in the bays
Use caulking or something else to seal

Gary in WA 08-22-2009 02:59 PM

"If I understand correctly, the proper way to insulate is to install rigid foam insulation boards on the foundation walls, seal seams with the proper tape, then install non-faced fiberglass batts in the stud cavities and then drywall. Is this correct?" -------- This is correct. Then unfaced glass batts, so the moisture slowly released (if any) getting past the foam board can dry to the inside of the room. I think with some patience you can tape it effectively. Use a house wrap tape or a sticky window tape, either the 2" wide or the 4",6" will work. Cut some blue 1" thick board into pieces small enough to just fit behind wiring, etc., install one piece (with spray can foam, non-expanding) next to or just behind the stud with tape already on 1/2 of it, length-wise. Install another piece to fill that bay, then tape the joint. So the first piec in every bay goes behind the stud face, while the next is cut to butt the first and be flush with the side of the next stud. So "A" might be 9-1/2" wide, and "B" will be 6-1/4" wide. As you install "A", feed the flap of tape behind the stud and onto the previous "B" panel. You may need 4 pieces in each bay to cover the bottom and top plates area. Use wide tape if unable to install behind the studs, the tape will seal the air gap. Narrow-2" tape if able to butt the foam. The foam only needs to go on the concrete below the dirt grade. Then, un-faced batts in the wood stud wall. In the future, a thermal break should be under the pressure treated bottom plate of the wall ( poly sill sealer or tar paper). A 1" air space is good to promote air/moisture drying between the foam and the wood wall. Use latex paint to allow the wall to breathe.
And don't forget the rim joists with the scraps:
Be safe, G

ncgrogan 08-22-2009 06:53 PM

I did the same thing your describing. Rigid extruded polystyrene between the studs and non faced fiberglass. See pictures below

VelvetFoot 08-24-2009 08:23 AM

Maybe get a spray foam contractor in. Not cheap but good.

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