What the F is the correct way to insulate a finished basement?

PingSpike

Lifer
Feb 25, 2004
21,675
487
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My google searches seem to indicate...no one fucking knows.

Plastic vapor barrier on warm side or cold side? Some say warm, some say cold, and many said don't use a vapor barrier at all.

Fiberglass batts? Seems the answer is no, although people did use them forever and apparently we've now discovered they can cause mold issues.

Frame flush to wall? No, put rigid board down first. Yes, its fine. No, leave an air gap for drying?

There didn't seem to be any consensus on this, after reading tons of shit. A good case was made for first installing 1.5" of XPS/rigid foam board directly to the basement wall, then framing, insulating the framed wall with fiberglass and using no vapor barriers other than the XPS. Some places suggested an additional 1" air gap, many places suggested a pressure treated 2x4 for the bottom (other places said this is illegal).

I'd just do that, but...2x4s are already framed up flush against my basement walls and nailed in with concrete nails as well at attached around the windows. I had some free labor (and free lumber) show up rather abruptly and...well...its already done. My basement has never had sweating walls or any water intrusion save sweating cold water pipes in the summer. I'm now inclined to just install enough rigid foam/XPS between the studs to get up to R-10, fill in any gaps with those small cans of spray foam and throw some drywall up.

But I thought I'd ask here so I can become even more unsure of the correct way to do this.
 

HydroSqueegee

Golden Member
Oct 27, 2005
1,709
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vapor barrier on walls, frame the wall with 2x4's with regular fiberglass insulation (wont be an issue with the vapor barrier), or furring strips attached directly to the walls with extruded foam insulation.

pretty simple...
 

HydroSqueegee

Golden Member
Oct 27, 2005
1,709
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if the framing is already up, throw in the rigid foam if it makes you feel safer. you could staple the vapor barrier to the inside of every opening in the frame, but thats not really proper. It will give moisture protection, but not 100%.


when we did our basement, we threw up the vapor barrier, framed with 2x4s (pressure treated on the floors), put in regular insulation then dry walled. no problems yet, but its only been a year.
 

PingSpike

Lifer
Feb 25, 2004
21,675
487
126
Well, the studs aren't entirely flush with the wall...it seems possible I might be able to pull plastic behind the studs after the fact, although it would be a pain in the ass. That said, I've read a number of things that say to install no vapor barrier since it allows insulation and parts of the wall to dry out when if they do get damp. Ugh.
 

PingSpike

Lifer
Feb 25, 2004
21,675
487
126
What do I need for spray foam...all I see for sale on Lowes website are these small cans that say they are for filling cracks...and an $8000 machine. I guess I'm not sure what I'm looking for with spray foam.
 

Anubis

No Lifer
Aug 31, 2001
78,716
414
126
What do I need for spray foam...all I see for sale on Lowes website are these small cans that say they are for filling cracks...and an $8000 machine. I guess I'm not sure what I'm looking for with spray foam.
call someone to do it for you is what you want to look for
 

highland145

Lifer
Oct 12, 2009
42,468
4,916
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What do I need for spray foam...all I see for sale on Lowes website are these small cans that say they are for filling cracks...and an $8000 machine. I guess I'm not sure what I'm looking for with spray foam.
Could paint the walls with a tar water barrier. Have seen this done on the outside of concrete block that's going to get a brick veneer. Call an expert.

Chemist friend says the foam out gasses formaldehyde like what happened in all the fema trailers after katrina. Again, find an expert. Which I'm not.
 

PingSpike

Lifer
Feb 25, 2004
21,675
487
126
call someone to do it for you is what you want to look for
I figured that was what you were getting at. Hiring a crew to insulate two walls of a room seems expensive given the small scope of this project.
 

MagnusTheBrewer

IN MEMORIAM
Jun 19, 2004
24,135
1,592
126
I would direct you to one of the diy sites for the best info. This Old House won't steer you wrong. The vapor barrier in the basement goes on the warm side over the insulation. Use 6 mil or better. If you use foam, you don't need it. Lowes can order foam for you in larger containers but, truthfully for a whole basement, it would be just as cheap to have a contractor do it.
 

bruceb

Diamond Member
Aug 20, 2004
8,874
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Agree, spray foam is best and preference of choice by Mike Holmes on the TV show Holmes On Homes
 

Squisher

Lifer
Aug 17, 2000
21,207
65
91
Your confusion is shared by many.

If it were me I'd do either rigid foam or spray foam, whichever is cheaper, and would stop the insulation 18 inches from the floor. That is assuming you have a normal 7 foot deep basement.

If you decide on spray foam, I would call someone in. They'd be much better figuring out how much to spray to fill a cavity without over filling it. I can't imagine the labor costs would be very much. They probably wouldn't be there for more than an hour.
 

endlessmike

Senior member
Jul 24, 2007
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Do not put any fiberglass against the foundation wall, no matter how dry it may seem. My suggestion from scratch would be maybe foil faced rigid insulation against the foundation, then metal framing, no insulataion between the studs.

In your case you could cut rigid to fit between studs, but I don't know what you'd do about a vapor barrier against the insulation.
 

PingSpike

Lifer
Feb 25, 2004
21,675
487
126
Your confusion is shared by many.

If it were me I'd do either rigid foam or spray foam, whichever is cheaper, and would stop the insulation 18 inches from the floor. That is assuming you have a normal 7 foot deep basement.

If you decide on spray foam, I would call someone in. They'd be much better figuring out how much to spray to fill a cavity without over filling it. I can't imagine the labor costs would be very much. They probably wouldn't be there for more than an hour.
Seems like the necessary amount of rigid foam to get R-10 to cover the two walls would be about 200 something dollars. The walls to the interior I might just stuff with fiber glass, use some cheaper rigid foam or maybe just not insulate at all. I can't get some one to bend over for less than $100 so I'm kind of leaning towards that.

I realize spray foam may be best, but the basement isn't completely finished and won't be for awhile. I'm just putting one room up with maybe plans to later do the rest.

I'm curious why you wouldn't do the bottom 18". The ceiling in mine is 7'6". I have a bunch of electric wiring already at the bottom and I admit that it would make things a lot easier if I didn't have to cut all the boards around the gang boxes and wire entry points.
 

BoberFett

Lifer
Oct 9, 1999
37,563
9
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See your local building regs, because there is likely the correct way, and then the government way. The first one will last longer, but it may not pass inspection.
 

Squisher

Lifer
Aug 17, 2000
21,207
65
91
I'm curious why you wouldn't do the bottom 18"
First, it is unnecessary. The cold seeping through the walls will be the most at the top and will be less and less as you get to the base. Doing the bottom of the walls provides little or no added benefit.

Second, any floods in the future will be more easily remedied if you don't have to contend with insulation. Even rigid foam could wick up moisture between itself and the wall or itself and the drywall. Spray foam less so, but there is the potential for problems.
 

Eli

Super Moderator | Elite Member
Super Moderator
Oct 9, 1999
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First, it is unnecessary. The cold seeping through the walls will be the most at the top and will be less and less as you get to the base. Doing the bottom of the walls provides little or no added benefit.

Second, any floods in the future will be more easily remedied if you don't have to contend with insulation. Even rigid foam could wick up moisture between itself and the wall or itself and the drywall. Spray foam less so, but there is the potential for problems.
Huh? I'm failing to understand this. If the ground outside is 40 degrees, and the basement temp is 72 degrees, isn't there a 32 degree difference from top to bottom, not counting the fact that the ceiling will be warmer than the floor because of the way heat rises?

I don't think that was the best way to explain it, but what you said does not seem right to me. Why is there "less and less cold seeping through the wall as you get to the base"?
 

EagleKeeper

Discussion Club Moderator<br>Elite Member
Staff member
Oct 30, 2000
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Much depends on how much below ground level the basement floor is.

Temp at ground level is up to 10-20 degrees different than that at 4 ft below ground level.

The earth is a natural insulator.

I have a split level - 5 ft is below the ground surface.
Temp outside in the garage in the AM can be 80;
In the kids playroom (below ground level) is 60+.

And to my regret, I discovered that the finished downstairs has no insulation in the walls. That is my fall project.
 

StageLeft

No Lifer
Sep 29, 2000
70,150
2
0
I spent weeks reading up on this last year, literally. The best single source is buildingscience.com. A lot of people don't know how to do it properly including MOST contractors, who are using old science. The best way to insulate a basement in a cold climate (e.g. northern US) is with spray foam. The second best way is with extruded polystyrene insulation. The least effective way is with fiberglass bats followed by a vapor barrier.

The last approach is the cheapest way and it's how most basements have been done. It's not always bad but is more likely than the foam approaches to cause condensation and/or mold issues.
If it were me I'd do either rigid foam or spray foam, whichever is cheaper, and would stop the insulation 18 inches from the floor.
The point of this being to allow some heat to leak out from the bottom into the surrounding soil and limit frosting damage. Unfortunately there is little science on this particular matter either way. There was a study done in the 80's in Saskatoon or something indicating it's not necessary, though. Virtually all foundation damage is not caused by frost damage alone from excess cold but rather excessive moisture and an expansive soil with sub-freezing temps (three things contributing). If you have poor site drainage AND your soil is susceptible to expansion AND it's particularly cold, then you may have an issue. However, if you have a wall made out of masonry units with weeps holes on them anyway it's going to be a very drafty wall unless you seal down to the bottom. I'd prefer to just go thinner on the insulation overall in such a situation but still seal it down to the bottom. I know my holes in the winter I can feel a surprising amount of air move through, even with the top of them sealed to reduce chimney effect. With a pure concrete wall things can change a bit, though.

Further to eaglekeeper, earth has something like an Rvalue of 1 per foot depending on its type. When I removed the old insulation from my basement walls, if I put my hand on the section that is only half-submerged (walkout basement) it was colder on the top than the bottom because on the top I had on the other side of that concrete brick freezing temps, but down low it was earth, so it's a well documented fact that the majority of heat loss in a basement is at the top half of the wall.

I am "in the middle" of doing our basement and put down R10 extruded foam everywhere (2" thick pink stuff by owens corning is a touch cheaper than DOW) with a crapload of foam safe glue and have been framing in front of that foam. I had to confirm with inspector first he'd ok it since it is a "new" approach going heavy on foam than insulation and frankly most people do not bother with it, including most contractors, but all the science I read said it was superior so I took this approach.


 
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PingSpike

Lifer
Feb 25, 2004
21,675
487
126
Thanks for the info guys. Glad I'm not the only one driven a little nuts by this. My basement is poured concrete, only the top 1-2 feet are above ground all the way around. Yeah, it seems a wall of XPS with the framing on top is the way to go. I'm kind of in a bind though as my framing is already all up flush to the wall with the electrical done and nailed to the floor with concrete nails. I think I'm just going to buy some XPS, cut the sheets to fit between the studs and install that. Obviously not perfect...but given my general lack of water problems I don't feel like I'll get burned.

I *could* potentially pull a sheet of plastic behind the installed studs before putting up the XPS in between them. It isn't attached directly to the wall in any way right now and there is a tiny gap. I don't know if that would help or hurt though.

This room is only in one corner of the basement. If and when I finish the rest...I'll learn from my mistakes and remember to use pressure treated on the bottom and install the XPS first.

Has anyone used those cans of spray foam for filling cracks and stuff? Are those any good for filling in smallish areas that might have a lot of wiring?
 
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