exterior basement wall insulation
I’m in the process of finishing my basement. I have gotten all the interior walls up and now am to the point of getting the exterior walls up. I am planning on using 1 inch xps rigid foam insulation against the poured concrete walls then stand my walls against the foam for a complete thermal break (seams will be taped and likely sealed with spray foam). My question is can/should I also use R-13 fiberglass in the walls too to up the R value? I think the 1 inch rigid is about 4.5R (but not positive) so I would like a higher R. I plan on this being the main living/entertaining/play area for kids, space of the house when done. There are no bedrooms and only a half bath. Total finished space will be about 1000sg/ft of the 1500sg/ft total size of the basement.
I live in S/E lower Michigan and house is 6 years old, no water problems from the foundation and house has forced heat/central air. I am going to add some heat and return vents in the basement also (of which there are only 4 heat and 1 return in 1500sg/ft unfinished area). I am not planning on using a vapor barrier between the concrete and foam or between the conditioned space inside the wall, thinking that if there ever was moisture in the wall it would evaporate better with out a barrier causing less problems.
Anything seem hughly out of wack with my thinking? Anyone have any suggestions/advice/criticismism/etc? It would be appreciated. I plan on starting framing the exterior walls next week and will likley have it done in 1 day (nice long straight uncomplicated walls).
Thanks in advance,
J-
Replies
Interior walls first then exterior walls?
That's a little backwards but hey, if it works for you...
;-)
With the foam/fiberglass, I wouldn't do the fiberglass unless you up your foam thickness. I don't know the specifics for your location but you probably need more like 2" in order for the inside of the foam to be above the dew point when it's cold outside and your warm, moist interior air is trying to make its way outside.
Good thought with skipping the plastic. Don't want a double vapor barrier.
Ok thanks, Yeah probably should have said that the "interior" walls were actually just dividing my basement into finished and un-finished sides and a small 1/2 bath, I did kind of make it sound like I was doing some major building. I just needed to partition it off first to have a place for all my #### when then finished side was done.
J-
Use Min. 2" XPS or spray foam with close cell foam. No Vapor barrier. No need for the bat insulation.
More info.
http://www.taunton.com/finehomebuilding/how-to/articles/no-mold-finished-basement.aspx?LangType=1033&ac=fp
http://www.taunton.com/finehomebuilding/how-to/articles/built-wrong-from-start.aspx?LangType=1033&ac=fp
Hi J.
I have a virtually identical situation, except that I used 1.5" XPS (R7.5), glued the seems and taped. I also framed 2x4 walls on the exterior walls iand plan to use R13 batt insulation (I live in Toronto, Canada). The building inspector came by a while back and loved it, except that he told me to remove the tape along the seams to allow for trapped moisture to escape. Given that 1.5" XPS is semi-vapour permeable, I will be using 6mil poly vapour barrier behind the drywall. I recommend you doing the same since you're only using 1"XPS, which is not considered a vapour barrier. Besides, the VB should be on the warm-in-the-winter side of the building envelope.
Danny
Whatever you do, please DO tape or caulk the seams on your XPS foam and DO NOT use a plastic vapor barrier. You want the basement wall to be semi-permeable to water vapor. Otherwise it cannot dry and the only way for an underground wall to dry is to the inside of the structure, unless you have a basement built in a dry dry desert.
But don't take my word for it -- trust these folks here who will explain it all logically and clearly:
http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements/?topic=/designguidance/buildingscienceanddesign/below-gradeenclosure/main_topic
Here's Joe Lstiburek's bio:
http://www.buildingscienceconsulting.com/about/who/biographies/About_Joe.pdf
Billy
Edited 4/16/2008 9:20 pm ET by Billy
Edited 4/16/2008 9:21 pm ET by Billy
Hi Billy,
Thanks for the useful comment. Intuitively, I am in full agreement with you that the wall needs to dry out. Except that the rule has always been that the VB should always be on the warm-in-winter side, which means behind the drywall. What's you're take on that??
Danny
Here's my take. The reason for the "rule" is that you don't want the vapor to get halfway through the wall and either reach its dew point and condense or freeze in the wall. By placing the VB on the warm side this is prevented in winter.
But a basement wall is decisively different from an above-ground wall.
First, the basement wall is made of masonry and your above ground wall may or may not be masonry (but this is not the main factor). Second, in winter the outside of a basement wall in your climate is at a higher temperature than the outside of an above-ground wall. Third, in winter the outside of a basement wall is wetter than the inside air and the outside of an above-ground wall is drier than the inside air. Therefore the situation are entirely different. In an above ground wall in winter in your climate the vapor is driven from indoors to out. In your basement wall it is different -- the vapor drive it from outside to in as the wall tries to dry to the inside (because it cannot dry to the outside). If you place a vapor barrier as you suggest it will trap water vapor and rot your walls and wet your insulation. Look at the photos in the Building Science materials which show exactly this situation.
Maybe the experts will have a different take on this, but this is my understanding of how it works.
Billy
Wonderful recommendations gentlemen!!
thanks for the info guys. I have read the building science article and a couple of others. I am gonna go up to 2 inch xps and will reconsider the fiberglass once the rigid insulation and stud walls are up. I'll add r13 if needed before going with 1/2inch dry wall.
I am also likely gonna have my rim joist/joist bay ends sprayed with foam before the walls etc go up.
Thanks for the suggestions.
J-
Good deal. I think you'll find the XPS is easier to work with in this application because it is more rigid.
The EPS is kind of floppy and it won't always stay where you put it, if you know what I mean... at least that's what people tell me because I've never had that floppy experience!
Billy
Sounds good J. Let us know how it all works out!
Guess what guys?
My building inspector (Supervisor) came by today for a final inspection (I live in Toronto). Everything was fine except for the basement insulation. I have 1.5" XPS along the exterior walls, behind 2x4 studs. My plan was to have Roxul batts put in to bring the wall to R-20, then drywall as Billy mentione earlier. The inspector didn't like or pass it.
He said anytime I use batts I need VB. Can't have XPS and batts together, said XPS will trap moisture in stud cavity. showed him article by Building Science, explained that moisture will dry to the inside, he said NO, must have VB. Also explained that 1.5" XPS is semi-vapour permeable. His response was that because I had clued seams, vapour can no longer excape (BS!!). He even whipped out the code book and pointed to the section/sub-sections to prove it. Said I must either:
A) Rip out XPS in order to use batts + VB (traditional method) ... OR
B) Leave XPS, but double it to get required necessary R-value, and MUST drywall to hide XPS. He said XPS is highly toxic and must be protected from fire before final inspection can be passed.
I'm choosing option B). - Bastard!!!!
What do y'all think??
I think he's wrong, but he's the man and you got to do what he says. I would cut 2" xps to fit about 1/2" undersize, and gun foam to the studs. It's going to be time consuming, but I bet in the end you'll have a better system anyway,if it's any consolation.
FYI - I've seen code language that says any polystyrene insulation must be "in substantial contact with" the sheetrock ... i.e. no air spaces allowed. This was standard language forever in e.g. UBC and subsequent codes, I believe.
Basements can be strange. My guess is that moisture is moving inward ... not outward in a basement ... was this covered by someone that quoted Joe L.?
XPS is a vapor barrier, I believe (i.e. < 1 perm rating).
Seems like the basement insulation detail problem has been around forever and we still don't have some standard details/science that give us some options that are acceptable. Go figure. Such is construction, I guess.
Is this approach acceptable for basement walls:
- furr out 2x4 stud wall 1" from the block wall
- spray 2lb closed cell foam so that it fills between block and studs
- 1/2" drywall over topWould produce R-24 value (key number for energy rebate in Ontario).I'm unsure of how I want to finish my floor but I thought dimple membrane like SuperSeal and then 3/4" T&G OSB would be adequate for carpet finished floor.
Closed cell foam ... that is e.g. urethane you refer to? (or a close cousin?). Your approach sounds fine. As an option, maybe 2x2 w/ e.g. 2x4 top/bottom plate push the plates up against the wall ... and do foam. Maybe not the highest R-value as w/ your option? I did 2x2 w/ 2 inch full top/bottom plate on a half basement wall in my own house. Glued the top plate to the wall and bottom to the floor. No issues. Can't remember if I insulated, too ... but already had 3inches EPS on the exterior continuous, so it didn't matter, really.
I need to hit R23 in the basement which is 3.333" of the spray foam I'm looking at using (R6.9/inch)What R Value does 1/2" finished drywall add? .45Let's call it 3.3" of foam to hit the target number.
I am working on building a duplex and we will use cinder blocks for the 2 center garage and 2 x 6 framing for the rest. I worked on and ICF house a couple of years ago and got on the company mailing list. Quad lock now makes a retrofit system I think I will use on the house to garage wall, It's called E-retro. No furring strips required.
so a 2x4 top/bottom plate up against the wall gives you 3.5 inches ... R-24+ at 6.9 per inch. Do they want you to install insulation of R23 or have an overall construction of R23? Sounds like you are there. I wouldn't lose any more floor space than I had to.
Overall construction of R23 of better. Had 2 spray foam trades through today and they are about $1000 more than I estimated. If I revert back to batton or board insulation, what's the best R value I can get in a 3.5" space? 1.5" space?
Well ... you could do polyiso ... at about R 7 per inch. EPS at R 5 per inch. You can cut both accurately on a table saw ... clean, messy, but not dirty. Polyiso is more messy, I think but they are both bad (but still clean :) ). The EPS won't get you there in 3 1/2 inches. Personally, I think you get what you pay for ... polyiso is more expensive, but you get better R-value for each inch.
R 7 per inch is about the practical/economical limit for insulation R-value.
Edited 11/26/2008 12:21 am ET by Clewless1
If its on the interior, you can use EPS (white foamboard). Just a matter of thicker foam to get the desired R.
FHB # 169 had an article on finishing a basement.
jt8
So what was that like?
It was like coming THIS close to your dreams. And then watch them brush past you like a stranger in a crowd. At the time you don’t think much of it. You know we just don’t recognize the most significant moments of our lives while they’re happening. Back then I thought, “well, there’ll be other days.†I didn’t realize that that was the only day. --FoD
The Building Science site that others have referenced is an excellent resource. And there have been FHB articles as well; I seem to remember another on besides the one referenced, but I'm not a home to look it up.
One thing you might consider after deciding on the type and thickness of foam is metal studs, which should make straight walls easier to construct, and you don't have to worry about water absorbtion (adsorb, absorb, whatever) if you do happen to get some water in the basement.
1" XPS is R-5. You might consider 2" and then go t 2x2 walls. You knew that the XPS is a vapor barrier in and of itself, right?
Moisture is a strange animal and while the science tends to be simple, predicting the outcome in the miriad of possibilities can be daunting.