Insulating Joist End Spaces
Question I have is in regards to insulating joist spaces at the rim joist. By brothers house has the floor joists set into the concrete foundation so that the joists end spaces are essentially filled with concrete. We are going to be insulating his basement with 2″ SM on the inside on the concrete wall, framing a 2×4 wall about an 1″ in front of the SM and then insulating that with R-14 Roxul. It’s a fairly common insulating practice for this area. I recommended to my brother having his joist end spaces insulated with spray foam, but after doing some reading I’ve found conflicting articles on whether to insulate or not insulate joist end spaces where the joists are set in the concrete as opposed to on top of the concrete. The problem apparently is that there would be a potential for moisture buildup behind the insulation causing the joists in the concrete to rot. I’ve encountered a few houses with joists set in this way and have never been a fan of this method as I was always taught wood touching concrete is bad chicken.
Your thoughts?
Replies
I will be reading with interest as I have the same problem.
I am thinking that i will insulate the outside of the walls with ridgid foam
And I thought I'd heard and seen just about everything. Wow. Joists IN concrete. Your intuition is your friend. I'm no expert here ... just giving a couple cents of comments. If you insulate the inside ... either batt or something more substantial like foam, it seems to me you allow moisture from outside to come into the concrete/around the joists. Theoretically, the insulation doesn't make it better or worse (or does it?), but the fact that you allow the moisture migration ... hmmm.
I'd have a tendency to warm up to the other poster ... insulate tob of concrete down on the exterior like you would a standard foundation wall.
IMO yes, I agree, wood (untreated)/concrete contact can be a bad thing ... although I did interior wall bottom plates untreated (and code compliant) in my new house ... but you are talking exterior concrete application, here.
I guess I'd do the exteior blue board and flash/protect it from weather/damage. Maybe someone else will offer additional food for thought.
Vapor barrier will prevent condensation, which is the real issue. No condensation, no problem in those areas between the joists or anywhere else. So I'd say go ahead and insulate between the joists right up to the crete, and take the time to add a proper vapor barier before drywalling it in.
Where are you located? I'm writing from a midwest cold climate perspective.
~ Ted W ~
Cheap Tools! - MyToolbox.net
See my work - TedsCarpentry.com
The house is in Winnipeg, Manitoba, Canada - definitely a cold climate.I also just learned that the exterior foundation wall including rim is insulated on the exterior with 2"SM. Not sure how far down the wall it actually goes, but it does go below grade.
I would be tempted to leave it alone then.
For info sake, my house was built in the 1880's by German immigrants. This concrete or brick in my case is not the only odd framing technique I have seen. I have a combination of early platform framming and balloon framing. This makes it extreemly hard to get proper insulation in all the right places!
Brrrr... yep, that's cold.
Personally, I'd still do the insulation. The issue with moisture is going to come from the inside, so vapor barrier under the drywall will negate that. And in cold climates you can never have enough (properly installed) insulation. That 2" SM on the outside is a good thing, especially if it goes the full depth of the foundation, but it's still only 2" thick.
Bear in mind, I'm a handyman/carpenter. Others more knowledgeable on these matters will be along soon with more exacting info, so don't take my word for it just yet. I'm just thinking along the line of "Insulate like crazy and keep the interior moisture inside the rooms." Let's see what others have to say about it.~ Ted W ~
Cheap Tools! - MyToolbox.netSee my work - TedsCarpentry.com
I'm probably just having a senior moment, but what is "SM"?
Jon Blakemore
RappahannockINC.com Fredericksburg, VA
Jon, I guess we're both getting old then, as I had to google it. Sounds like it's a term for t&g XPS.
SM is Dow Styrofoam extruded foam board.
Jeff
Edited 4/9/2009 2:29 pm ET by Jeff_Clarke
Go figure!?? SM?? EPS - Extruded PolyStyrene ... XPS - eXpanded PolyStyrene ... standards in the industry ... from my experience. NEVER heard of SM used (OK, until now). I guess I'm old too.
I used the term SM because that's what OP said, and I didn't want to sound ignorant. Figured it meant one or other type of foam insulation. I guess that improv backfired on me :-p
SM = Styrene Methinks
~ Ted W ~
Cheap Tools! - MyToolbox.netSee my work - TedsCarpentry.com
Edited 4/8/2009 11:26 pm by Ted W.
In my line of work, SM (if we are talking styrene) would mean "styrene monomer." If that is what is meant by the OP, then it is wrong, as the materials made from styrene are all polymers. EPS and XPS are just polystyrene. ABS is Acrylonitrile-Butadiene-Styrene (all monomers by themselves, until polymerized to link up the simple molecules into supermolecules).
I think this is what the OP was referring to:
Dow SM styrofoam insulation
I doesn't make any sense, but blame Dow.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
It's a CANADIAN term. Now it makes sense. Not.
I still don't like the idea of wood on concrete. Water will wick 2' up, but more than that, the concrete will be a condensation point.
Mike, like it or not, the oldtimers built lots of houses this way. Some are even still standing!
I don't believe it ;-)
Canadian, eh?
That's the stuff. Never realized it's a "Canadian" term. I always thought of it as a product label...
LOL!! Yeah, no prob ... I was just being particular about lingo. To me, the lingo is important in good communication and I tend to be a stickler in an effort to be clear ... crystal. :)
One of the first jobs we did as a company was fixing a home that had a tree fall on it. Roofing, sheathing, replace some rafters, etc.Anyways, the next door neighbor was a contractor but our client said they didn't feel comfortable having them any closer than the lot line. Strange looking people...The name of their company was "S&M Construction". I don't know if the double entendre was intentional or not, but it raised our eyebrows a few times.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
okay, to answer the orig question there is nothing inherently wrong with wood and concrete, nor is condensation a problem. The reason wood/conc are usually seperated is because the conc is usually in contact with the ground and it wicks moisture which keeps the wood perpetually damp. Many structures were built this way. The reason it is SMd outside is that with this style of building that's the best way to do it, you get a better bang for the Rvalue if it's on the outside, you should not insulate any further on the inside. Frame it 1" away, but don't insulate.
Edited 4/9/2009 8:38 am ET by wane
Under the IRC structural members cannot bear directly on masonry (in the US this would be a code violation).
Jeff
Under the IRC structural members cannot bear directly on masonry (in the US this would be a code violation).
Jeff>>>>>>>>>>>>>..
Jeff, this is an existing house.
In my case ove 130 yrs old.
Edited 4/9/2009 9:16 am ET by frammer52
In my case ove 130 yrs old.
I like that "Grandfathering in" of pre-existing condtions, especially when basement stairs have 8" deep treads and about 30" wide. Just don't ask BI if the joist are okay, or he might make you lift 'em up and put a piece of isolating material under each joist.~ Ted W ~
Cheap Tools! - MyToolbox.netSee my work - TedsCarpentry.com
I understand that - just noting that you can't do it today. Likely that the older growth joists are something reasonbly resistant to rot.
Jeff
Or the more likely, the amount of water at 2' in the air, is minimul!
I placed an untreated bottom wall plate on a slab ... approved/met code. Interior partition, not exterior wall. Not sure if the current IRC agrees w/ that ... For a long time, it was a common misnomer that ALL wood/concrete contact had to use treated wood.
The IRC says you need an "impervious moisture barrier" in order to not have to use PT (or other treatments or a naturally rot resistant wood).I've tossed the idea around before (not using PT), but how to prove to the inspector there is poly under the slab and what will others think about it (who don't understand the code and intent).
Jon Blakemore RappahannockINC.com Fredericksburg, VA
BTW - 6 mil poly is not an "impervious moisture barrier". It has a perm rating under 1.0, I believe. Aluminum foil, by definition is "impervious".
Maybe that is how I got mine OK'd ... I did have a 6 mil VB under the slab and it was inspected. But I vaguely recall code language saying something to the affect of wood contact with concrete exposed to the exterior or somtehing like that ... so any contact elsewhere was implied to be OK.
We use 8 mil poly only for just this reason.
There's a special reinforced 10 mil product if remaining exposed in crawl spaces over dirt (something I'd prefer to avoid).
Jeff
Really. Sounds a bit much. I've often seen 6 mil black poly in crawls. Outside of unusual situations, don't know if the additional expense would be worth it ... but I'm used to a fairly arrid climate where soils tend to be much drier ... not sure if that makes a difference, but I asume it might.
6 mil = vapor retarder (due to perms)
8 mil and thicker = vapor barrier
It's a little more and you usually have to order it - but that's no problem.
It's much less tearable than 6 mil.
Jeff
Edited 4/13/2009 9:40 am ET by Jeff_Clarke
"6 mil = vapor retarder (due to perms)
8 mil and thicker = vapor barrier"
What is the difference between the perm ratings?
In this article, Joe Lsitburek states that polyethylene is impermeable, but he doesn't specifically note what the perm rating is.
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Impermeable ... by definition, the perm rating should be 0.0, right? BTW good article. Didn't read all of it, yet, but Joe L gets a few good points out right off the batt. The principles are easy, but it gets complex fast and what is good for one thing may not be good for another. There is no such thing as one size fits all ... which is what a lot of people try to do (naturally).
Doesn't 6 mil. poly have a perm rating of .06? Definitely a Class I vapor retarder and, for all practical purposes, impervious.Am I wrong?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
I'm not sure what the perm rating is ... just know it's under the 1.0 I'm used to seeing in the code. Don't know if that is considered "impervious for all practical purposes". I really don't know too much about perm ratings, just that codes typically consider 1.0 the reasonable threshold.
"Question I have is in regards to insulating joist spaces at the rim joist. By brothers house has the floor joists set into the concrete foundation so that the joists end spaces are essentially filled with concrete. We are going to be insulating his basement with 2" SM on the inside on the concrete wall, framing a 2x4 wall about an 1" in front of the SM and then insulating that with R-14 Roxul. It's a fairly common insulating practice for this area. I recommended to my brother having his joist end spaces insulated with spray foam, but after doing some reading I've found conflicting articles on whether to insulate or not insulate joist end spaces where the joists are set in the concrete as opposed to on top of the concrete. The problem apparently is that there would be a potential for moisture buildup behind the insulation causing the joists in the concrete to rot. I've encountered a few houses with joists set in this way and have never been a fan of this method as I was always taught wood touching concrete is bad chicken.
Your thoughts?"
Every old house around here has the joists concreted in. In spite of what the code says about wood and concrete, I have not seen a single case of rot, UNLESS there was some other source of moisture, generally an exterior source (like a roof or siding leak). I would think spray foam would be fine in this case, as long as exterior moisture is controlled.
Many of the houses we work on have had fiberglass insulation crudely tucked in this space. Even in those cases, I haven't seen a problem. I say go for it.
Insulate the outside...You are sooooo right about the rot issue..I've seen and fixxed this problem numerous times...
All I ever wanted in life was an unfair advantage...
Hi hammerthumbouch,
Question I have is in regards to insulating joist spaces at the rim joist. By brothers house has the floor joists set into the concrete foundation so that the joists end spaces are essentially filled with concrete. We are going to be insulating his basement with 2" SM on the inside on the concrete wall, framing a 2x4 wall about an 1" in front of the SM and then insulating that with R-14 Roxul. It's a fairly common insulating practice for this area. I recommended to my brother having his joist end spaces insulated with spray foam, but after doing some reading I've found conflicting articles on whether to insulate or not insulate joist end spaces where the joists are set in the concrete as opposed to on top of the concrete. The problem apparently is that there would be a potential for moisture buildup behind the insulation causing the joists in the concrete to rot. I've encountered a few houses with joists set in this way and have never been a fan of this method as I was always taught wood touching concrete is bad chicken.
Well, here's my idea......seal the concrete floor to help keep moisture out of the enclosure/room......build your wall with a treated sill plate and carefully wedge your top second top plate between the framing and the bottom of your existing floor joists...."if" any moisture weakens the ends of your joists at the outer wall, they'll be resting on the wood framed wall as a backup.....spray the wall with closed cell foam from the floor up the wall and all around your joists creating a thin seal preventing water movement.....finish with your choice of unfaced batt insulation and drywall.
I realize the vapor barrier is backwards in this case.....but you're working with existing construction....building new, I'd have the vapor barrier on the inside surface for your region....but again you're working with what you're given.
Pedro the Mule - One stubborn idea after another
If your moisture is coming from outside than no sprayfoam but if its from warm air on the inside than yes.