# Rim joist insulation



## davemax (Jan 15, 2010)

Just some final thoughts on this :

I am about to insulate the rim joists in my unconditioned basement... I guess it's not really unconditioned.. there is one vent from the furnace that is open and does supply some heat in the basement.. Block walls, some water in springtime, however there is a dehumidifier running at all times.. the de- humidifier is what i was told is the key here.
As for the rim joists, there are a million comments and videos out there on this subject.. some say rigid with foam is best, some say batt is fine. Some have had mold and condensation with the rigid, some with batt. Owens corning, which sells both, so they don't care as long as product sells so to speak, however, they state batt insulation, is acceptable in the joists as long as they are caulked.. now it's never an airtight seal, but that is what they recommend.

I have heard some say they have seen moisture in the joist when rigid board was removed also..condensation can build up behind the board as well, so there is no fool proof definitive method here. All depends on your condition of the space.

In my very old home, i caulked the joists, many openings due to age of home.. and plan on using the batt, faced for ease of product.. this way, I can monitor over time by simply removing the batt at times to monitor for mold, and also, because there can still be condensation between the joist and the rigid board, you wouldn't know until you break apart the board , scrape away the hardened foam, etc..... 

Final note. this home is in central NY, that I own and try to maintain, never had insulation in joists. My home in PA , around 20 year old home, the builder put batts in all the joists, and anywhere along the top of the poured foundation...no water in basement ever, don't think sump pump has ever run. I pulled some away to examine last week.. and although there are some dark spots on the fiberglass from airflow, etc.. after being there for 20 years there is absolutely no mold growth in site, no odor, nothing on the joists.. So I think it depends on condition of the space.


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## joecaption (Nov 30, 2011)

Fiberglass batts is not going to stop air flow as you have seen by the dirt in the fibers.
Seal any gaps with expanding foam, install 2" thick foam and once again seal any gaps with expanding foam.


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## davemax (Jan 15, 2010)

Thanks, but as i said above, if the joist is caulked sufficiently to begin with, there shouldn't be any airflow . , or very minimal, and the batt would give you the better r value. Way too much overload with the rigid board foam thing.


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## Windows on Wash (Aug 30, 2011)

This is not as much unsettled science as your first post infers.

The reality is that insulating with batt insulation can create problems were problems do not exist currently. By blocking some of the radiant energy from inside the home and therefore making the ribbon board colder you are increasing the condensation potential as warm moist air will diffuse settle and more likely to condense on the now colder board.

Spray foam is the easiest to use as most people don't like to be forced to cut and cobble the rigid foam in there. Rigid foam works extremely well is your most cost-effective option from a material stand point. 

Use rigid foam, spray foam or do nothing at all in my opinion.


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## davemax (Jan 15, 2010)

Thank you, appreciate it. It's just that I have seen mold and rot in joists that had the rigid/spray foam as well. So, yes, not an exact science here.


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## joecaption (Nov 30, 2011)

Only time I've seen it is when there's a problem outside causing it.
Deck pored right up against the siding.
Attached deck with no flashing.
Deck built to close to the door threshold.
Siding to close to grade.
No gutters.
Mulch piled up against the foundation.
ECT.


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## HomeSealed (Jan 3, 2008)

Joe is right on with the moisture issue. If the rigid board is tight to the rim joist and properly sealed (or sealed with foam), interior moisture cannot reach it. It would have to be an issue from the exterior which would be completely unrelated to the insulation. WOW's advice is perfect as well. It IS actual an exact science when done correctly.


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## davemax (Jan 15, 2010)

the warm rigid board (from being in the warmer basement), can and does cause condensation against the wood. rigid board , even done properly, still has resulted in moisture and mold on the joist... so, all depends on the enviroment.. a warm but dry environment usually doesn't result in mold. and again, have had batts in place for decades, and never any mold. so there is no perfect solution.


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## SeniorSitizen (Sep 10, 2012)

Quote:Rigid board tight to the rim joist and properly sealed, interior moisture cannot reach it.
************************************************************

That's a figment of people's imagination, far from realistic and that's exactly why there is condensation in that area in some instances. If it isn't hermetically sealed it isn't sealed.


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## Daniel Holzman (Mar 10, 2009)

Davemax, I do not understand your argument. You say that warm, rigid board causes condensation on the joist. Let's examine the physics of this to see how that would work.

Moisture condenses on any surface that is below dew point temperature. So moisture will condense on the rim joist when the temperature of the rim joist, on the side facing the basement, drops below the dew point temperature in the basement. For an uninsulated joist, the temperature of the side facing the basement should be approximately basement temperature, since the rim joist will equilibrate with the basement temperature. If the basement temperature drops below dewpoint in the basement, which can certainly happen in the winter if the basement is moist, then you will definitely get condensation on an uninsulated rim joist.

Now let's see what happens with a rim joist insulated with fiberglass batts. The temperature of the batt side facing the basement will equilibrate to basement temperature. The outside temperature in the winter will typically be colder than the basement temperature. So a temperature gradient will be set up between the basement and the outside air, thus heat will move from the basement to the outside. This means that the termperature of the inside face of the rim joist will be colder than the basement temperature. The actual temperature can be computed if you know the R value of the batts and the rim joist, and the temperature of the outside air and the inside air. But the temperature of the rim joist inside face will be colder than the temperature of an identical, uninsulated rim joist. Total heat loss will be much less, because of the batt insulation.

Moisture can move through the fiberglass batt, and on days when the temperature of the inside face of the rim joist is lower than the dew point in the basement, moisture will condense on the rim joist.

If you use rigid foam, or spray on foam, and are successful in sealing the foam (this is not very difficult, I did it in my basement), the temperature of the inside face of the rim joist can drop below the basement dew point, but no moisture will condense, since moisture and air cannot move through the rigid or sprayed foam. There is a chance that moisture will condense on the inside surface of the rigid foam, but this should not cause mold, since there is nothing for the mold to eat (mold needs correct temperature, food source, and water to grow).

My basement rim joist was insulated with fiberglass batts when I moved in. These were about R6 batts, but they were in poor condition, so I eventually replaced all the batts with rigid foam 3 inches thick (about R12), sealed with expanding foam. My basement is relatively dry in the winter, and I have never had a condensation problem either with the batts or the foam, but my analysis suggests that a moist basement is more likely to have issues with batts than sealed foam. But enjoy your project, and good luck.


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## HomeSealed (Jan 3, 2008)

Great explanation Daniel.

@Fairview,while a 100% seal may not be possible, all that we can do is seal things as best we can, and follow best practices based on science. Sealing with foam and/or a combo with rigid board is that. It most certainly WILL reduce the amount of interior moisture that could reach that area as much as possible, and therefore provide the lowest risk for condensation and best insulation/seal. By your logic we should not airseal attics either, as they cannot be "hermetically sealed" and they will still develop condensation. Obviously that is not the case.

@Dave, as stated above, you are right in that there is no PERFECT solution, however there is a best solution. The advice you are getting on this thread is from experienced and certified professionals that do this type of thing day in and day out. It is the best practice as recognized by the building science/home performance community, and there very much is a science behind it. Ultimately it is your home so you can do what you choose, but the best way to do it is as described. 
Best of luck.


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## SeniorSitizen (Sep 10, 2012)

If we put insulation against the sheathing between the rafters I could see a correlation between insulating rim joist and an attic but that isn't done.

The problem with the rim is we've stopped trapped air from circulating with the room air and by insulating the rim the interior rim surface is allowed to cool to dew point temperature. Once that temperature is reached it could very well hold moisture in that space for weeks or possibly all winter.

I fear for those with walk out basements in cold climates who are attempting to conserve energy by insulating the interior and allowing that portion above grade to become close to outdoor ambient.


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## HomeSealed (Jan 3, 2008)

@fairview, there is no trapped air. The insulation board or sprayfoam is sealed tight to the rim joist. If one were to improperly leave an airspace between the insulation and the rim joist that could possibly be a concern, but again, you are still sealing it off from the interior moisture. The condition that you are describing is essentially what happens on windows when you keep the blinds/drapes closed, but does not apply here. In fact, along those same lines, a good example would be properly sealed window plastic on the interior, or a good interior storm that seals well. The window itself will be much colder, but it will not develop condensation unless the seal is poor and the warm, moist interior air escapes. Ironically enough, the situation that you are describing is pretty much what would happen with fiberglass batting. It willl keep that surface colder yet not stop the moist airflow toward the cold surface. Batting in the rim joist = closed drapes on your windows. No, it will not guarantee condensation, but it will increase the risk. Does that make sense?
Regarding the attic analogy, your example would only be accurate on a vaulted sealing, and yes, it is normal procedure to seal/insulate against the underside of the roof deck in that case. 

@ Davemax, one other thing that I just noticed was your reference of the basememt as "unconditioned space". Is the temperature in your basement closer to the temperature in your main living space, or to the outside? If the former (like most basements), it is conditioned space and should be treated as such. Whether or not you have heat supply lines down there is irrelevant.


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## SeniorSitizen (Sep 10, 2012)

QUOTE: @fairview, there is no trapped air.

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That's where I respectfully disagree and that's why we see examples of this problem here on the forum whether it be wood or concrete.


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## Windows on Wash (Aug 30, 2011)

davemax said:


> Thank you, appreciate it. It's just that I have seen mold and rot in joists that had the rigid/spray foam as well. So, yes, not an exact science here.


Anything done improperly doesn't exactly invalidate the observations and largely settled insulation science when it comes to ribbon boards.

Most ribbon board rot is due to exterior moisture from bad flashing details and drainage plane design. 



Fairview said:


> QUOTE: @fairview, there is no trapped air.
> 
> ***********************************************
> That's where I respectfully disagree and that's why we see examples of this problem here on the forum whether it be wood or concrete.


With all due respect Fairview, many of the posters on here are first time DIY'ers and may not make the right material choices or consumable applications. 

This is nothing that isn't done by a myriad of contractors as well because I see highly regarded basement companies insulating finished spaces terribly as well.

Once the sheetrock and mud it is becomes far less of a potential issue but still done poorly. 

At this point, I think the original poster has the input of several folks that do this for a living and have incorporated the evolution of building science into their business model. 

If he or she wants to use batt insulation, it very likely will work for them. Often times the bands are so leaky and so much air is moving that the moisture is carried to outside


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## davemax (Jan 15, 2010)

Thank you. Regarding the conditioned /unconditioned. I guess it would be considered conditioned as per above, the temp is closer to the living space. It could be 10 degrees outside vs. 50 in the basement. Does this adjust what needs to be done regarding insulating the rim joists?


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## SeniorSitizen (Sep 10, 2012)

davemax said:


> Thank you. Regarding the conditioned /unconditioned.  I guess it would be considered conditioned as per above, the temp is closer to the living space. It could be 10 degrees outside vs. 50 in the basement. Does this adjust what needs to be done regarding insulating the rim joists?


 I recommend a dehumidifier to keep the humidity somewhere in the 30 percent range with the temperatures of 10 ° outdoor and 50° in the basement. 

Use the link and click the dot that solves for dew point. Set the temp. scale at 50 and run the humidity up and down its scale to see what happens to the dew point temperature. 

If the humidity is adjusted into the 50 percent range at those temperatures given there is likely to be moisture condense on the interior surface if the rim reaches temperatures in the 30'ish range. As the outdoor temperatures increase for the better it may be possible to shut down the dehumidifier some days.
If the insulation is still on go I would devise a method to monitor rim temperature during cold spells and use the link to advantage.

http://dpcalc.org/


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## davemax (Jan 15, 2010)

Thanks


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## davemax (Jan 15, 2010)

*crawl space?*

Along with the rim joist insulation, I have one more item if anyone can assist?

Had an old porch, unheated, over a dirt floor, but the siding went all the way to the ground, not a lattice or open sides.
They gutted the porch, insulated it top to floor, fiberglass in ceiling, walls (new studs), in the floor joists, put subfloor, linoleum.
Have a small 1/2 bath in the porch now, and another area for a freezer, shelving. So there is plumbing that put in for the sink and toilet through one of the joists in basement, and waste pipe comes back through joist to sewer line. Also, there was old basement window apparently under the porch that was boarded up when porch was added to home in the 1940's. They used this window to run 2 flex ducts through there, and sealed up best they could. So there are 2 ducts that are under the porch as well that go to floor vents in new insulated porch.

Problem: contractor did not use actual boots at floor above, lots of gaps and spray foamed. We always smelled the dirt floor below. He did not use any vapor barrier in subfloor, or at ducts either.
He came back and put some plastic on the dirt floor underneath, put some batt in the joists above , and put some rigid board on walls underneath. Not the best/sealed job, but said due to some plumbing under there, needed to keep it warmer. There no vents in this porch underneath, never were.

I have an actual HVAC company coming out to put new boots on the ducts, and said they could use a vapor barrier at that point and up to the floor and that should seal off any drafts we felt and smelled. I looked into having all the ducts ripped out and just putting in electric baseboard heat ( cheap electric rates where the house is). 

So, is this considered a crawl space underneath? Should i pull out his cheap thin uneven covered plastic and put some stronger 6mil or higher vapor barrier on floor? any insulation issues here? was it a waste to even put insulation under there? Do I need to not have plastic on floor to let floor "air Out" constantly? am i just trapping odor under there? You guys get the picture. Again, where the siding meets the ground, there is a about an inch or so open at bottom. Thinking if it was a room with no plumbing/pipes, no insulation would be need under there, again, not sure if this is actually a crawl space, just an exposed porch so to speak.

Any thoughts? Seen lots of sites about crawl space encapsulation, really getting it air tight?


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## davemax (Jan 15, 2010)

And by the way, your help is invaluable to people like me, and I do realize i am getting professional advice. Did not mean to come off like i knew more than anyone. Trying to block out all the nonsense that's out there and just rely on you guys.

I think my dilema with the rim joist insulation thing was, frustration knowing i had to cut board and spray foam all of them.. i know the fiberglass is so much easier, but now afraid of the moisture issue. The basement will and does have a de-humdifier running at all times. 

And another frustration was with the spray foam. everytime i used spray foam, it's such a large expansion? didn't then want to have to go around and trim the mess after it dries!
Should i use the one that doesn't expand the that much? they have some for windows and doors? the only other one i see is for gaps less than 1 inch. What's the trick, just keep the can moving and spray lightly? 

Thanks again.


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## HomeSealed (Jan 3, 2008)

No worries Dave 

Why the concern about over expansion in the rim joists? Doesn't need to be pretty. I would not use the low expansion stuff there. 

Fairview's advice about running a dehumidifier is solid, regardless of what you put in the rim joists. Two things to add: 1) It probably won't run much in the winter if you live in a cold climate as it gets pretty dry (around here people run HUMIDIFIERS in the winter to keep the moisture level comfortable). Really depends on your specific home though. 2) If you have windows in your basement, simply monitoring those will let you know if condensation will be an issue. They will condense first before any other area. 

On the "crawlspace" it should really be treated as exactly that. It sounds like the guys that did it the first time had the right concept, possibly just shoddy workmanship/materials? What is the height from your homes finished floor to the ground in this area?


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## davemax (Jan 15, 2010)

"What is the height from your homes finished floor to the ground in this area? "
no more than 4 feet from the dirt floor to the joist in the "crawl space", more like 3 1/2 feet.


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## HomeSealed (Jan 3, 2008)

definitely would be classified as a crawlspace in that case. As I mentioned earlier, the concept as done currently is correct for a sealed crawlspace, just may need to be done better per your description. Are there vents into the space? If so, seal them up.


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## davemax (Jan 15, 2010)

No vents, however, I questioned whether or crawl space because it is nothing more than the space under a once open porch, it has a "hatch door" about 2 feet high where you could access underneath the porch. My father used to open the hatches in the summer to "air out" underneath.

and again, the siding goes down 3 sides about 2 inches from the ground, so in essence it is relatively open at the bottom. Wondering if I should get the vapor barrier and pull up the sides to block out everything. 

PS. just got a second set of short term radon test results, 6.5 in basement, 2.7 in living room. Basement as I stated before has issues.. open sump, some cracks in walls, floor does leak water at center due to hydrostatic pressure mostly in winter/spring.

seriously considering the French drain to be put in (sides /wall sealed at floor), enclosed sump, and cement the holes, and the deal with the floor. Flat level floor in center, then slopes on 4 sides to walls. The seam where it slopes is where the water comes in. Going to use grinder at those points, caulk, etc..


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## High Gear (Nov 30, 2009)

In regards to the OP question ...You can get wireless temp monitors that have wires with probes for not a lot of $$.
You could put the probe right against the rim and then foam over it.
I have 4" of foam on the rim and 1" of foam board on the walls with r13 glass in studs past that.
Now that the basement is finished temps average 66f with a high of 68F in summer and 63f in the dead of winter.( uninsulated ceramic tile floor even).
That's with no heat ducts on and I have plenty.
Air sealing and insulating made a big difference from what it used to be.
Main level floor is warmer now ..no cold stocking feet.
I do run a dehumidifier in the basement except in the winter.


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## Windows on Wash (Aug 30, 2011)

Post up some pics Dave.

No worries about the back and forth. I appreciate a lively debate as does HomeSealed.

You should have seen some of ours back in the day....epic battles.


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## davemax (Jan 15, 2010)

Thanks


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## HomeSealed (Jan 3, 2008)

To clarify, it is now conditioned living space above this "crawlspace" in question, correct? How much above grade vs below grade? 
Pics would be helpful.

@Wow, epic to be sure. Something like this: https://www.youtube.com/watch?v=M-4ZNsw16-g


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## Windows on Wash (Aug 30, 2011)

HomeSealed said:


> To clarify, it is now conditioned living space above this "crawlspace" in question, correct? How much above grade vs below grade? Pics would be helpful. @Wow, epic to be sure. Something like this: https://www.youtube.com/watch?v=M-4ZNsw16-g


Too funny!!


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## davemax (Jan 15, 2010)

Yes, it is conditioned above the crawl space, it was an uninsulated porch /enclosed; had a door off the kitchen that went into this room. Now the room is completely insulated, has a half bath, door to kitchen removed, and it has 2 ducts running to it from under the crawl space. all above grade, even the crawl space is above grade.. as the "crawl space" was merely the area under the porch. 


Okay, the pics are of the other side of the house.. same set up.. this is the "front porch" not insulated, but you can see where the block foundation ends, and the siding of the porch begins and goes to ground (dirt floor under there). ON the other side of house, no pic available now, is same set up.. only no windows.


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## SeniorSitizen (Sep 10, 2012)

Running a humidifier in the winter because of dry air could be part of the problem in many instances. Using the windows as a monitoring device, we see that at an indoor temperature of 65°F , RH of 45 percent we have a dew point temperature of 42°. If we lower the RH to 25 percent at that same indoor temperature of 65° we have lowered the DP temperature to 28°. That 14° difference in DP temperature can be significant in preventing moisture on cold surfaces. 

Every time the subject of relative humidity appears I can't help but recall the advise of many doctors and health personnel in the 1950s.

IF YOU HAVE HEALTH PROBLEMS, MOVE TO Arizona.:laughing:


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## davemax (Jan 15, 2010)

Think i just found something that is going to make my rigid board and spray foam in the rim joists alot easier, cause you know i dreaded this instead of using fiberglass.. I hate spray foam,
Anyways, I'm going to use the spray foam but am going to get one of the professional nozzles for the cans! 
Saw some videos on them; foam stops when you stop, can control the flow a bit better, and easier on the clean up. Although i know they can run a few bucks, but i'll always have the nozzle. Saw them for under $50


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## Windows on Wash (Aug 30, 2011)

+1

Spend the few bucks and it will save you hours. 

We buy them in bulk and can get them for $25 a pop.


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## french_guy (Sep 11, 2012)

And what about rockwool instead of fiberglass?
Will rockwool generates the exact same issue than fiberglass, meaning rigid foam is the ONLY option?


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## Windows on Wash (Aug 30, 2011)

Rockwool is better than FG about keeping air from moving through it but it is still permeable.


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## french_guy (Sep 11, 2012)

so with proper caulking of all 4 seams, it's an acceptable option?


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## High Gear (Nov 30, 2009)

http://www.buildingscience.com/documents/information-sheets/basement-insulation


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