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Old 10-10-2014, 08:54 AM   #1
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Rim joist insulation


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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Old 10-10-2014, 09:08 AM   #2
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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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Old 10-10-2014, 09:11 AM   #3
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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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Old 10-10-2014, 09:41 AM   #4
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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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Old 10-10-2014, 09:57 AM   #5
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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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Old 10-10-2014, 10:40 AM   #6
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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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Old 10-10-2014, 11:47 AM   #7
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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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Old 10-10-2014, 11:55 AM   #8
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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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Old 10-10-2014, 12:57 PM   #9
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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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Old 10-10-2014, 01:01 PM   #10
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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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Old 10-10-2014, 04:29 PM   #11
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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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Old 10-10-2014, 05:58 PM   #12
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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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Old 10-10-2014, 07:18 PM   #13
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@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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Old 10-10-2014, 11:40 PM   #14
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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.
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Old 10-11-2014, 08:39 AM   #15
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Quote:
Originally Posted by davemax View Post
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.

Quote:
Originally Posted by Fairview View Post
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

Last edited by Windows on Wash; 10-11-2014 at 08:44 AM.
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