Insulating Basement Rim Joist

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  #1  
Old 10-19-15, 05:22 AM
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Insulating Basement Rim Joist

Hello, my name is Dave, new homeowner, and first post to DIY. The home my family purchased has an unfinished basement and with winter coming soon to the upstate New York area, I wanted to insulate my rim joists. After doing some research I purchased some rigid foam insulation (FOAMULAR 250 2 in., R-10) and, to the best of my ability, cut it to fit each approximately 9"x14" area. Next I understand that I should use expanding foam (Great Stuff) to fill the gaps between the edges of the sill, joists, and ceiling. Before I get too far into the job I have a couple of questions:
1) The most basic question - using the rigid foam plus expanding foam a good approach?
2) Between a couple of the joists I have air ducts. Currently I have just stuffed some rock wool insulation (Roxul) - please see attached photo. Is that OK? Do I need some sort of vapor barrier?
3) Due to some obstructions (pipes, etc.) I am having a hard time getting the Great Stuff application gun close enough for application. I'm thinking I could get a caulking gun into those spaces, is there another sealant I could use with a caulking gun?
I appreciate you taking the time to read my post and hope you can provide some much needed guidance.
Dave
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  #2  
Old 10-19-15, 06:19 AM
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Hi Dave and welcome to the forum.
Despite its convenience rigid foam has some drawbacks, like often needing an ignition/thermal covering. Local code requirements may specify what is needed and the location and area my be factors. You can put that on your to-do list to research.

House to foundation is often a source of much air leakage. I prefer a good caulk to the foam for covering small cracks. If using the can foam you might try a short length of 1/4" plastic tubing (I found one size that fit nicely over the straw of my can foam. Tape it securly as the pressure can blow it off. But the additional flexible length really makes getting the foam where you want it much easier.

Your temperatures are cold enough to be a problem with exposed mineral wool allowing air to seep through and reach the cold rim. I would fabricate a vapor barrier, or glue (caulk) several pieces of the rigid in place to insulate those cavities. Example, you could stuff the mineral wool in behind the duct, but then cut a piece to cover farther out with the hole in it for the duct. Then cut it into 3 or 4 pieces to get it around the duct. Hard to see so I'll leave the fabrication to you.

As a note, the typical r-value of an 8" concrete is "one" That's not r=1 per inch, that's r=1 for the entire 8". Here are some numbers minus the actual calculations (available if you are interested).
Using 2' of exposed concrete (include one foot below grade) and a perimeter of 140 feet, that's 280 ft² of concrete. If I assume your basement is only indirectly heated, not at a full 70°, then your energy loss from just the concrete, not the rim, would cost over $500 per year. For a unintentionally heated space, that is huge. Just 1" (they require 2") of rigid covering the upper 3 or 4 feet of foundation would save 80% of those lost dollars. Again, more for your to-do list. Better calculations available with more real information. My numbers were a conservative guess.

Later,
Bud
 
  #3  
Old 10-19-15, 07:47 AM
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Thanks Bud, this is very helpful. I will get some multi-purpose caulk and use that for the smaller gaps and supplement with the expanding foam for larger gaps. I will do my best to construct a vapor barrier around the air ducts - maybe a silly question, but would plastic sheeting be an option? So keep the rock wool in place and staple some plastic sheeting around the air duct.

Concerning the rock wool insulation, do you know if I should I be at all concerned about using the rock wool insulation around the air ducts? After is pushed that up around the duct I was wondering if any of the fiber could become airborne when the heat (or AC) turns on.

Thanks again for all the help! Dave
 
  #4  
Old 10-19-15, 08:01 AM
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Two things about those air ducts, there should be no air leaking from them and the rock wool next to them should not be a problem. But seams and junctions on the ducts system should all be sealed, optional somewhat when the ducts are inside the thermal envelope, however any leaks can result is pressure balancing issues that contribute to increased air leakage to the outside.

Plastic should be fine just caulk it as well as staple it.

BTW, the use of can foam for long term air sealing is getting a closer look as much of it has been in place foe many years. But subsequent leakage testing is showing older air sealing projects are not holding up as well as expected.

Bud
 
  #5  
Old 10-19-15, 10:06 AM
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I really appreciate that you are taking the time to help me out. One last question on the rock wool insulation. I understand I should put a vapor barrier between the rock wool and the room, but do I need to put anything between the rock wool and rim joist wood? Just want to be sure before I button things up.
Thanks again Bud, nice to know there are folks out there willing to help a stranger.
 
  #6  
Old 10-19-15, 10:19 AM
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No, the rock wool can go directly against the wood as long as you air/vapor seal to the inside. Do your best at sealing all of the seams on those ducts where they venture out next to the cold rim joist, even at the floor level to be sure it isn't blowing air back down into the cavity.

Bud
 
  #7  
Old 10-19-15, 07:42 PM
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great stuff is air tight, it's not water tight though, just water resistant. caulk small joints, greatstuff larger ones. The foam board should be a vapor/draft barrier already. I'd put foam board against rim joist and caulk/greatstuff to seal them and then to increase the R value, you can stick pieces of fiberglass batts in there but I think the rim joist insulation has to be paperfaced for code with the paper facing the room (fire retardant but regular paper faced should do).
 
  #8  
Old 10-20-15, 05:07 AM
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Thanks Gunner, I hadn't thought of using insulation batts too, but that seems like a good idea considering how cold it gets here in the winter.
I spent some time last night cutting the rigid foam and sealing with expanding foam and overall I think everything is going well. However, considering that this I my first home and I am a worrier, I was wondering if there is a possibility I could end up doing more harm than good? I am doing my best to get the foam board snug against the back wall and sealing with the caulk/expanding foam, but if the seal is not perfect, is there a possibility that it could cause a molding issue between the wall and the foam board?
I should mention that we keep the basement between 60-70 F and run a dehumidifier 24/7 which keeps the humidity between 35-45%. Also, we installed a new perimeter drain and sump system after we purchased the home. Up to this point we have not had any noticeable moisture issues. The previous owner had just stuffed faced fiberglass insulation within the rim joist areas and upon inspection when I removed it there was no sign of mold or mildew.
Guess that even though I knew we were throwing some money away heating/cooling our home with the unsealed rim joist, at least I could see that there was no mold or mildew issues. I have to admit it causes me some anxiety now that I cannot see behind the foam board. Obviously you cannot see what I am doing to assess how good the seal is, but am I worrying too much?
Thanks again for your time and patience!
 
  #9  
Old 10-20-15, 05:35 AM
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I don't know what your definition of "not perfect" is? I suspect, given the concern you are showing, it is better than most peoples "good job".

The idea behind using only one vapor barrier is that an assembly will always have a direction to which it can dry. If the rim behind your rigid foam were to get damp, it simply would dry to the outside, thus the use of vapor permeable house wrap and even tar paper will let the moisture pass.

I'm not sure what else you have done to improve energy efficiency, but a good energy audit that will allow you to accompany them the whole time and use an infrared camera would be a big plus. In addition to reducing energy costs, one of the main objectives is to improve air quality and comfort.

Take pictures as you make improvements to have if and when it comes time to sell. Once covered others may not understand what was done.

Bud
 
  #10  
Old 10-20-15, 10:01 AM
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as the poster before me said and you are implying, double vapor barrier can lock in moisture, I was just suggesting it for draft, just caulking the wood joints should keep draft out but you never know how good the house wrap is because air can go through wood. Double vapor barriers are sort of two different schools of thought if they are needed or not. Most latex (I think) paints are a vapor barrier and thus most walls have double vapor barrier though. As someone mentioned, the foam board might need to be covered with something flame resistant, even sheet rock will do and they have a flame resistant paint even but I don't think your house is going to burn down without it and the paper faced fiberglass might be enough for code, there's also fire resistant wool-like fiberglass stuff I don't know the name except it's called rotten cotton sometimes, might be what you already used. They also have foam board with a silver face that when heat meets the silver, it actually bounces back into the basement, good for winter, not good for summer. Vapor barriers and insulating are admitting tricky to me, I hope I helped.

By 'perfect' I think you just meant didn't miss a spot when sealing with caulk/greatstuff.
 
  #11  
Old 10-21-15, 02:49 AM
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I'm sure I might be overthinking or just it might just be overkill, but what do you think about spraying the foam sealed edges with one of those liquidized rubber sprays (e.g., Rustoleum LeakSeal or Flex Seal) to help ensure the edges are air tight and waterproof.
 
  #12  
Old 10-21-15, 03:15 AM
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LOL, both, "overthinking... and... overkill".
One of the details I have learned from energy auditing is, the first step makes the biggest improvement. It would be better to allocate that money and time towards other areas. What is covering the concrete below the rim joist?

Bud
 
  #13  
Old 10-21-15, 03:51 AM
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You might be able to see from my initial photo, we have exposed block wall. At the top of the block I placed Owens Corning Foam SealR Sill Plate Gasket, held in place by weatherized 1x4. I realize that is not much of an insulator, but it has helped (I think) and helps to keep the spiders that inhabit the block in check (I hope).
 
  #14  
Old 10-21-15, 06:10 AM
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In that picture it looks like the block wall is exposed on the inside. Where it is exposed on inside and outside (plus one or two feet below grade) it performs like a piece of sheet metal. In post #2 I mentioned it has an r-value of R=1, which is the same as a single pane of glass or plane sheet metal. In other words, it loses heat 10 to 20 times faster than the walls in your home. This area becomes especially important when you are heating that basement.

Blocks are often installed without filling the cores. That results in air currents within the blocks that extends that heat loss all the way to the floor. For your to-do list, consider some rigid foam on the inside.

Bud
 
  #15  
Old 10-21-15, 06:55 AM
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You are correct, all we have done is paint the block wall and the block cores are not filled. Framing and insulating the walls is definitely on the to-do list, but that is something that I think I would need to get a professional contractor to do and funds are a bit tight right now. Thanks again for all the good advice.
 
  #16  
Old 10-21-15, 10:52 AM
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SyracuseDave. -

You are at a good point now because the cores are not filled completely. Filling all the cores is a big mistake because the empty cores offer flexibility down the road. The only cores that should be filled are the ones that require rebars AND grout. Arbitrarily filling the other cores does little structurally since the vertical loads on a basement walls are really minimal since your home is just 1 or 2 stories of lightweight frame construction with light floor load that actually get to the basement walls. You probably have some interior columns and/or walls that do see a great portion of those loads.

Also, the block walls below grade see much warmer temperatures than the walls above grade in most northern climates. In MN, the normal temperature below grade at about 4-5 feet is about 54F (close to the average annual air temperature of the location. The frost depth is just an arbitrary structural code construction requirement to eliminate frost damage in the next 50 or 100 years and has little to do with the actual daily soil temperatures.

Bud, - If you attempt to calculate the insulation value of a wall section, you would add the "R" values of the various material layers of the wall section including the "R" value of the surface air films (.68 or .85) that is a beneficial item for a realistic number. With that, you can use the reciprocal of the sum to determine the "U" value of the wall assembly.

Dick
 
  #17  
Old 10-21-15, 12:21 PM
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Hi Dick,
I was just trying to emphasis the terrible insulation qualities of a concrete or block wall without going into a lot of detail. Air films are a detail that gets mostly ignored, but you are correct, when the rest of the r-value gets down to one, they do have some value.

Bud
 
  #18  
Old 10-22-15, 04:01 AM
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To increase the R-value, at least in the rim joist area, I was considering adding insulation batts now that the rigid foam insulation has been installed and sealed. First off, is it a good idea to install faced batts in the rim joist area with the rigid foam (not sure if I followed the discussion on having two vapor barriers). If so, I have seen the paper/foil vapor barrier simply stapled to the joist, is that OK or should I glue it (if so what type). Thanks again, I realize this project would not be going so well without your guidance.
 
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Old 10-22-15, 07:18 AM
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@Dave "(not sure if I followed the discussion on having two vapor barriers)."
With two vapor barriers, any moisture that gets in can't get out. Thus the better designs will always one one vapor barrier, which then allows drying to either side. The confusion comes in when we, correctly, consider that many materials we once considered "vapor barriers" are in reality "vapor retarders" having some permeance. That means that moisture may dry right through them, but VERY slowly.
Vapor Barriers or Vapor Diffusion Retarders | Department of Energy

Given the location, the rim joist, and the newly installed rigid insulation, stapling up some additional Kraft faced fiberglass insulation should not create a problem. The normal guidance is to use unfaced, but I hate mice and mice love fiberglass. Absent the kraft facing one could cover it with house wrap.

Now, how much fiberglass? Here is a link that may help or add more confusion. I've read his previous related links but not this one, just added to my reading list. But it seems to deal directly with the foam and fiberglass marriage. let me know.

Bud
The 2012 Code Encourages Risky Wall Strategies | GreenBuildingAdvisor.com
 
  #20  
Old 10-23-15, 04:58 AM
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Thanks Bud, this is good information - but still trying to get my head around it.

With respect to faced vs. unfaced insulation, my concern is the potential impact of unfaced insulation on the basement air quality (specifically suspended fibers). My son loves to play in the basement and I don't want him breathing anything that might harm him. That was why I was considering using the faced, to help place a barrier between the room and the insulation. Maybe I will just double-up the rigid foam board or consider fiber insulation plus drywall (kind of like framing in a wall).
 
  #21  
Old 10-23-15, 05:12 AM
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Except for our attics, most fiberglass insulation is enclosed on all 6 sides.

As I said, given the location, I don't see using Kraft as being a high risk. If you prefer, cover some unfaced with house wrap, which is vapor open, yet will control any fibers.

Limit your fiberglass to r-13, r-15 max. Too much fiberglass will allow the inside surface of the rigid to get cold enough to form condensation.

Bud
 
  #22  
Old 10-23-15, 06:41 AM
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Bud - here's another possibly silly question, but I was looking at the cost of house wrap or Tyvek sheeting and was wondering if I could just use the weed barrier for the same purpose (Polypropylene Landscape Fabric)? I have a bunch of that in the garage. It would allow air to move but should do a decent job keeping the insulation at bay. Thanks, Dave
 
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