Caulk vs. spray in expanding foam insulation

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  #1  
Old 12-08-16, 08:28 PM
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Caulk vs. spray in expanding foam insulation

Hi all,

I have a few small seams between the wood and the concrete in the ceiling of my basement where I can feel cold air coming in. I'm planning to put some fiberglass insulation in the areas between the joists but thinking I should also make it more airtight first. Is the best way to do this with caulk of some sort or would the cans of spray insulation that expand out work? Any pros/cons to either? Thanks!
 
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Old 12-09-16, 02:47 AM
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This is not the answer you want, but here goes. I believe you're planning to insulate your rim joists - the rectangular sections between the joists. If so, fiberglass insulation is a bad idea and should be avoided. You see fiberglass insulation is air permeable so warmer interior air easily passes toward the exterior. In winter, the rim joists are cold and condensation forms from the warmer air leading to mold, plus the moisture makes the insulation less effective.

Instead, use 2" rigid foam sealed with expanding foam or use spray foam insulation. I think you'll find the spray foam an easier but more costly choice. If you want, add fiberglass over that.
 

Last edited by Tony P.; 12-09-16 at 03:55 AM.
  #3  
Old 12-09-16, 04:58 AM
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Between wood and concrete = the crack under the sill plate. This is a good place for caulk, which does a better job of air sealing tight gaps.
 
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Old 12-09-16, 06:21 AM
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Great info, thank you both! I didn't realize that about fiberglass in the rim joists (thanks for figuring out my non-technical terminology ).

So I will caulk the air spaces first. Then it sounds like I need to rethink my plan. I have a 1" foam insulation board already - could I use that against the wood and then put the fiberglass (R-19) in front of it? Or would using just the 2" board be more effective?

Also - if I do use the fiberglass, does it matter if it's faced/not?
 
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Old 12-09-16, 10:03 AM
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The concept is that 2" of rigid insulation provides sufficient vapor protection for the rim joists. You can get by adding a total of 1.5" of rigid insulation but the industry standard is 2", yours plus another 1'.

Unfaced fiberglass could be added to that.up to the recommended rim joist R-value for your area.
 
  #6  
Old 12-09-16, 03:45 PM
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The use of rigid insulation against the rim joist or a cold exterior wall is to prevent warm moist air from reaching a cold surface, as Tony explained. But, there is a ratio of rigid to fiber insulation necessary to ensure the surface of the rigid remains above the dew point, the condensation temperature. Adding too much fiber insulation to the inside of too little rigid can take you right back to a cols surface. Here is a link that discusses this ratio and another link for the 09 minimum insulation code requirements for WI. Note, you need to confirm if 09 is still appropriate or whether WI has moved on to 2012 or 2015 requirements.
Calculating the Minimum Thickness of Rigid Foam Sheathing | GreenBuildingAdvisor.com
https://energycode.pnl.gov/EnergyCod...tate=Wisconsin

Bud
 
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Old 12-09-16, 09:14 PM
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Bud, I must say you've confused me. I can see this applying to rigid insulation applied to the exterior but in this case its applied to the interior. I'm not following how adding more insulation will impact the exterior side surface temperature in a meaningful way.
 

Last edited by Tony P.; 12-09-16 at 09:32 PM.
  #8  
Old 12-10-16, 12:03 AM
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The article referenced is the only source I can find that discusses the ratio of fiber insulation to rigid, but that ratio applies whether the rigid is applied to the exterior of the house or the interior surface of the rim joist. In either case the objective is to keep the first interior surface that warm air can reach above the dew point.

To be complete we can add the r-value of the rim joist and exterior siding to the r-value of the rigid so 2" of r-5 plus approximately R-2 would give about R-12. That would meet the climate zone 6 guidelines for WI but fall short for climate zone 7. If in zone 7 he would need more rigid or less fiber as in a 2x4 wall.

It's 3 am so I may have still missed with the explanation so kick it back up later when I'm awake and I'll try again.

Bud
 
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Old 12-10-16, 02:51 AM
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Bud, I hope you had a good sleep and certainly don't disagree with what you're saying but it's considerably different from my basic understanding of rigid insulation in rim joists. My understanding has been that stopping moisture is the focus and that takes me to permeability.

Sticking with XPS, 1" has a perm rating of 1.1 making it a Class III permeable vapor retarder. Alternatively, 2" has a rating of .7 (not proportional) making it a more desirable Class II semi-permeable. This has been my basis for recommending 2" rigid insulation.

XPS has an R-value of 5 per inch. This means that, per the table, 1" XPS may be used in zones 5, where I live. While I get that from an R-value perspective, it seems to disregard the permeability aspect. I mean, where the point is moisture flow, why use a permeable level of XPS anywhere?

Finally, amateur, I'm sorry if I temporarily hijacked your question.

Tony P.
 
  #10  
Old 12-10-16, 06:47 AM
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Hi Tony,
To avoid more drifting I started another thread on this issue:
http://www.doityourself.com/forum/en...ml#post2584465

Bud
 
  #11  
Old 12-11-16, 04:03 PM
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No worries, Bud, I don't mind at all - just reading the conversations here are helpful and interesting. My plan right now is to get the 2" foam board and some of the spray in (to spray around the perimeter of the 2" board) and then put the faced fiberglass in front of that. I did already add caulk to all the areas where I could feel / see any gaps but that's all I've done so far.
 
  #12  
Old 12-11-16, 04:19 PM
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In your climate the 2" will be good. When I do an Infrared scan during an energy audit the leaks between the house and foundation are often major. Turning on your dryer and bath/kitchen exhaust fans will simulate the leaks when the wind is blowing, easier to find especially when cold.

Bud
 
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