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Old 11-30-2011, 12:18 PM   #1
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Yes, Another Basement Insulation Question


I am getting ready to finish off my basement in Minneapolis. I have read all the Building Science pieces in all the posts I have searched over.

My question has to do with the size of polystyrene sheet to use. The building inspector said I could just use the 2" board and no further insulation in the framing. That gives me an R value of 10.

Pricing the styrene (Rigid Foamular Insulation) and unfaced insulation bats at my favorite home improvement store (HD, NOT Menards) I have determined that it would be more cost effective to use a 1" poly board and use unfaced insulation in the framing. The 1" board R value is 5 and the unfaced insulation R value is 13. That would give me a total R value of 18 at a rough estimate savings of 17%.

The landscape slopes towards the street and I believe we have the drainage pretty good at the rear.

MY QUESTION: Do you think it would be OK to go the 1" board and unfaced insulation as opposed to 2" board?

From Building Science:

Walls with 3.5 inches of extruded polystyrene (XPS) and no
vapor barrier performed the best in this analysis. However,
walls with 0.75 inches of extruded polystyrene and 3.5 inches of
fiberglass batt insulation in the cavity would perform well as
long as interior humidity was controlled below 50 percent
during the summer. Increasing the extruded polystyrene to 1.0
or 1.5 inches would improve performance even with higher
interior relative humidity during the summer months. This
part of the analysis assumed that the concrete wall had a
relative humidity of 100 percent at the exterior temperature.
Since these studies were for a climate location similar to
Minnesota, the thickness of rigid insulation (R-value) could be
proportionately reduced in milder climates.

Thanks for any input.

B



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Old 11-30-2011, 02:50 PM   #2
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Yes, Another Basement Insulation Question


Or just go with Sprayfoam and forgive the combo batt & foam board.

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Old 11-30-2011, 02:56 PM   #3
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Yes, Another Basement Insulation Question


Do you think that would be more cost effective than either of the options I have mentioned?

I may use this procedure to seal up the rim joists once all the mechanical, electrical, and plumbling have been done...depending on price.

B
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Old 11-30-2011, 03:58 PM   #4
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Yes, Another Basement Insulation Question


In the long term, yes.
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Old 11-30-2011, 04:28 PM   #5
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Yes, Another Basement Insulation Question


1" foam and R 13 here.

I believe BS says to keep humidity levels below 50% with this combo.

I did the 2" XPS at the sill band , doubled up for 4" to = R20

The basement is considerably warmer without opening any heat ducts even, no regrets.
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Old 11-30-2011, 04:59 PM   #6
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Yes, Another Basement Insulation Question


Quote:
Originally Posted by High Gear View Post
1" foam and R 13 here.

I believe BS says to keep humidity levels below 50% with this combo.

I did the 2" XPS at the sill band , doubled up for 4" to = R20

The basement is considerably warmer without opening any heat ducts even, no regrets.
After you doubled up to 4" did you then 2 x 4 stick frame? I like to have the framing for the electric, DVW, and water supply. Heating ducts are no problem as the main trunk runs overhead of the bedroom bathroom and utility room on one side of the load bearing wall and is next to the living room on the other side.

I want to do this properly w/o overkill ($$$).

B
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Old 11-30-2011, 06:15 PM   #7
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Yes, Another Basement Insulation Question


The framing is 2x4 up tight against the 1" xps , so R5 foam + R13 glass.

Sill joist band area only was 4" of xps.

Lots of threads on this using the search feature.

Sure its overkill but I want my comfort in the winter.

Main floor 72f , basement 66.5f ( with basement heating ducts closed)

outside 37.6f gotta luv those remote temp moniters.
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Old 11-30-2011, 06:30 PM   #8
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Yes, Another Basement Insulation Question


Sorry High Gear, I misunderstood, what you have done sounds exactly what I want to do. I thought you had 4" of XPS all the way down the wall.

I want to have a 1" poly board and unfaced insulation in the framing(R5 + R13). I was asking if 1" of poly instead of 2" will create a moisture problem on the inside of the board (condensation). I will monitor the humidity levels in the summer to see if I need a humidifier.

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