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Old 09-23-2016, 09:30 PM   #1
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Rigid Foam Insulation in Minnesota

I am finishing my basement in my 2015 build home in Minnesota. It is a walk out so part of it is above grade, part is partially above grade and part is obviously below grade.

I have several questions I am having trouble getting answers on.

1) If I use rigid foam insulation sheets rated for R10 attached directly to the concrete blocks with 2x4 framing in front, do I still need to use insulation batting between the studs?, if not do I just need a vapor barrier?

2) On the left side of the picture you can see how half is concrete block and half is already framed and insulated. The half that is concrete sticks out from the framing by about 1.5". If just use the 4x8 sheets of 2" rigid foam insulation here, can I go the full height of the wall and just use 2x4's to make a point of contact for the sheets? Or do I just use the sheets on the concrete and use the 2x4s to make it flush for drywall?

Any help is appreciated.

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Old 09-23-2016, 10:31 PM   #2
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Re: Rigid Foam Insulation in Minnesota

If this is a code question, I pulled the following from the internet for Minnesota.

Looks like you're on some form on the 2012 IECC.

So here's from Minnesota's adoption of that code, copied and pasted.

Looks like the code requirement would be R15 continuous insulation.

R402.1.1.4 Interior foundation insulation requirements. Any insulation assembly installed on the interior of foundation walls shall meet the following requirements:
1. Masonry foundation walls shall be drained through each masonry block core to an approved interior drainage system.
2. If a frame wall is installed, it shall not be in direct contact with the foundation wall.
3. The insulation assembly shall comply with the interior air barrier requirements of section R402.4.
4. The insulation assembly shall comply with section R402.1.1.5, R402.1.1.6, or R402.1.1.7, as applicable.
R402.1.1.5 Rigid interior insulation. Rigid interior insulation shall comply with ASTM C578 or ASTM C1289 and the following requirements:
1. For installation:
a. the insulation shall be in contact with the foundation wall surface;
b. vertical edges shall be sealed with acoustic sealant;
c. all interior joints, edges, and penetrations shall be sealed against air and water vapor penetration;
d. continuous acoustic sealant shall be applied horizontally between the foundation wall and the insulation at the top of the foundation wall; and
e. continuous acoustic sealant shall be applied horizontally between the basement floor and the bottom insulation edge.
2. The insulation shall not be penetrated by the placement of utilities, fasteners, or connectors used to install a frame wall, with the exception of through penetrations.
3. Through penetrations shall be sealed around the penetrating products.

If this is just a general question and inspectors won't be involved, yeah, you can install the R10 directly on the walls and additional cavity (between-the-studs) insulation is not necessarily required but it won't hurt. Still install the insulation with the vapor retarder installed to the conditioned (warm) side of the insulation.

What does tend to happen when interior insulation is installed on foundation walls in a cold climate (especially vapor impermeable or nearly impermeable such as rigid foam sheets) is that the foundation wall behind it becomes cold since it is thermally broken from the conditioned air on the inside. However, any humidity will tend to condensate on that cold wall since it is below the dewpoint temp and make the wall behind it wet. That's why the code shows as part of the installation requirements that the foam sheeting be sealed it it's joints and at it's edges where it is in contact with the wall.

Hope that helps.
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Old 09-24-2016, 06:33 AM   #3
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Re: Rigid Foam Insulation in Minnesota

1. I will pass this one along. You will see Gary mention it several times in here and it has application.


2. Are you talking about the alignment of the wall here for continuity? Depends how much of the walls are framed. At the end of the day, you can always have more insulation and goodness knows that you could use it in Minnesota. If you aren't worried about the small footprint losses, frame the whole thing up. Be sure to insulate under the sill plate as well.
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