Construction Design-Below Grade Insulating-New Construction

In new construction, Minnesota building codes require insulating basements and foundations, but this insulation was not required in some older homes. Over the next couple blog postings, I will be discussing below grade options for foundation insulation.  This first blog will discuss new construction code requirements and options with the second posting dealing with existing structures.  For those builders outside of Minnesota, these codes are probably much different than codes in other parts of the country.  I personally would never use fiberglass or open cell spray foam for below grade applications, but they are both allowed in Minnesota.

An uninsulated concrete block basement at -10 degrees.

Let’s start with the State of Minnesota’s requirements for new construction basement walls. The code reads:

R402.2.8 Basement Walls. Walls associated with conditioned basements shall be insulated from the top of the basement wall down to 10 feet below grade or to the top to the footing, whichever is less. Foundation insulation shall be installed according to the manufacturer’s installation instructions. Walls associated with unconditioned basements shall meet the requirements of this section unless the floor overhead is insulated in accordance with sections R402.1.1 and R402.2.7 and the following requirements:

1.  R-15 insulation for concrete and masonry foundations shall be installed according to R402.1.1.1 of R402.1.1.8 and a minimum of R-10 shall be installed on the exterior of the wall. Interior insulation, other than closed  cell spray foam, shall not exceed R-11. Foundations shall be waterproofed in accordance with the applicable provisions of the International Residential Code (IRC).

Exception: R-10 continuous insulation on the exterior of each foundation wall shall be permitted to comply with this code if the tested air leakage rate required in section R402.4.1.2 does not exceed 2.6 air changes per hour and the total square feet between the finished grade and the top of each foundation wall does not exceed 1.5 multiplied by the total lineal feet of each foundation wall that encloses conditioned space.  Interior insulation, other than closed cell spray foam, shall not exceed R-11. See footnote c to Table R402.2.1

2.  Minimum R-19 cavity insulation is required in wood foundation walls.  See footnote 1 to table R402.2.1

Isn’t code language fun!  Basically, if you are building a new home and it has a basement, you need to insulate the foundation wall to R-15 from the top of the foundation to the footing. The insulation can be placed on the exterior or both the interior and exterior.  You can eliminate foundation insulation if the foundation space is unconditioned and the floor above the basement is insulated to at least R-38 in climate zone 7 and R-30 in zone 6.

If all the insulation is on the exterior, it must be R-15 and must have a water proofing strategy or some other approved exterior drainage assembly. Waterproofing must extend from the interior top edge of the foundation wall to the top of the footing.  I would also include the entire top of  footing as part of the waterproofing strategy (the connection between the footing and foundation wall is a concrete “cold joint” and can be a source of basement water issues).

Any wood interior walls built must be held 1 inch away from the masonry.  The insulation assembly must comply with the air barrier requirements of the Minnesota code.  There are additional requirements depending on which insulation type is being used.  Your choices are rigid foam, closed cell spray foam, open cell spray foam (I would not use this product at all in Minnesota, but it is allowed), or fiberglass batts (in my opinion, another poor choice for basement foundation wall insulation).

If there is a combination of interior and exterior insulation, at least R-10 must be exterior with no more than R-11 interior, unless using closed cell spray foam for interior insulation.

Now, there is an exception to all this, you can use R-10 exterior if you can achieve a blower door test of 2.6 ACH50, which most builders in Minnesota are achieving.  All the water proofing and drainage provisions are still required.

Let’s go over the interior foundation insulation choices. First, the basic requirements;

R402.1.1.4 Interior foundation requirements.  Any insulation assembly installed on the interior of the 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 framed wall is installed, it shall not be indirect 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.

This first section deals with drainage, air sealing and framed walls not being in direct contact with masonry.  The next three sections deal with the insulation types.

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 acoustical sealant
c. All interior joints, edges, and penetrations shall be sealed against air and water vapor penetrations
d. Continuous acoustical sealant shall be applied horizontally between the foundation wall and the insulation at the top of the foundation wall; and
e. Continuous acoustical 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.

Straight forward enough.  The insulation needs to be in direct contact with the foundation wall and air sealed.  The only penetrations allowed are through penetrations, such as a sewage discharge or an electrical conduit.  The two rigid foam products that would work best would be XPS and EPS foam.  Rigid rockwool insulation would be a good choice but it is not air or vapor tight.  The extra work that would be required would cost more than the benefits.  Next up is spray foam.

R402.1.1.6 Spray-applied interior foam insulation. Spray-applied interior foam insulation shall comply with the following:

1. Closed cell foam:
a. The foam shall comply with ASTM C1029 and have a permeance not greater than .08, in accordance with ASTM E96 procedure A, and a permeance of not less than 0.3, in accordance a with ASTM E96 procedure.
b. The foam shall be sprayed directly onto the foundation wall surface. There shall be a 1-inch minimum gap between the foundation wall surface and any framing
c. The insulation surface shall not be penetrated by the placement of utilities, fasteners, or connectors used to install a frame wall, with the exception of through penetrations.
d. Through penetrations shall be sealed around the penetrating products.

2. Open-cell foam:

2 inches of closed cell spray foam in a basement wall cavity. (R-14)

The requirements for open cell spray foam are similar to the closed cell requirements, except that a vapor retarder of 1 perm of less using the dry cup testing method is required.  Do not use open cell spray foam below grade!  It has the ability to absorb water and the requirements of a vapor retarder may not allow any drying potential.  If open cell must be used, be sure to use a smart vapor retarder such as Membrane, Intello or Marjex.

The last type of foundation insulation that is allowed is fiberglass batts.

R402.1.1.8 Fiberglass batt interior insulation. Fiberglass batt insulation shall comply with the following:

1. The above grade exposed foundation wall height shall not exceed 1.5 ft.

2. The top and bottom plates shall be air sealed to the foundation wall surface and the basement floor.

3. A vapor retarder and air barrier shall be applied to the warm in winter side of the wall with a permeance not greater than .08, in accordance with ASTM E96 procedure A, and a permeance of not less than 0.3, in accordance a with ASTM E96 procedure B meeting the following requirements.
a. The vapor and air barrier shall be sealed to the framing with construction adhesive or equivalent at the top and bottom plates and where the adjacent wall is insulated.
b. The vapor and air barrier shall be sealed around utility boxes and other penetrations; and
c. All seams in the vapor and air barrier shall be overlapped at least 6 inches and sealed with compatible sealing tape or equivalent.

Can you say dirty diaper?  The entire assembly needs to be perfect both inside and out. Any air leaks or mistakes on the exterior drainage and waterproofing will trap moisture between the vapor barrier and foundation wall, soaking the insulation.  Very similar to the open cell spray foam requirements, I personally would not use fiberglass insulation below grade.

So, what would I choose if I were building a new home with a basement today, I would use Insulated Concrete Forms (ICF), which falls under R402.1.1.1 Integral foundation insulation requirements.  I have built several of these systems myself, not hard to do.  ICF’s usually have R-20 or better of EPS insulation and come in several concrete wall thicknesses.  They are more expensive to install than a concrete block wall but have many benefits that justify the added costs.  One of the problems with block foundations in my area is the limited amount of masonry contractors, especially block layers. There are a few around but are in demand and hard to schedule.

An ICF foundation under construction. This is a home I built for a customer in 2011.

I’ve left out one type of foundation that also has codes requiring it’s insulating, frost protected shallow foundations.  I’ve built a lot of homes using this foundation and is probably my favorite type on price point builds.  I will write a dedicated blog for insulating shallow foundations in the future.

Again, I am in a very cold climate, (climate zone 7) with roughly 10,000 heating degree days.  I don’t agree with all of Minnesota’s foundation insulation requirements (open cell foam and fiberglass batt), but all you have to do is look at the first photo to realize foundation insulation is a good idea.  Depending on where you are located, your below grade insulation requirements may be much different. Always follow local codes.

Next week, foundation insulation for existing foundations.

5 Replies to “Construction Design-Below Grade Insulating-New Construction”

    1. we are building a church with flat floor construction, footings at 4 ft with a 4ft poured wall, the plans is to use 2 inch styrofoam on the interior walls but what to keep it down 3.5 inches and pour the floor over the top of it, can we or are there codes saying we can’t and if so where do we find the code, I have looked with no success. Thanks

  1. I want to Seal and paint my unfinished areas in my basement.

    I wondered what the builder put on the interior walls?
    Is it a spray they use over the Poured walls to absorb moisture?
    It adds a white rough textured finish to the walls vs. the smooth poured look after the forms are removed.

    I was thinking of sealing these walls with Aqua Lock then painting with a Bath or High humidity Latex paint.
    I would appreciate your professional opinion as to my proposed plan to make these areas look new and pristine.
    I planned on painting the concrete floors with InslX Floor paint when the walls are finished.
    Thank you for any recommendations.

    1. Hi Carl,
      Basements can be tricky when finishing on the interior. There are lots of variables, how old is the home? Has the basement ever leaked water through the walls or slab in the past? Many older homes lack a good water management strategy on the exterior when it comes to basements. A basement without any water management product or sealer on the exterior will move some moisture through the concrete. This moisture will mostly dry to the interior side, this is the reason older basements have high humidity and musty smell. Most products painted on the interior concrete will eventually bubble and peel off, my basement has this issue. If your basement has a water management system on the exterior, such as a drainage mat, or at a minimum, some sort of painted or sprayed on product designed to reduce moisture intrusion from the exterior, you shouldn’t have any issues with finishing the interior walls.
      As far as existing products, anything currently on your basement walls is most likely some sort of concrete sealer. I have been in basements that have the rough surface you described; I do not know which product produces that finish. There are no products I’m aware of that absorb moisture from the concrete walls. Again, as long as the basement walls or floor do not become wet or damp at any time of the year, you should be able to go ahead with your idea.

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