Building Science-Building Shell Layers-Thermal Control Layer

Last week we talked about interior moisture drive through the building shell, the vapor control layer. This week we will continue with the building shell layers discussion, number 4 on our list is the thermal barrier. The four layers:

1. The rain control layer
2. The air control layer
3. The vapor control layer
4. The thermal control layer

The thermal control layer, or insulations in our roofs, walls, foundations, and floors, is what keeps our homes warm in the winter and cool in the summer. Thermal performance effects heating and cooling costs, comfort, and can provide or reduce moisture drying potential within building assemblies.  Let’s start with the code requirements in zone 7, northern Minnesota.

Ceiling insulation values are required to be R-49, or approximately 15 inches of fiberglass bat insulation.  There are a couple of exceptions to the code where less R-value is allowed, usually in situations where ceiling framing choices limits the thickness of the insulation.  My suggestion has always been to install insulation levels over code requirements.  Remember, code is minimum, you can always go above code requirements.  R-60, or around 18 inches of fiberglass is better.  Minnesota building code also requires a 6 inch raised energy heel on all ceiling/roof assemblies.  This code requirement allows for a greater amount of insulation at the roof eave, reducing the threat of ice damming at the eave.

Fiberglass wall insulation.

Zone 7 wall insulation requirements changed with the adoption of the 2012 addition of the IRC code, which was adopted in 2015 by the state of Minnesota.  R-21 is now the minimum.  I still see a lot of R-19 fiberglass insulation batting at the big box stores.  This would be allowed for sound reduction in 2 x 6 interior walls, not exterior wall insulation unless another insulation method is also installed, such as exterior insulation.  The code will also allow 2 x 4 exterior wall framing with R-13 wall insulation and an additional R-10 exterior insulation.  When choosing this insulation method, you need to change the interior vapor barrier from a class I or class II to a class III.  2 x 6 exterior wall framing with R-20 wall insulation and an additional R-5 exterior insulation is also allowed, but still requires a class I or II vapor barrier.  I recommend 2 x 6 exterior wall framing with R-21 wall insulation and an additional R-15 exterior insulation, well above code minimums, you will then need to use a class III vapor barrier.  See the previous blog discussion for more information about the vapor control layer.

Zip-R sheathing. Rain control layer, air control layer, and thermal control layer all in one product.

Basement insulation for northern Minnesota is required to be R-15 to a depth of 10 feet or to the top of the slab, which ever is less.  The state of Minnesota has amended a large section of the code for basement insulation requirements along with vapor, air sealing, and exterior drainage and water management.  These code changes are confusing to a lot of contractors.  If there is a question, talk to your local inspector.  If I were building today, I would use ICF, or insulated concrete forms for foundation walls.  Many of these forms have insulation levels of R-25 or more, well above code minimums.  I will be writing a future blog talking about ICF’s.

Insulation is also required below concrete slabs, with R-10 being the minimum. This insulation must also cover any exposed concrete edge, which is common in slab on grade foundations.  Doubling this insulation level has a dramatic affect on comfort within the home. The temperature of the slab becomes much warmer with additional insulation, even without in-floor heat.  Code will allow for a frost protected slab on grade foundation which only has a 12-inch footing at grade level. (Typical footing depth is 5 feet for our climate.)  It is best to extend the insulation 4 feet out from the slab around the perimeter of the foundation.  A good option for someone looking to reduce the cost of the foundation, or where a basement is not practical.

The last insulation code requirement is for crawl spaces and floors over crawl spaces.  Two common types of crawl spaces are vented and un-vented.  A vented crawl space will require insulating the floor joists to R-38 minimum in zone 7.  (R-30 is allowed in zone 6.)  Because the crawl space has vents to the outside, the space is considered outside the thermal boundary of the home. There are codes regulating forced air heating and ductwork when they are located outside the building’s shell, a topic for a future blog.  This construction method is common in manufactured homes.  An un-vented crawl space will have the walls (and possibly the floor) of the crawl space insulated. The space is treated like any other space within the home.  The space is typically heated and may have an air exchanger supply and exhaust duct installed.  Minimum insulation value for crawl space wall is R-15.

I did not talk about the different types of insulation in this posting, I will save that discussion for another day. Just be aware that there are limitations when using certain types of insulation. You would never use fiberglass batt insulation under a concrete slab.

Earlier in this discussion I mentioned the effects insulation can have on a building’s ability to dry.  In a climate such as Minnesota’s, heating is required to maintain comfort for over half of the year.  (Some years it seems we heat for 9 months.)  Insulating a building results in a resistance in heat flow to the outer building shell components.  Colder building materials tend to be wetter and can more easily reach dew point temperatures.  Water and most building materials do not get along.  Building codes are not going to reduce R-values in walls, ceilings and floors.  In my opinion, what will be required in the future is insulating the exterior of our structures.  This is a major change to how we are building today.

This concludes the discussion on the four layers of the building shell.  Questions, comments or concerns?  Send me a message below.

One Reply to “Building Science-Building Shell Layers-Thermal Control Layer”

Leave a Reply

Your email address will not be published. Required fields are marked *