Construction Design-Below Grade Insulation-Existing Homes

Last week we talked about Minnesota code requirements for insulating foundations in new construction. This week we’ll hit existing homes.

There are a lot of potential issues with insulating existing foundations.  What kind of foundation?  Block, poured, stone or even brick?  Do they have waterproofing on the exterior?  Are they structurally sound?  There is not a one insulating technique that fits all when dealing with existing foundations and it’s important the right choices are made.

Heat loss through foundation walls will the keep soils along the foundation from freezing in a cold or very cold climate.  Frozen soils can push against a foundation, causing structural damage.  I have seen several bank owned homes that were winterized and allowed to freeze during the past financial crisis.  Some of these homes had foundations that had been frost damaged.  Bulges and cracks in the concrete that were not present prior to freezing.  All the damage I have seen has been in concrete block or CMU constructed foundations, most were not constructed correctly.  Insulating existing foundations can have similar effects, reducing the heat loss to the soils.

A damaged foundation wall. Is this freeze damage or damage caused during back-fill?

What are the options?  I’ll start with the most expensive, but depending on how the foundation is constructed, may not be the best.

Insulating from the footing to the mudsill on the exterior is the most expensive (and most work) option.  This will require excavating the foundation to the footing.  Be sure water proofing is present on the foundation, if it’s not, add it.  I would then follow the Minnesota (or your local) codes for new construction, at least R-15 total interior and exterior with no less than R-10 on the exterior in Minnesota.  If there is no footing drainage, this is a good time to add it.   The foundation needs to be sound, the soil will probably freeze along the foundation, problems worst than elevated heating costs and cool basements can result.

The next option would require some excavation, but only a foot or two below existing grade.  Adding insulation four feet horizontally away from the foundation and then insulating the foundation wall above.  This technique is similar to a frost protected shallow foundation, slightly raising the soil temperatures below the insulation to prevent the soils from freezing.  (Frost protected shallow foundations will be the topic of a future blog posting.)  The horizontal insulation should have a slight pitch to allow any moisture to move away from the foundation.  In my opinion, this is the best option if you are worried about foundation damage due to freezing soil movement.  One additional benefit is the soils below the horizontal “wing” insulation should stay dryer, which may help improve issues with a damp basement.

Frost protected foundation with horizontal insulation.

The last exterior option, and the least expensive, is to cover the exposed above grade foundation to around a foot below grade.  The exposed above grade portion of a foundation has the highest heat loss due to the greatest temperature differential.  Expanded Polystyrene (XPS), Extruded Polystyrene (EPS), and rigid Rockwool all work well.  I would try to achieve at least R-10.  It would be best to flash the insulation to the water resistive barrier of the above grade framing.  The insulation needs to be covered to protect from degradation due to UV and atmospheric pollutants. (and the weed whip).  This above grade insulation protection is a requirement of all three exterior insulation options.

An uninsulated concrete block basement at -10 degrees.

One additional option which I have seen used once is below grade exterior spray foam.  The foundation was entirely exposed and sprayed on the exterior with closed cell spray foam.  An additional coating material was spray applied to protect the foam.  I do not know a lot about this process, may be a topic for a future blog posting.

If your trying to insulate the foundation of an existing home, interior insulation is also an option, but to tell the truth, it scares me.  There are a few potential problems.  The first, freezing moisture that is present within the concrete.  Concrete could care less if it’s wet or dry, as long as wet concrete doesn’t freeze.  Lowering the temperature of the foundation by insulating the interior can cause the moisture trapped within the concrete to freeze and damage the foundation walls.  For this reason, exterior insulation is always the first choice.

The second I already discussed, lowering the heat loss through a foundation wall which reduces the temperature of the soil to the point where it freezes.  Soil movement in cold climates can damage foundations.

Lastly, when you bury one side of concrete with soil that is wet, (and there is no or a poor water proofing strategy on this exterior foundation wall), the moisture will move through the concrete to a dryer area, which will be the basement.  The second law of thermodynamics wet to dry!  Capillary and hydraulic pressures may also move moisture through concrete.  Minnesota requires extensive air sealing measures to be completed when insulating foundation walls.  Are we trapping that moisture behind a product that may have a low perm rating?  Codes in my area require the use of low perm vapor retarders when insulating with either fiberglass or open cell spray foam.  The retarders often end up being polyethylene sheeting.  Not a good idea.  Never insulate a basement on the interior that has moisture issues.

For these reasons, I will only use two methods to insulate the interior of a basement.

Rigid sheet insulation, either EPS or XPS, sealed on all four sides, and less than R-11 is the first method.  Neither of these insulations are supposed to promote mold growth, but they do create a toxin when burned.  Almost all rigid foams will need to be covered with a fire resistant covering, such as drywall.  Install the foam against the concrete foundation wall and build a wall against the foam.

The second option is closed cell spray foam, which I try to keep under two inches.  Again, I don’t want to reduce heat loss to the soil to the point of freeze concerns, a couple inches is plenty.  The foam needs to be covered.  Build a wall one inch from the concrete, then add the spray foam and cover with an approved material, such as drywall.  Neither closed cell spray foam or rigid insulation are supposed to absorb moisture, though I have heard instances where XPS has.

Closed cell spray foam in a basement wall cavity. 

This covers the basic strategies for masonry foundation insulation.  Two foundations that would be very difficult to insulate are a rubble or stone foundation or a structural brick (which I have never seen).  There are other foundations that are required to be insulated. Pressure treated wood and frost protected shallow foundations are a couple that come to mind.  As I said earlier, there is not one strategy that will apply to all foundation insulation.  Be sure to follow any of your local codes.

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