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Foundations

What is the right way to insulate a basement? Why?

Basements have a reputation for being cold damp places where we put junk and let it accumulate. Lately though, more and more people are finishing basements into living spaces. To make that more viable, builders are applying spray on waterproofing membranes and exterior drainage planes with gravel backfill on the outside of a foundation, and they are applying stud walls and insulation on the inside of foundations. That all sounds well and good, but any attempt to “waterproof’ an object is likely to fail. Ships need bilge pumps. Sometimes the ‘drain to daylight’ is a dream. Sometimes you are faced with a spring in the corner of your customer’s desired site.

Water is a material we all must deal with, and it is one of the most difficult. Water can be ice, water, or vapor. Sometimes it can be all three in the same place on the same day. Bummer! The best way to deal with water is to let it flow. You cannot stop the flow; you can direct it, using drainage strategies, pumps, and pipes. You can take advantage of the phase changes to direct it by allowing a shift from liquid to gas then allow the gas to escape. This is something Architects and Engineers spend hours thinking about, and design accordingly, but occasionally a builder or occupant will do a small change that totally trashes the entire concept the Architect or Engineer developed. Make sure you think about all the consequences of changing a design element on drainage before you make any change. Can you add that driveway space without destroying a drain path the guttering system was designed to work with? Will terminating that gutter there result in massive water intrusion when the 200 square of roof funnels down to that 4 inch downspout? Is the risk worth the cosmetic gain? Can you make that potential problem evident to the owner who is asking for this change?

In cases where exterior water can be controlled, or adequate drying can happen on the outside of the foundation, it is best to insulate basement walls with foam. Applications of rigid foam and spray foam have good success, with spray foam being slightly better because it is fully adhered, so no moisture can condense on the cement behind the foam. In these applications R values of 10 to 14 are common and effective. Fiberglass and cellulose should be avoided in basements because there are too many paths for water to get between the cement or block and the insulation, and adding porous insulation will just amplify this problem.

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