Should I Insulate My Basement (We Answered How…Here’s The Why)?

by Erik North on October 8, 2011

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I realized while writing a radon post last week that I’ve addressed in some detail ‘How’ to do certain insulation projects, but not why.

I mean other than insulation as an unalterable good like U.S., apple pie and being able to DVR Red Sox games. Which for energy auditors, it is. But seriously…should I insulate my basement? The answer is a strong but very qualified yes.

Should I Insulate My Basement – The Strong Yes Part

I’ve covered basements in several previous articles, marching forward with the answers to a question that was never asked. Why should you insulate your basement? Because it loses a huge amount of heat and provides a great payback and return on investment for a larger project. (I like to differentiate between small projects which may impact heating bills but may have less effect on comfort and larger projects which can greatly change a home’s heat dynamics.)

You should insulate your basement for the simplest reason: your basement loses a ton of heat and costs you a lot of money. Consider the thermal dynamics for a minute…start considering, I’ll wait *starts whistling*. Suppose the outside air temperature is 10 F, the ground temp below the frost line is 50 to 55 F and the temp in your cozy, conditioned house is 70 F. Heat energy always moves from higher to lower (unless you live in Los Angeles) and heat moves faster when the temperature difference across a material is greater. Following the heat’s likely path:

  • The conditioned 70 F living space will radiate heat into the basement.
  • The basement (which between the boiler, hot water tank, hot water pipes and heat radiating from the floor is probably around 55 – 58 F) will radiate heat slowly into the below-grade soil.
  • The basement heat will move very aggressively through the above-grade concrete/brick/rubble foundation wall toward the 10 F outside air.

Where is your foundation/basement losing the most heat? Probably through the outside edge of your floor and out the above grade section of the foundation wall. And as an added bonus, this is probably where your uninsulated hot water pipes and baseboard heating elements run. Hooray!

My Basement is Losing How Much Heat?

A lot. The building enclosure heat leaves pretty aggressively through the foundation wall and guess what? Concrete, brick and rubble are just about the poorest insulating materials possible. 8″ of concrete has an R-value of 1, meaning your foundation has the same approximate heat retention as a single-paned window. Would we be having this conversation if when you walked into your basement, you were confronted with 150 feet of glass? Probably not, you’d have already called an insulation contractor.

It’s a conundrum of construction. Materials which make good foundations need a very high compressive strength, like concrete (nerdy aside: compressive strength is measured in megaPascals, which sounds like a superhero version of a normal Pascal!). But high compressive strength materials tend to be very dense, meaning they’re good conductors aka really bad insulators. This isn’t universally true (aerogels both have high compressive strength and are a fantastic insulator), it is certainly the case in your foundation walls.

That’s why you should insulate: because you’re losing a boatload of money.

Shouldn’t I Insulate My Basement – The Qualified Yes Part

Are there any reasons you shouldn’t insulate? I’ll say up front, there are a bunch of considerations. But none of them are worth subsidizing OPEC for.

Shouldn’t I Insulate My Basement – Moisture

Moisture is a big issue in most basements. Here’s my handy moisture list I hauled over from a previous post:

  • Your concrete foundation contains a few thousand pounds of water (if freshly poured, make that a few tons of water).
  • The traditional approach to basement water/damp proofing was to apply an asphalt exterior coating and allow the concrete to dry to the interior.
  • Rain and groundwater diffuses into the concrete (or if there are cracks, it just flows right in).
  • Water moves up the foundation wall through capillary action (and if there isn’t a capillary break between the foundation and sill plate, up into the building walls).
  • Lastly, interior moisture can condense on the portion near or above grade. At last some good news.

Head over to the same article on ‘Insulating Your Basements’ for information about the moisture management necessary for basement insulation.

Shouldn’t I Insulate My Basement – Freezing Damage

Another factor is the potential for freezing damage. Concrete, rubble mortar and brick, especially older brick, can be susceptible to damage in the freeze-thaw cycle. If you insulate the foundation, the outside will be colder. Not too radical a statement. And the less warmth/heat energy, the more likely that any present moisture will freeze.

As benign a question as this may seem, it is an extraordinarily complex one. It depends on the type of foundations as brick or rubble and mortar foundations are more susceptible than concrete ones, the presence of gutters, if the gutters move the water far enough away from the foundation, whether the surrounding soil is saturated with water, the drainage potential of the soil, the presence of french drains, the surface drainage and whether there is exterior damp-proofing (usually a tar coating). Whew…it’s pretty complicated.

The very short answer: If the water is controlled and your foundation is dry, there shouldn’t be a problem insulating. No water, no water to freeze and cause damage.

Longer Answer: Here are a few links to read some extended technical discussions on the issue.

From Building Science, an article on the possibility of freezing damage with bricks (the same logic/physics applies to mortar and concrete): Thick As a Brick

From Green Building Advisor, a simple question about water and buildings which lead to a very long comments discussion on the issue: Water – The Wonder and The Danger

Shouldn’t I Insulate My Basement – Radon And Other Contaminants

Finally, basement spaces sometimes have gas contamination issues like radon or VOCs. These can be exacerbated by insulation work (for example: applying closed cell spray foam on the basement ceiling can reduce air flow and increase gas concentrations). In this case, the first step would be testing for contaminants and if there’s an issue, address the problem such as a radon mitigation before insulating.

Basements deal with numerous conflicting energy and material dynamics. Moisture, water flows, chemicals, natural differences and seasonal temp fluctuations all affect the foundation. Ultimately, the enormous savings potential in insulating your foundation makes it too big an opportunity to pass up.

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