Should I Insulate My Crawl Space (Yes, it’s A Mini-Basement)?

by Erik North on November 9, 2011

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A sister question to a previous post, should you insulate your basement crawl space? Are there savings and comfort benefits that make such a large project worthwhile?

Through my energy auditor eyes, good insulation ranks right with a nice brown ale – Newcastle, maybe – within a list of the world’s great things. Should you insulate your crawlspace…the answer, like a full basement, is a strong but qualified yes.

Should I Insulate My Crawl Space – The Strong Yes Part

I covered basement insulation in a previous post and concrete crawl spaces are similar but in ways quite different. Why should you insulate your crawl space?

For the same reasons as your basement: it loses a huge amount of heat, provides a great payback and return on investment for a large project. I’ve joked in the past that crawl spaces have all the drawbacks of basements with none of the storage. Fantastic!

(OK, I am totally copying this section from my basement post and crossing out ‘basement’ and inserting ‘crawl space’) You should insulate your crawl space for the simplest reason: your crawl space 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*.

Should I Insulate My Crawl Space – Where The Heat Goes

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 crawl space (which between the boiler, hot water tank, hot water pipes and heat radiating from the floor is probably around 50 – 55 F) will radiate heat slowly into the below-grade soil.
  • The crawl space heat will move very aggressively through the above-grade concrete/brick/rubble foundation wall toward the 10 F outside air.

Where is your crawl space losing the most heat? Through the outer edge of the building enclosure and out the above grade crawl space wall. And often the uninsulated pipes, hot water tank or heating system live unattended in these spaces.

Should I Insulate My Crawl Space – How Much Heat Am I Losing?

A lot. Probably more than a basement. Crawl spaces lose every bit as much heat as an uninsulated basement … and a little more. Concrete walls are miserable insulators and crawl spaces have greater height allowances – ya know, enough space to crawl. Generally, this means a greater portion of the crawl space walls is above grade, meaning more heat loss. Exciting.

The building enclosure and thermal enclosure in a crawl space is the concrete wall. Heat energy moves most aggressively at the largest temperature difference (in this case through above grade concrete walls). Concrete has almost no insulation capacity, having an R-value of 1 per 8 inches. And, while not universal, most crawl spaces have more above grade concrete than do basements.

What is the difference between a crawl space and a basement? Simply a crawl space is a much shorter basement. Structurally, they are the same with a footer, concrete foundation wall and sill assembly. The space is usually 3-4 feet, allowing access underneath and have dirt floors more often than standard basements. This will be an important point later! They experience the same heat loss and moisture issues as basements with the fun added bonus that problems can linger since no one sees them.

Should I Insulate My Crawl Space – Moisture Concerns

Moisture is a big issue in basements but can be an even bigger one in a crawl space. The issues of freezing damage and soil gases (see this post for the longer discussion on freezing damage and soil gases in the basement) apply equally to crawl spaces but moisture deserves more attention.

First, is the ‘out of sight’ problem alluded to earlier. Basements are more regularly trafficked and any problems such as a leaky foundation crack will be spotted quickly. Homeowners sometimes go years without venturing into their home’s crawl space. If a damaging water incursion persisted, it could go unaddressed for ages.

The prevalence of dirt floors in crawl spaces add another layer to the moisture issues. Open dirt evaporates huge volumes of water vapor into the building enclosure. The elevated humidity levels can cause huge problems when we start mucking around with insulation. For example, an uninsulated crawl space stays north of the dew point, because of heat radiating from the living space. Add fiberglass batts into the crawl space joists and the crawl space temperature could drop to where condensation becomes possible.

The insulation approaches used must account for the present moisture. Homeowners usually implement moisture mitigation (see this article on moisture control for some non-dehumidifier thoughts) and use a water impervious insulation like closed cell spray foam. And, like basements, moisture, water flow, chemicals and material differences affect the crawl space. There are bushels of money radiating from an uninsulated crawl space making it a savvy and green investment.

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