Moisture Control for Crawlspace

Foundation Assemblies : Crawlspaces

A crawl space or crawlspace is an unoccupied, unfinished, narrow space within a building, between the ground and the first (or ground) floor. The crawl space is so named because there is typically only enough room to crawl rather than stand. [1]

In Residential Construction there are three foundation assemblies:

  1. Foundation slabs

  2. Crawlspaces

  3. Basements

In this topic, we are focusing on crawlspaces and its varying fundamental approaches to construction and use of moisture control.

History behind crawlspaces

Crawlspaces became popular in the 20th century US residential homes as it was a cost-effective alternative to basements in areas where the climate or ground conditions made basements impractical or unnecessary.

Historically, crawlspaces provided a convenient space for the installation of plumbing, electrical systems, and HVAC components, keeping these elements protected yet accessible for maintenance. They also elevated the home off the ground, which helped prevent issues related to moisture and pests, particularly in damp or termite-prone regions.

The choice of a crawlspace over a basement or a slab foundation typically depends on regional practices, climate, and economic factors prevalent at the time of construction.

Traditional crawlspaces often have simple ductless vents that connect the outside environment to the inside crawlspace by a “grill” barrier to prevent animals from getting in. This causes the environment within the crawlspace to be non-conditioned. [2]

Problems with Traditional crawlspaces

Traditional crawlspaces allow air circulation underneath houses. Heat from residents and heating systems warms the subfloor, keeping the floor framing dry and warm. However, this is not efficient because existing insulation is often inadequate and the occupant’s will often pay more in energy costs.

Even by increasing the insulation in the walls and subfloor, we are making the crawlspace more susceptible to increasing vapor pressure, moisture content, and leaving the non-conditioned crawlspace vulnerable to rot.

Signs of moisture problems include mold growth, musty odors, condensation, wood rot, and pest infestations.

By performing regular inspections to detect any of these signs, we can then find solutions to prevent a future disaster from occurring.

two types of crawlspaces to consider

As a homeowner, what do you want out of your existing crawlspace?

There are two main solutions to updating your crawlspace:

  1. Keep the crawlspace vented and non-conditioned (air exchange is flowing freely from the crawlspace and its outside environment).

  2. Convert crawlspace as “not vented” and conditioned (air exchange is connected internally between crawlspace and the house or by dehumidification).

The most cost-effective improvement is number 1. If you see the crawlspace for it’s utility use, such as easy access for future plumbing and electrical work. You can save money by keeping your crawlspace vented and non-conditioned.

The second option, by converting your crawlspace into a conditioned space, you are thus controlling its temperature and relative humidity. This will allow you to stow away luggage, equipment, and any belonging you have without needing to worry about mold growth.

preventive measures

The number one issue to divert groundwater away from the home.

  • Proper exterior drainage systems (gutters, downspouts, grading).

    • Are the downspouts that connect to your gutters are properly connected to underground pipes that lead the flow of rainwater away from your home?

    • If the ends of the downspouts are exposed, is there a solid concrete padding that is sloped so that the run-off can lead away from the house and not directly to its foundation?

  • Install a 10 mm vapor barrier inside the crawlspace to fully encapsulate the ground floor from the crawlspace.

  • Insulate the floor joists with insulation recommended by local code and then cover with rigid board plus a protective board to prevent pests and fire.

Vented Crawlspace Detail


vented crawlspace improvement by viking wood

Insulate the Subfloor

Here we added R19 insulation in the floor joists in the crawlspace area.

Rigid insulation boards were then nailed below.

10 mm Vapor Barrier over Ground Floor

Once the rigid insulation boards were all nailed up and sealed, the vapor barrier was then laid out and taped.

Footnotes:

  1. “Crawl space,” Wikipedia. Last modified April 28th 2024. https://en.wikipedia.org/wiki/Crawl_space

  2. Joseph W. Lstiburek. “Moisture Control for Residential Buildings,” Building Science Press. 137.