Attic & Crawlspace Mold

Attic and crawl space mold can be the most difficult and costliest of mold removal projects due to space restrictions and the time and tools it takes to properly remove mold from these areas (basement mold remediation is often no picnic either, especially after heavy flooding).

Making matters worse, attics and crawl spaces are also highly susceptible to mold growth because moisture can easily accumulate in these spaces, often unbeknownst to the homeowner since very little time is spent in these areas of the home. If you want to avoid a costly mold remediation project in your attic or crawl space, consider the following:


Most attic mold problems arise from:

  • Roof leaks;
  • Improper ventilation; and/or
  • Improper venting from pipes and/or vents.

Let’s take a look at each one of these culprits individually for preventing attic mold.

Roof Leaks

Below are a few ways to check for possible roof leaks:

  • Check for discoloration of insulation and wood (e.g. rafters, sheathing, joists, attic side of fascia boards, etc.).
  • Check roof valleys (i.e. where two roofs join at an angle), which are highly susceptible to roof leaks.
  • Observe skylights, chimneys, attic windows and any portion of the attic/roof where dissimilar materials join each other (including flashings). These places are hotbeds for potential moisture intrusion.
  • If you have a vapor barrier installed, check for condensation. Although this is not really a roof leak, it is nevertheless a sign of a moisture problem. And moisture problems lead to attic mold problems!
  • Make sure there are no leaks coming from and around attic plumbing stacks.


  • If you have a vapor barrier installed (such as polyethylene plastic) and your vents are close to the roof then you need 1 square foot of ventilation for every 300 square feet of attic floor space. Otherwise, 1 square foot per 150 feet square feet is needed.
  • Air travels up to the attic. So activities such as cooking, bathing, showering, etc will produce moisture that will make it way up to the attic. If there is poor ventilation, the moisture gets trapped in the attic and can lead mold problems.
  • Do not cover up your vents with insulation!!! We see this all the time and it is a surefire way to encourage moisture buildup and attic mold growth.


Plumbing stacks, dryer vents, vents from bathroom fans and kitchens should NEVER terminate in the attic. If they do, condensation can easily form (or in the case of plumbing stacks, hazardous gases can amass if venting terminates in the attic).

Crawl spaces

Crawl spaces are as vulnerable to mold growth as any part of your home due to the high moisture and humidity levels often found in these spaces. We’ve read studies that indicate that an estimated whopping 60% of homes have crawl space mold!

Why is moisture accumulation so common in crawl spaces?

Consider the location of the crawl space: it sits underneath your home right on top of the earth as your home’s foundation. Soil and dirt from the earth contain moisture. And moisture is one of two requirements for mold growth (the other being a food source such as the wood found in crawl space subflooring). And since moisture forms so easily in confined areas, dirt crawl spaces are regular mold breeding grounds.

Moisture from dirt normally will evaporate into the atmosphere but can get trapped in a closed area like a crawl space. When moisture is not vented to the outside, it can start to condense on foundation walls or subflooring, which can be especially a problem if the subflooring is wood (wood is a food source for mold).

Two other points to keep in mind about crawl spaces:

  • Air travels from bottom to top. Thus, any moisture formed in crawl spaces will be carried by the moist air upwards to your living spaces naturally (or mechanically through the HVAC system), which can lead to mold growth beyond just the crawl space.
  • You do not need standing water in crawl spaces to trigger mold growth. High humidity levels (above 60%) are enough to cause crawl space mold growth, And high humidity levels are very common in crawl spaces even in the absence of flooding!

What can be done to limit moisture build-up and crawlspace mold?

  • Check the foundation walls and make sure there are no cracks where water can enter. Along these lines, check that there are no holes or cracks in the ceiling of your crawl space. These holes can be a gateway for mold and water transport from the crawl space to your living area.
  • Make sure gutters and downspouts channel away from the house. Otherwise, water can accumulate near the house and eventually find its way into your crawl space.
  • Check for adequate ventilation. At a minimum, there should be 1 square foot of ventilation for every 150 square feet of crawl space area. (Many building professionals now advocate closing vents and controlling moisture through other means. They argue that open vents makes it difficult to keep crawl spaces warm in winter and can lead to increase in moisture build up in the summer when the hot humid air is brought into the crawl space. These are good points but it can be challenging to control moisture through other means without the use of vents. In addition, once mold starts to grow, it is going to spread much easier in closed spaces without vents relative to closed areas with vents, all other things being equal. The outside fresh air coming from the vents can also help dry out a structure and dilute/reduce the concentrations of mold in the air once the outbreak occurs.)
  • Vented crawlspaces insulate tightly against the subfloor. Secure the insulation snugly with mechanical fasteners. Do not just stuff the insulation up in the subfloor between the joists and hope it stays put. It will likely fall out eventually without fasteners. The vapor barrier portion of the insulation (e.g. the paper side) should face the heated source, which typically means upward against the subfloor.
  • Make sure existing insulation is not missing or sagging, both of which can suggest you have a water problem.
  • Make sure plastic sheeting covers the entire dirt crawl space and goes up a few inches alongside the foundation walls. There should be plenty of overlap between the layers of sheeting and the sheeting should be absent of any standing water. If done correctly, plastic sheeting will serve as an important vapor barrier in your crawl space.
  • Look for leaks around HVAC ducts and plumbing components.
  • Dryer vents should also terminate to the outside of the house and NOT into the crawl space.

Safety First

Follow the tips and tricks above to prevent attic mold and crawl space mold but please be extra careful! The crawl space and attic present a number of potential safety hazards that can cause serious bodily harm and even death to you and your loved ones. We are not trying to be dramatic but the potential dangers are real, especially if you do not know what you are doing! (See Attic Dangers and Crawl Space Dangers for more info.)

If you lack experience working in attics or crawl spaces or are hesitant to do so for any reason, hire Summit Environmental Solutions because it’s simply not worth risking your well being to save a few bucks.

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