By adam
September 18, 2015   Comments Off on Bare-Minimum-Sealed-Crawlspace

Bare Minimum Sealed Crawlspace

9 steps: Stop moisture intrusion. Air-seal the floor-plane. Install your smooth, even, Vapor Barrier. Plug and air-seal foundation vents, exterior leaks, and crawlspace door. Install your temperature and humidity monitor. Install an adjustable drying mechanism. Learn to use DPCALC or one of our charts; there is one below.

Note: If you have old or poorly built duct-work in your crawlspace then the energy being lost via the duct-work might alone be enough to keep the crawlspace out of the “danger zone” without an intentional Drying Mechanism such as those pictured on our site with 1, 2 or 3 flaps open / screwed shut.

Also Note: Sometimes replacing your old duct-work eliminates leakage and thermal loss that has been keeping the crawlspace air dry where it would have otherwise been very wet. If there is a little bit of moisture getting into a crawlspace, some leaky duct work can easily cover that. So warning, if you replace your old leaky ducts you might want to do a really good job with the vapor barrier and lots of air-sealing work to combat potential problems.

– Vented crawlspaces are often have “operable vents” and people may open and close them seasonally. We often test/open¬† the operable vents before we go crawling around, as they provide valuable light for inspecting.

– Sometimes a sealed crawlspace is a band-aid for the symptoms of bad water-proofing or design. There should really be nothing but dry dirt inside a crawlspace. Mud is not a building material. Mud should not be tolerated in crawlspaces or under the footprint of a building.

– There is nothing stopping residents from plugging their vents, but residents should learn to use a temperature and humidity monitor and understand our charts about humidity, before closing their vents.

– Here at Stetten Home Services we tend to believe that any crawlspace without a Temperature and Humidity Monitor is a bad idea. For about $50 dollars you can own an Accu-Rite Wireless Indoor & Outdoor Temperature and Relative Humidity Monitor with interior back-lit display. The “outdoor” readings would thus actually be “crawlspace readings”. All crawlspaces need a sensor and the best place to put it is not in the center, not along the perimeter, but somewhere between to get an “average” reading. The elevation of the remote sensor should be just below the floor system or a little higher. We want to know if the wooden structure is wet, the “highest strata”. You also want the device to be easy to find given it requires annual batteries. Flag it or make a sign like we do if your are DIY.

– A clean, navigable, well-lit crawlspace is crucial. If there is liquid moisture or wet-earth, get the moisture out, even if it means shoveling the dirt into bins and hauling it away. The resulting surface of only compacted or virgin soils is ideal for conveyance of moisture to a low-point drain or sump-pump. The main purpose for an interior drain or even the vapor barrier itself is to convey liquid from pipe burst or other leaks to the low-point drain, without wetting the dry soils, in the crawlspace. For this reason, special effort has to be made so that the seams overlap properly, yes, your vapor barrier should be properly shingled unless it is taped, even in a vented crawlspace. Building code mandates crawlspace vents unless a sealed crawlspace is installed, but even the vented crawlspaces often have “operable” vents that can be opened or closed. For this reason, we see no problem with plugging the vents with Insulated Plugs that can be taken out easily to re-ventilate with outside air again. If there is duct-work in the crawlspace, it may be leaking enough air down there to keep the space dry. Our question is: Why not have a dry, comfortable, accessible, well-lit crawlspace?

The above photo depicts our STANDARD VAPOR BARRIER

Given the above photo, what difference would simply plugging the vents cause?  We would rather not allow ambient humidity and dust into the crawlspace.

The above photo depicts our SEALED VAPOR BARRIER

So, when asked if a crawlspace is “right” or “wrong”, I often ask a series of questions, and run into a dead end. Generally nobody has done the most obvious thing; Install a Humidity Monitor. Rather than a fancy data logger, our monitors allow the user to see the real time crawlspace conditions from the inside. So to answer this question; “Is my crawlspace done right? or “Is my sealed-crawlspace done right?” I answer, with questions.

Is your crawlspace dry?
Is your crawlspace more comfortable than the ambient environment? (On average of course)
Is your crawlspace clean, well lit, and easily navigable?

With a little effort we can exclude the ambient humidity and temperature swings from affecting your home’s temperature, moisture levels, and energy consumption.

Vented crawlspaces are often more humid than the ambient conditions in the summer, and they can tend to lag behind on winter days when the ambient temps rise. Closing vents can offer lots of benefits, but if water is getting inside and getting trapped in the “island soils” then with a sealed crawlspace this moisture will get trapped in the crawlspace air, and you can have a horrible situation. Knowledge is power. A monitor is a cheap way to quantify the problem. You do not want to install a dehumidifier or use the existing HVAC equipment to dry out the ground or the city you live in.


In this crawlspace just one third of the drying potential is needed to keep the crawlspace well within the target zone.
In this crawlspace just one third of the drying potential is needed to keep the crawlspace well within the target zone.

Of course none of these devices are specifically made for crawlspaces, so “outdoor” = “crawlspace”.

It will become really useful to either print and retain this chart or commit the numbers to memory if you will be sealing your vents and taking on that responsibility.
Basic Crawlspace True Humidity Chart

One of the most important aspects of a sealed crawlspace, especially if your crawlspace had/has “topical biologicals” is floor system air-sealing.

To learn more about our crawlspace lighting, see the following post: Lights, Camera, Action

Floor System Insulation Strategies – There are many different options, learn more. Even in sealed crawlspaces,
sometimes the best place for insulation is against the floor of the home.