Knowledge Builders

should a crawl space be vented or not

by Prof. Brant Grady Published 8 months ago Updated 2 months ago
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Building codes generally require working vents in the crawlspace to allow outside air to circulate under the floor in the summer to prevent moisture buildup that, among mildew and mold, encourages wood rot.Apr 13, 2017

Full Answer

Should you vent your crawl space?

The crawl space must be also ventilated during the winter. The idea is to prevent moisture and other harmful particle condensation. These destructive and invisible molecules can accumulate in unventilated spaces and are a serious health threat that may be very dangerous to the occupants of the building.

Why to vent a crawl space?

A crawl space vent well is only going to keep out already-condensed moisture. It’s not going to do anything about the new moisture this process is constantly creating. Humidity Is Going to Come In . If you have crawl space vents, even the best crawl space vents out there, they’re going to let in humidity. After all, that’s sort of the point.

Does my crawl space need venting?

The absolute best way to protect your home, your crawl space, and your air quality is through encapsulation. But if your crawl space has no encapsulation, or only has a vapor barrier, you should have open vents. Vapors, gases, and air continuously evaporate out of the ground; these need a way to vent out of the crawl space.

Do I need to vent my crawl space?

The absolute best way to protect your home, your crawl space, and your air quality is through encapsulation. But if your crawl space has no encapsulation, or only has a vapor barrier, you should have open vents. Vapors, gases, and air continuously evaporate out of the ground; these need a way to vent out of the crawl space.

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Which is better vented or unvented crawl space?

The principal perceived advantage of a vented crawl space over an unvented one is that venting may limit radon and moisture-related decay hazards by diluting the crawl space air. Additionally, providing a vented crawl space may make sense in flood-prone areas such as coastal zones subject to hurricanes.

Should crawl spaces be vented in winter?

When temperatures drop below the freezing point, water pipes in the crawl space are bound to freeze if the vents remain open. It's a good practice to close off the vents in winter. Doing so prevents the dry, cold winter air from freezing the pipes inside the crawl space.

What is the best way to ventilate a crawl space?

Airing out your Crawl SpaceTransfer a small supply of heated air from the HVAC system into the crawl space.Use an exhaust fan to move air from the crawl space to the outside.Install an air intake or dehumidifier system.

How many vents do I need in my crawl space?

Most building codes require 1 square foot of open ventilation area for every 150 square feet of crawlspace. Generally, Automatic Foundation Vents have 50 inches of net free area per vent. Therefore, install one vent for every 50 square feet of crawlspace.

Should crawl space vents be open or closed in the summer?

Sealing your crawl space vents is the most sensible thing to do especially in summer when the outside air is dense with moisture. While it may take years for people to agree unanimously to close their crawl space vents, the facts remain. An open crawl space vent is bad news for your home.

When should crawl space vents be closed?

winterThese vents allow outside air to circulate under the floor in summer to prevent the moisture buildup that encourages mildew and rot. In winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawl space might freeze.

What are the cons of encapsulating a crawl space?

Cons of Crawl Space EncapsulationUpfront Investment. According to Home Advisor, the average homeowner can expect to pay $5,500 to install this system. ... Cost of Foundation Wall Insulation. ... Additional Maintenance. ... Might Need To Upgrade Your HVAC System.

Can you put a dehumidifier in a vented crawl space?

The idea is correct, except the vents in the crawl space need to be closed off first. If you place a dehumidifier in your crawl space without sealing the crawl space vents, you are essentially trying to dehumidify all air inside and outside of the crawl space, which is impossible.

How do you rodent proof crawl space vents?

The one and only tried and true way of keeping your crawlspace clear of unwanted critters permanently is to seal up the vents like Fort Knox. We recommend using premade expanded metal mesh grates. The mesh in these grates is made from an actual metal plate that is slotted then stretched to create a strong mesh.

Is a vapor barrier necessary in a crawl space?

Yes. You need a vapor barrier in your crawl space. What's more, a moisture barrier is the minimum amount of protection that you should have in your crawl space. A vapor barrier helps you to avoid expensive repairs.

How much ventilation does under a house need?

The minimum net area of ventilation openings shall be not less than 1 square foot for each 150 square feet of under-floor space area, unless the ground surface is covered by Class 1 vapor retarder material.

Will a fan help with humidity in crawl space?

However, it's important to note that putting a fan in your crawl space definitely won't fix moisture and humidity problems. In fact, many vent fans actually allow external air into the crawl space, which often leads to added moisture and even worse problems.

What temperature do you close crawl space vents?

40 degreesAutomatic crawl space vents close when it is 40 degrees or colder to lower the chances of freezing pipes; they open when it is warmer.

How do you insulate a crawl space vent for winter?

1:012:38Best Materials to Seal Crawl Space Vents | Foam Board and Spray FoamYouTubeStart of suggested clipEnd of suggested clipThe vent with an r10 foam board not only are you creating that thermal barrier you're also sealingMoreThe vent with an r10 foam board not only are you creating that thermal barrier you're also sealing around the foam board with our one component spray foam that's also available on our diy.

Should vents be open or closed in winter?

Air vents should be open to prevent damage Increased air pressure can create cracks and holes in the ductwork. It can also cause your furnace's heat exchanger to crack. Leave all air vents open to prevent extensive damage to your home's heating system this winter.

Should crawl space have vapor barrier?

Yes. You need a vapor barrier in your crawl space. What's more, a moisture barrier is the minimum amount of protection that you should have in your crawl space. A vapor barrier helps you to avoid expensive repairs.

Why not vent a crawl space?

Choosing not to ventilate a crawlspace with outdoor air has two major advantages: energy savings and control and prevention of moisture problems. Compared with the minimum ventilation allowed by code, the energy savings from not installing crawlspace vents may be small in a well-built and well-sealed home.

What is the problem with enclosed crawlspaces?

Enclosed crawlspaces are usually contained by a solid concrete or masonry foundation wall that extends down to footings just below the frost line. Porous materials such as wood and insulation can absorb a lot of water. And when these materials get wet, they tend to stay wet.

What is a crawlspace louver?

Thermostatically controlled automatic louvers and manually operated louvers are available that can close off the crawlspace vents when the temperature drops below freezing. Although louvers allow crawlspace ventilation without incurring large energy losses, it is unlikely that such devices can be kept in good working order for the life of the home, and indeed, there are many reports of failures. If the devices fail in the open position, they are useless; if they fail in the closed position, the owner will not have ventilation when it's needed. Unless a relatively foolproof device is available, or high levels of maintenance are expected, build an unvented crawlspace that is mechanically conditioned instead.

How does a crawl space affect the air quality?

If the floor structure above the crawlspace is uninsulated, heating and cooling losses will result. Gaps in the flooring will also admit moisture from the crawlspace below , which will affect indoor air quality. Careful design and construction to mitigate the entrance of moisture and condition the space is your best line of defense.

What is the best way to insulate a crawl space?

When insulating, provide access strips to allow inspection of the wall plate for termites and wetness. Rigid insulation rated for crawlspace exposure tightly adhered to the foundation is better than insulation that creates hidden void spaces where mildew can grow.

Why do you need to seal a crawl space?

By far the most important reason for sealing a crawlspace is to avoid condensation in humid climates. This condition can arise in cold climates, but it may occur in almost any humid climate if the temperature of the crawlspace floor falls below the dewpoint of the outside air. During the winter, the ground under the home gets cold. A sudden warming spell in the spring will send in warm, humid air, which will condense on the cold ground.

Does a gap in a floor affect air quality?

Gaps in the flooring will also admit moisture from the crawlspace below, which will affect indoor air quality. Careful design and construction to mitigate the entrance of moisture and condition the space is your best line of defense.

Why do crawl space vents need to be surrounded by wells?

Perhaps they are surrounded by wells to avoid water flowing directly into them . The vents are meant to ventilate the crawl space in order to prevent mold growth .

When should crawl space vents be closed?

The traditional line of thinking is – the vents should be open in the summer and closed in the winter . But there’s a newer approach that says the vents should be sealed shut at all times.

What is stack effect in crawl space?

The “ stack effect ” draws the air from your crawl space into your home.

How much savings can a sealed crawl space give?

Along with insulation, a sealed crawl space can give the homeowner around 18% in savings for heating and cooling costs.

Why is crawl space important?

The crawl space is a deceptively important section of your house. The fact that it can be damp and unsightly leads a lot of people to avoid crawl space maintenance until it becomes a major issue. However, the crawl space affects the quality of life inside your home every day.

What percentage of air in a house comes from a crawl space?

Here’s a quick snapshot of why you should pay attention to the conditions down there: 40% of the air inside your home comes directly from your crawl space. So if your crawl space is musty, dirty, and contaminated with mold — so is your home.

What is the first step in a closed water system?

The first step to a closed system is dealing with any standing water issues. Crawl spaces are occasionally dug below the water line and are apt to flooding when heavy rains hit. Proper drainage is essential to eliminating this moisture.

What was the purpose of foundation vents in a house?

The thinking was that the vents allowed air to circulate under the house, keeping it dry.

How hot does an air vent need to be?

Automatic vents are less fuss. Air Vent makes ones that open at about 70 degrees F, close about 40, and don’t require any electricity.

What happens when you put moisture in a vent?

However, what happens in many locations is that moist outside air coming through the vents condensed or collected, causing corrosion, mold, mildew, and rot.

Do unvented crawl spaces stay drier than vented crawl spaces?

The report concluded that closed, unvented crawl spaces stay considerably drier than vented ones, regardless of the climates. On the other hand, some of the energy-use results were surprising.

Do foundation vents need to be changed?

A debate ensued, and because some builders are slow to change, many foundation vents continue to be installed. Some building departments are also slow to change and still require foundation vents as part of their building code.

Should we block our crawl-space vents or keep them open?

Tom Silva replies: I think working vents in crawl spaces are a good idea, and so do the building codes, which generally require them. These vents allow outside air to circulate under the floor in summer to prevent the moisture buildup that encourages mildew and rot. In winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawl space might freeze.

Issues With Enclosed Crawl Spaces

Enclosed crawl spaces are typically built with solid concrete or masonry foundation walls. These extend to the structure’s footings, where crawl spaces come into contact with porous materials, like wood or insulation, that trap water when they get wet.

The Assumed Benefits of Ventilating a Crawl Space

Crawl space ventilation allows air to circulate under the floor, potentially preventing the buildup of moisture. However, ventilating a crawl space can increase a home’s energy bills while not keeping it as dry as many believe.

To Vent or Not to Vent

Research conducted by Advanced Energy, a global energy company, yielded surprising results about the effects of ventilating a crawl space. They monitored a modular housing development in Baton Rouge, LA, which is hot and humid for most of the year. They also monitored 12 stick-framed homes in the colder, drier climate of Flagstaff, AZ.

Crawl Space Encapsulation

Because a significant amount of air exchange takes place in crawl spaces, having mold or mildew in this part of the home can seriously affect indoor air quality. Encapsulating a crawl space in a polyethylene vapor barrier can drastically reduce moisture infiltration without relying on expensive ventilation options.

How many square feet of vents should be in a crawl space?

There are standard requirements for ventilating crawl spaces, one including the types of crawl space vents that can be used. Each vent should be at least one square foot and made up of certain materials such as:

Why seal a crawl space vent?

Some homeowners prefer to seal their crawl space vents throughout the year to allow heat and air conditioners from their HVAC system to seep in. If you plan to go with this option, it is best to be sure about it beforehand.

What is the most destructive element in crawl space?

For crawl spaces, the most destructive element is moisture. Moisture bears responsibility for multiple problems that occur in a home such as different mold and mildew strains, and wood rot.

What happens if you keep your vents open?

If you have a dirt floor crawl space and keep your vents open throughout the year, the floor will become a never-ending source of moisture. Even if the surface of the dirt looks dry, a few inches down the Earth is moist.

What happens if you mold your crawl space?

If your crawl space becomes the victim of mold, you’ll be risking irreplaceable home damage and a dangerous home environment. You can diminish this risk by optimizing your crawl space ventilation.

Why do foundations have crawl spaces?

A foundation with a crawl space in it is more common in warm, moist climates. The advantage of raising the structure includes avoiding moisture.

Why is it important to have a positive surface drain?

Although less likely, it is also possible for standing water to foster deadly microorganism growth. This is why having positive surface draining is so important.

Why is it important to ventilate a crawl space?

Because it is difficult to provide light and heat to a crawl space, promoting good airflow is the best solution. To avoid damage from moisture that might rot away beams, joists, and flooring, building codes have long required that crawl spaces be properly vented so that good airflow can help keep these spaces dry.

How many square feet of vent space is required for a crawl space?

Translated, this code requirement means that you need 1 square foot of screened vent space penetrating the perimeter foundation for every 150 square feet of space in the crawl space.

How many square feet of underfloor space is required for a ventilation system?

The minimum net area of ventilation openings shall be not less than 1 square foot for each 150 square feet of under-floor space area , unless the ground surface is covered by Class 1 vapor retarder material. Where a Class 1 vapor retarder material is used, the minimum net area of ventilation openings shall be not less than 1 square foot for each 1,500 square feet of under-floor space area. One such ventilating opening shall be within 3 feet of each corner of the building.

What section of IRC is venting crawl space?

The IRC prescriptions for venting crawl spaces are contained in section R408, in several paragraphs. The first paragraph of IRC section 408 provides the standard requirement for ventilating crawl spaces:

What are the vents on the side of the house?

These are crawl space ventilation vents, and they are crucial to the proper functioning of your house. Your comfort—and your home's long-term maintenance —hinges on how well your crawl space is ventilated.

What is crawl space floor sealed with?

The crawl space floor area must be completely sealed with a vapor-retarding material.

What is a crawl space foundation?

A crawl space type of foundation is common in warm, moist climates where it is advantageous to raise the structure slightly off the ground to avoid moisture. Crawl spaces are also sometimes found in conjunction with basement foundations where a portion of the house—such as a porch—is separate from the main structure and has an independent foundation.

What is required for an unvented crawl space?

To work well, an unvented crawlspace needs proper drainage, air-sealing, insulation, and hvac tie-ins. The insulation can go either inside or outside the walls. The building code in most areas call for R-15 to R-19 on crawlspace walls.

How to reduce moisture load in crawl space?

This starts with grading around the foundation, which should slope away from the building at a minimum of ½ in. per foot. This carries water from rain and snow melt away from the building. If it’s not possible to slope the soil away from the building, you can have a landscaper create a subsurface drainage system.

What is the best insulation for a crawl space?

The most common, and cheapest, crawlspace insulation is fiberglass batts, which tend to collect moisture and provide housing for mice and other critters.

What is the best way to cover a crawl space?

Ground Cover. No matter what approach you use in a crawlspace, vented or non-vented, it is critical to cover the ground with a durable moisture barrier. Even ground that appears dry can generate a lot of water vapor by evaporation through the soil.

Where are crawl spaces popular?

Crawlspaces are a regional building practice, popular mainly in the Southeast and the West Coast. They are less expensive to produce than a full basement and useful where a high water table would make a full basement impractical. Traditional vented crawlspaces are prone to moisture problems in hot, humid climates.

Is a vented crawl space good for hot weather?

A lot of research has been done to determine the causes and cures for damp crawlspaces. The consensus is that a vented crawlspace is not a good building detail for parts of the country with hot, humid weather.

Do you have to ventilate a crawl space?

Once you’ve sealed and insulated the walls, and covered the ground with a durable vapor barrier, the building code (IRC) requires that you either ventilate the crawlspace to the outdoors or heat and cool it with your hvac system.

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