When Should I Open The Vents Under My House?

If you have vents under your home, you have probably wondered whether you should open or close them at particular times. You’re not alone. One of the most popular questions on many home maintenance sites is When should I open the vents under my house?

Google the question, and you’ll have page after page of conflicting advice. Much of it from companies that may have a vested interest in providing you with a particular point of view.

So, When Should I Open The Vents Under My House?

Unfortunately, there isn’t a one-size-fits-all answer to this question. In fact, there isn’t a one-size-fits-the-majority answer either.

For some people, the best advice is that you should open your vents every year in the spring, then close or block them again in the fall. 

However, the average humidity where you live, whether or not you have air conditioning components in your crawlspace, the presence of moisture barriers, and many, many other variables will impact your decision.

To help decide precisely if and when to open your vents, first, you’ll need to know why they are there.

What Is A Crawlspace?

A crawlspace is an area under the home, between the bottom of the building and the ground. The floor of the home itself sits on foundational piers, which, in turn, rest on the rough ground. 

The home’s underside can be accessed from the crawlspace, but you are unable to stand up and walk around. Instead, you have to crawl around under the house, hence the name crawlspace.

On the other hand, a basement is an area under the home where the walls run down to a finished floor space. A basement may be finished or unfinished, but it is generally tall enough for a fully grown adult to stand up and walk about with ease. 

Why Do Some Homes Have A Crawlspace?

So, why are some homes built with a crawlspace instead of a basement?

Crawlspaces Save Money

Crawlspaces reduce the costs of building a home. When you build a house, you can either:

  1. Level the ground, pour a foundation and build.
  2. Place piers of different heights on the ground to enable a level floor, and build.

Option “A” requires more time and resources. As a result, building a home with a crawlspace – option B – is more cost-effective. 

Crawlspaces Are Preferable In Some Climates

A crawlspace minimizes the amount of time the structure of a home is exposed to moisture when building houses in:

  • Coastal areas, 
  • Locations with a high water table, 
  • Potential flood planes, or
  • Other places where the ground can be very wet

Why Do Crawlspaces Have Vents?

To understand why crawl spaces have vents, you need to know how air flows through a building.

The Stack Effect

The stack effect is the name given to how air moves into, through, and out of a building. 

As warm air rises in a structure, it draws in cooler air. The cooler air will find it’s way into the building through the point of least resistance. This is why you feel a draft when you have gaps around window frames.

The Role Of Moisture

Even apparently, dry soil will contain some moisture. As the stack effect draws cooler air into a home, moisture can seep into the area underneath. If this moisture builds-up in a crawlspace, it can eventually cause rot and mold.

The Role Of Vents

Vents provide a route of low resistance for the moisture in the air to escape a crawlspace, thus preventing a build-up. As a result, most building codes include the requirement for vents

To Vent, Or Not To Vent

Now you know why crawlspaces have vents, it’s time to decide whether or not to open and close the vents in your home. To make an informed decision you need to know about the variables that can impact your decision.

Venting Variables

Let’s start by looking at the things you cannot control when deciding whether or not to use your crawlspace vents.

Local Climate

For those living in a particularly humid climate, whether or not to open your vents can be more complicated. 

If the ambient air being drawn into the crawlspace is warm and moist, when it hits colder surfaces on the underside of the home, the moisture can condense. As a result, a build-up of water in the crawlspace is possible.

Air Conditioning

If your home has air-conditioning and the HAVC ducts run through the crawlspace, there is a potential issue.

The cold, conditioned air running through the system will chill your air ducts. When the warmer, more moist air in the crawlspace meets this chilled ductwork, the moisture will condense.

As a result, you can end up with sweating ducts increasing the moisture level or even small pools of water in your crawlspace.

HVAC Upgrades

I a similar way to A/C, a new furnace and/or ductwork can impact the moisture levels in your crawlspace. 

Moisture Barriers

Some crawlspaces have a moisture barrier installed. These range from 6 mils to 20 mils in thickness, and most are made from polythene or other plastics.

Covering the floor with a barrier can reduce the amount of moisture seeping into your crawlspace.

What You Are Storing

The contents of your crawlspace also impact the moisture levels. Space, which has a large number of items, will have less efficient airflow than one which is empty.  Consequently, a cluttered space may increase the likelihood of damp.

Even if you have only one or two items in storage down there, the items themselves may affect your air quality. 

For example, some homeowners store their mower in the crawlspace. If the mower has grass cuttings or other debris, this can increase the moisture levels as well as encourage the growth of certain molds.

The Venting Decision

Any, all, or none of these variables may impact your vent opening decision and timing. As a result, if you are in any doubt about whether to open your vents or not or if you are concerned there is a problem in your crawlspace, consider having the space inspected by a professional.

Not only will this give you peace of mind in the future but, if there are problems, the inspection report should offer you suitable solutions.

When To Open The Vents Under Your House

So, back to the original question!

Assuming there are no reasons for you not to open your vents, you should do it in spring.

Although there is an official calendar date for the start of spring, that doesn’t mean that spring has arrived from a weather perspective. For the purposes of opening your vents, wait until two weeks after the last frost.

Automatic Vents

There are many automatic vents on the market that can take the guesswork out of when to open your vents.

While some require connections to the home’s electricity supply, many do not. You simply remove your old vent and push the new vent into the same space. Job done.

Final Thoughts

If your home has a problem-free crawlspace, you should open the vents in the spring, preferably two weeks after the last frost. This will prevent any issues of cold air, freezing your pipes, or causing other damage in the crawlspace.

If you have moisture issues in your crawlspace, or if you live in a warmer, more humid area of the country, this decision is not so cut and dried.  It may be that you open your vents in the spring, and then close them again when the air begins to get humid.

But, no matter what areas of the country you live in, if you are in any doubt about the air quality of your crawlspace, it is worth getting a professional opinion.


Geoff Southworth is the creator of RealEstateInfoGuide.com, the site that helps new homeowners, investors, and homeowners-to-be successfully navigate the complex world of property ownership. Geoff is a real estate investor of 8 years has had experience as a manager of a debt-free, private real estate equity fund, as well as a Registered Nurse in Emergency Trauma and Cardiac Cath Lab Care. As a result, he has developed a unique “people first, business second” approach to real estate.

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