If you live in a house with a crawl space, upgrading it pays off in multiple ways

A properly sealed, moisture-protected, and insulated crawlspace will increase comfort, save on energy costs, improve the durability of the home, and reduce entry of moisture, radon, and other potential irritants or pollutants into the home. (Technology FactSheet: Crawlspace Insulation)

Let’s take those benefits one at a time:

1. Increase comfort: Vented crawlspaces allow drafty cold air to move under your home in the winter, and hot humid air to stagnate under the home in summer.  If your floor is not air-sealed and well-insulated from this air, the same temperatures and humidities will penetrate into your house.  Air sealing this space literally cuts off the bottom end of the stack effect in your home, which is a thermal force that works against your HVAC.

2. Energy cost: Letting this cold or hot and humid air into your home causes your HVAC system to work harder. Air-sealing enables insulation to work better, and truly seal the energy you are putting into your home to heat and cool it, to stay there.

3. Durability of the home: a sealed, insulated crawlspace is less likely to allow mold, rot, and pests like rodents and termites to damage the foundation of your home. Vented crawlspaces often cause homes to rot from the floor up…all that moisture from the ground and surrounding air can cover a home’s floor beams and joists in mold and weaken them, threatening the stability of the home. 

4. Reduce entry of moisture, radon and other pollutants:  We all know that humidity is a food for mold inside the home, and radon is another pollutant that can be siphoned away from the crawlspace, making the home healthier.  Byproducts of animals such as feces, food and carcasses are never healthy to live near!

This video by Crawl Space Ninja is excellent.  It highlights all the steps the company takes to turn a drafty, unhealthy crawlspace into a sealed, clean part of the home’s envelope.  The owner also encourages DIY homeowners to take the same steps using the company’s products and their own labor.  Here are the important steps:

#1: Air Seal the Subfloor: Crawl Space Ninja owner Michael Church recognizes that air sealing the envelope of your home is super-important, and his company does jobs that no one else wants to do–work in nasty, dirty crawlspaces.  They start by removing all the sub-floor insulation and air seal all gaps in the subfloor with foam.  The photo below (min. 1:27) is likely a bathtub water/drain area.  Plumbers like to cut nice big holes to work through…which let in big amounts of outside air!

#2: Air Seal the Rim Joists:  Rim joists sit on the foundation wall and transfer the load from the bottom plate to the sill plate.  If you were to look behind that white foam board in the photo below, you would see a flat board.  Why put insulation up against a board?  Because wood-to-wood joints do not make air seals.  Crawl Space Ninja, in adhering to all the building science evidence of air penetration, is not leaving any gaps unsealed. This includes where the sill plate touches the foundation wall.  (hidden by the white pipe below).  You’ll want to put a line of spray foam where the sill plate touches the foundation wall, all the way around the perimeter of the foundation.

Source: What is a Rim Joist?

#3 Air Seal Vents: These “windows” in your foundation are no longer necessary.  You can leave any nice-looking vents or grilles on the outside so that aethetics are not disturbed, but inside it’s necessary to block the air with custom-cut foam boards and spray foam to block the air currents.  A bonus is that the water pipes under your home will no longer be subject to freezing and bursting.  Also make sure to insulate any crawlspace doors.

#4 Insulate foundation wall AFTER air sealing

Crawl Space Ninja’s founder Michael Church has always recommended wall insulation over floor insulation when they are renovating crawl spaces.  He goes with the theory that placing the thermal break at the same place you make the air break (the perimeter of the foundation) makes the space more thermally efficient.  However, a newer video where he analyzes a study conducted in North Carolina from 2003-4 on three groups of homes shows that in colder climates, floor insulation outperforms wall insulation, but in warmer climates, wall insulation outperforms floor insulation.  Both options may be needed, but Return on Investment should be considered (the energy savings may not cover the cost of installing both).  For Crawl Space Ninja, the wall insulation is an insulation vapor barrier to control vapor as well as heat, because concrete and masonry are porous to vapor.

5.  Air Seal the Ductwork: According to energy.gov, the average home loses 20-30% of its heat through duct losses. If ducts are running through your crawlspace, then air from the crawlspace can be drawn directly into your living space through leaks in ductwork.

#6 (not mentioned in video): Install a dehumidification system

Once you seal up your crawlspace, you’re going to want to place some insurance that moisture will not build up in the space through an unexpected leak like a puncture or missed sealing area, or extreme thermal differences causing condensation.  Crawl Space Ninja has developed their own dehumidifier, but there are other brands of dehumidifiers that can handle the job as well.  These are permanent units that either use a gravity drain or a pump to remove the condensation they generate.  Once you place such a unit in service, it’s important to check it regularly, either manually every few months, or via a remote humidity sensor and app, to make sure all is working well!

As we mentioned earlier, this company recognizes that DIY homeowners can do a similar job, just substituting their own labor.  They have a DIY store with all the air-sealing, insulating, and dehumidification equipment needed to get the same effect, and a blog with plenty of articles on how to do it.    However, mold is a challenge that intimidates many homeowners.  We were particularly impressed when Crawl Space Ninja admitted that the typical DIY homeowner will not have the equipment or expertise to be able to remediate a moldy crawlspace using the company’s method, soda-blasting, but instead recommended a peroxide-based mold cleaner they supply (unfortunately it also contains a “Quat”, which is not so safe).  They also have solid advice concerning mold: it must not only be killed, but also removed.  If insulation is in contact with moldy surfaces, it has to be removed and disposed of. Moisture problems have to be fixed in order to truly get rid of the mold problem. 

At the beginning of the video, a homeowner stated that outside the temperature was at or below freezing, while under his home, the crawlspace was 53 degrees F.  This temperature difference was no doubt due to the air sealing and insulation treatment of the crawlspace, and he is reaping energy savings as compared to having a vented crawlspace.  If you want to go further after taking the previous steps to air seal and insulate, since in many cases the underside of the floor is now exposed, you could add underfloor heating in the form of radiant heating between the floor joists.  Here are some videos on several ways of installing radiant heat tubing underneath the floors:

• Here is a method using supplies from a radiant system manufacturer.  We recommend you review this installation first because radiant systems should be planned!

• Here is a good combination of DIY and manufactured supplies in a professional installation with a lot of helpful tips. 

• Here is a DIY method by a builder using leftover construction supplies.

The bottom line is that if your house is uncomfortable and is built over a drafty, dirty crawl space, don’t pour money into more HVAC capacity until you address what’s eating away at your current capacity: the condition of the crawl space!

Source: Basement & Crawlspace Air Sealing and Insulating Project