Top Worst Crawl Space Insulation Ideas

With winter right around the corner, many of us start looking at various ways to reduce heat loss and improve energy savings. One of the most common practices of improving energy savings is by installing or improving our homes insulation. This is especially true for those of us who have the luxury of living above a crawl space. Installing or improving crawl space insulation can be very beneficial. But it can also have expensive, negative effects if not done correctly. Here is a list of a few common crawl space insulation ideas that have the potential to cause much more harm than good.

1. Fiberglass Insulation Between The Floor Joist - This is one of the most common methods of insulating a crawl space. It is also one of the most effective ways of improving energy savings, but only if done correctly. Too often fiberglass insulation is installed between the floor joist in a crawl space without considering any other factors of the environment in the crawl space, mainly moisture. If your crawl space has standing water or excessive moisture, than I would recommend not installing fiberglass insulation between the floor joist until these moisture problems are solved permanently. This is because moisture in your crawl space eventually makes it's way up into the living area of your home by a process known as stack effect. Fiberglass insulation between your floor joist will work like a sponge, absorbing and holding this moisture. This will cause the insulation to lose it's R value. It will also cause it to become heavy due to the moisture and fall. Insulation the does not fall will continue to hold moisture between the floor joist which can cause an increase in the moisture content of the floor joist itself, leading to possible mold growth or wood rot. 

Fiberglass Insulation On The Foundation Walls - This is another common method used for crawl space insulation. Again, the problem with this method is moisture. If your crawl space is below grade, as most are, than the fiberglass insulation on your foundation walls will usually have direct contact with moisture. This is because crawl space foundation walls are porous and tend to allow water seepage. This moisture seeping in through your foundation walls will be absorbed by the fiberglass insulation. Again, this causes the insulation to lose it's R value and become heavy and fall. Fiberglass insulation will also wick moisture, allowing the moisture to move up into the structural wood components that the insulation is usually attached to. This increases the chance for mold growth or wood rot. 

Cellulose Insulation on Foundation Walls - Blown in cellulose insulation installed on your crawl space foundation is something I find very hard to ever recommend anyone doing. Cellulose insulation is primarily made up of paper fibers. To avoid moisture saturation it's recommend that this type of insulation be kept approx. 12" above the dirt crawl space floor when installing it on your foundation. But this doesn't address the moisture seepage that comes through your foundation walls. Again, this type of insulation will absorb moisture, become heavy, and fall. Once it has fallen, it will absorb moisture from the dirt crawl space floor, causing possible mold growth. 

Spray Foam Insulation - This type of insulation in a crawl space in seen mostly on foundation walls. It can be a great help with increasing energy efficiency. Spray foam insulation in your crawl space becomes a problem when it is installed around the structural wood components above your foundation wall. Any moisture that makes its way into the structural wood components from the outside is now encapsulated with the wood because of the spray foam. This increases the rate at which the wood will rot or grow mold. 

Insulating your crawl space can have many useful benefits. But other factors should be considered before installing insulation in your crawl space. The most effective way to get all the benefits of insulation in your crawl space is to have a dry, sealed, moisture free crawl space

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Comment by John Porter on January 6, 2015 at 8:07pm
Here in New England air sealing the crawl space is important since the crawl space stays cool all year round and the warm moist summer air that is drawn into the crawl space will result in high relative humidity and condensation. I generally consider that basements and crawl spaces are part of the house and should be inside the air barrier. This will allow the crawl to dry to the house. Dirt floors need to be covered with an impermeable moisture barrier, such as 6 mil poly. Air sealing the walls of the crawl space so that moist interior air does not come in contact with the cold wood members and if the top of the foundation and the rim joist area are far enough above grade they will generally dry to the exterior.
Comment by Kent Mitchell on November 22, 2014 at 8:40am

Hi Adam - It sounds like you may be referring to enclosed crawlspaces - Here in the NW we've steered away from them as moisture is a serious issue, so we vent nearly 100% of them.  We also have radon issues here so we use a combination of the vapor barrier and a radon vent system to minimize the effects radon can have on occupants.  Another key to good floor insulation installation, is to ensure the thermal barrier is in contact with the warm side (floor).  We often see 3 & 4" gaps between the subfloor and the insulation which nearly ruins the thermal performance trying to be installed.

Comment by Tom Mallard on November 6, 2014 at 7:55am

In the Northwest people use a vapor barrier on the dirt, thick black plastic the usual choice so you can crawl on it and not tear it; this may even be code here not sure on that but very common. This of course only deals with moisture and helps a lot with soaked batting, yet that issue is also dewpoint not just having the humid air.

Also, the things listed are common here so to fix where groundwater moisture is a problem is a rather large amount of work that usually will involve the foundation pore pressure of soils up against it which may require a rock blanket to a drain well below crawl space grade to keep pore water from rising to the surface, it's a case-by-case basis to evaluate what needs are but that's a typical solution.

So then you just spent some real cash and get a drier crawl space and that will help the heating bills, so it may make sense if you needed to do this and at times wanted to put your home off-grid consider taking extra steps with the crawl space to turn it into a thermal-mass to keep the room warm from below like some passive-solar designs.

If that's not a priority I'd suggest after dealing with pore pressure and active moisture transport to use insulation board below the joists to insulate, batting or no batting, many contractors used a layer of plastic below the joists and this does help but nothing like using the board which pretty much eliminates conduction and moves the dewpoint so moisture won't form in the space. Doing this will have a lot of benefit to reducing heat loss through the floor.

If going off-grid is a priority then using the crawl space for a thermal-mass makes sense, I recently figured out a way to use the air below the mass for existing homes that's reasonable for costs & labor if anyone is interested.

Comment by Eric Kjelshus on October 30, 2014 at 7:32am

6) Radiant barrier- under the gravel is just a barrier but with holes each square inch its not a barrier. 

7) Rolls of 4 foot x 300 foot fiberglass skraps used on metal building then used on floor or walls of foundation walls

8) scrap foam from packaging put in craw space with no seal no air barrier- just placed in space with 1/2" - gaps but 3 foot thick 

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