Trained Eye

 
 
 

Heating Cold Crawlspace

Question:

I have a cold Floor Problem. We have a house that had an addition built on to the south side in 1990, approximately forty feet by ten feet. The south wall is cantilevered and the construction is 2 x 6 with well insulated walls in the addition and in the crawlspace.

My actions taken to date includes caulking inside and out as well as digging down the exterior walls to a depth of two feet and sealing the foundation and applying two inches of rigid insulation. I have also insulated in between the floor joists with R20 fibreglass batts, just completed this April.

The house is heated with a mid-range gas horizontal furnace located beneath the older portion of the west side of the house. There are partially exposed heating ducts in this area. The east side has a conventional basement which is warm during the furnace season. 

Should I add baseboard electric heat in the crawlspace? If so should the supplemental heat be controlled by a thermostat or a timer?

Answer:

Instructions for methods of properly insulating & heating crawlspaces is one of the most common questions that I receive every year. This is often related to seasonal homes or cottages, but is also very relevant for many newer homes or additions. I will provide you with the only method that I have encountered to ensure a warmer floor.

There are different methods for heat transfer between different areas in a home, but when dealing with crawlspaces, only convection and conduction are the ones we will be concerned with. Basic theory of convection tells us that warm air rises and conduction relates to heated surfaces feeling warm and vice versa. Thinking about these two basic principles will help guide you in understanding the proper method for insulating & heating your crawlspace.

If your current crawlspace is unheated, there will be nothing you can do in the way of insulation that will make the floor above comfortable to walk on in the heating season. Insulating between the floor joists, as you have recently done, will only prevent a minimal amount of heat loss from the room above through the floor. This may slightly lower your heating bill, but will do little to warm the floor surface. Insulation of the perimeter grade beam from the exterior, as you have also done, is a smarter method to help warm the crawlspace, but is limited in effectiveness due to your current situation.

The first thing for you to do is go back in the crawlspace and remove all of the insulation from between the floor joists. While this may seem counterproductive, this insulation will only prevent heat from entering the building to the crawlspace, which is exactly what you want for a comfortable floor. The only place to leave the joist insulation is over top of the grade beam foundation, if you have not extended your exterior rigid insulation up that high.

While in the crawlspace, you should ensure that you have a proper polyethylene air-vapour barrier installed over the dirt floor and over the grade beam to the underside of the floor sheathing. If not in place, installation of this membrane should be your next accomplishment. After that is done & the floor insulation removed, and hopefully reused in an attic, the most critical component can be installed. A heat source must be added to the crawlspace to warm this newly insulated space. The best method is to extend the nearby ducting from the furnace to blow heated air into the area below the addition floor. This may require cutting or boring a hole through the concrete foundation, or installation of the ducting through an old basement window that may have been boarded up during construction of the addition. If this is too difficult, or the capacity of the furnace too low, installation of thermostatically controlled electric baseboard heaters in the crawlspace is the next best thing.

Thinking back to my opening statements about warm air rising, if the air in the crawlspace is heated it will naturally warm the floor sheathing in the addition. The warmed floor will now feel comfortable to the touch and heat loss from this newly conditioned space, and the building above, should be minimized by the rigid insulation you installed outside the addition foundation. Warm air loss from the crawlspace will also be minimized by the air-vapour barrier, which will also help prevent moisture intrusion through the soil on the floor of this area.

The only newly created problem that may occur from this retro-fit is a build-up of moisture in the newly heated crawlspace. This is particularly true if only electric baseboard heat is installed. The solution to this possible concern is to allow adequate air movement for ventilation of the crawlspace. In the heating season, this may require leaving any access hatches from the attached basement open. In the warmer months, screened vents can be installed in the grade beam or floor joist spaces to allow fresh air to circulated from the exterior. In the winter, these vents should be covered with insulation to prevent heat loss and cold spots in the floor.

If you follow all the suggestions above, you will most certainly improve the situation in your addition, often quite dramatically. The only area that may be cooler is the cantilevered section that you mentioned. Because that does not have an area below that may be heated, insulation with high density foam or other high quality insulation material may be your only option. This area may always feel cooler but, due to the Southern conformation, may not be a major inconvenience once the rest of the floor is made comfortable.