I’m currently in the middle of renovating the crawlspace in my home by insulating and heating it. I’m planning on using rigid foam for the floor but I’m confused as to where the vapour barrier should be for this type of application. The Owens Corning website says to “install moisture vapour retarder sheet after gravel fill has been built up to grade and thoroughly tamped. Lay FOAMULAR insulation in place with edges pressed together and butting foundation wall or adjacent vertical insulation”.
Is this correct? Do you not want to put the vapour barrier on the inside of the insulation? My initial plan was to lay the insulation on the gravel floor and then lay the vapour barrier over top of that.
Any help you can provide is greatly appreciated.
To answer your question we must explore the purpose of the “vapour retarder” mentioned in the instructions you cite. The purpose of this thin polyethylene sheathing should be to minimize water vapour intrusion into the crawlspace from the soil in the heated crawlspace. The location of this membrane will depend on the insulation used, if any, in your renovations.
The first thing to address is the term “vapour retarder”. This is commonly called vapour barrier or air-vapour barrier but we know that it is not a true barrier, as it can let air and moisture through seams, small holes and gaps, if not completely sealed. Whether we use the term retarder or barrier, the term air-vapour should be the prefix. The vapour is the moisture we are trying to inhibit, which is contained within the soil in the crawlspace. This moisture may be released into the warm air above the soil as water vapour when we heat the crawlspace. So, if we stop air movement between the soil and the crawlspace, we will stop the moisture intrusion. Thus, we should have an air-vapour barrier in place over the dirt floor.
Unfortunately, I am not familiar with the Foamular insulation you are planning on using, but it is likely rigid insulation that is either expanded or extruded polystyrene foam. If it is extruded polystyrene, it may have a good air-vapour rating and the location of your poly may not be much of a concern as the inner most surface of either material should provide a reasonable restriction to air movement. If the material is expanded foam, it may have small gaps between the foam pellets and may not have a good air-vapour rating. In that case, installation of the insulation underneath the poly is a better installation method. Both types of foam sheathings are very resistant to moisture, so burying it under the poly should not affect its insulation performance properties, but that is not the whole story. When addressing your dilemma, the complicated science of air movement and condensation within building materials should be considered.
Insulation in our homes is designed to prevent heat loss by slowing convection and conduction through the thickness of the insulation. With some types of insulation, such as fibreglass, this is done by trapping heated air within the insulation, as it escapes the living space. This slows heat, air and water vapour movement, but doesn’t stop it. If no air-vapour barrier is used with this insulation, the air trapped within the thick insulation may cool past its dew point, where the water vapour within it will condense and return to its liquid form. This water may freeze as it further cools, creating excessive frost within the fibreglass or in the cold area around the insulation. For this reason, a good air-vapour barrier must be installed on the warm side of the insulation to prevent air & vapour infiltration, for the desired results to be achieved. With several types of foam insulation, they may resist enough air infiltration to eliminate much of this condensation concern.
If that is not confusing enough, I will throw one more item at you for your consideration. Why are you even bothering to insulate the floor of your crawlspace?
Just as we know that warm air rises, the converse is true. Cool air falls, and if the perimeter grade beam or skirting of your crawlspace is insulated and air-vapour barriered a little below the outside grade, there should be little cold air infiltration from outside this heated space. The soil and snow around the crawlspace in the winter months, combined with the soil in the crawlspace should prevent cold air from causing much concern. The insulation around the perimeter will help stop heat loss, and the polyethylene sheathing will prevent warm air getting to the soil, so the floor insulation you propose installing will have little function.
If you are planning on using the crawlspace for storage and want a warmer surface to walk on or to prevent your items from coming into contact with the sand covering the poly, then the insulation may have value. Otherwise, you can save yourself the hassle and concentrate on installing a gap-proof polyethylene air-vapour barrier. This may be trickier than you think as it has to be sealed with acoustical sealant or other caulking at every seam, protrusion, hole, and to the grade beam poly. In my opinion, focusing your attention on that component may have a greater value than added insulation on the floor of your crawlspace.