QUESTION: I reside in a 60-year-old split-level home in North Kildonan. The home has a crawl space beneath the first level, about four feet high. The crawl space has a heat duct providing warm air to the space.
When I purchased the home in 1984, the original crawl space had a gravel base with tar paper on top. Over the years, I insulated the walls with blue foam-board insulation bonded to the concrete walls. Plus, on top of the board I used bubble/aluminum insulation wrap bonded to the foam-insulation board, then filled in the spaces where the joists are with panels of foam board.
I covered the gravel floor with poly and went up the walls about five inches, then I placed fibreglass reinforced poly tarps on top of the poly and sealed all seams with duct tape and aluminum metal tape. Where the floor poly met the walls I used 12-inch-wide aluminum backed with sticky black rubber to bond on the floor and the wall, thus sealing the envelope. This sticky aluminum/rubber tape is the type used for valley flashing on a roof and I thought that it would be the ideal seal between the floor and the wall, which it is.
The floor tarps billow upwards, indicating air entering through the gravel base. How can air enter from below the home with enough force to cause this billowing? This has become more apparent once I completed the seal with the valley flashing material.
I placed a hygrometer in the space, and it remains fairly constant at 35 to 37 per cent. What do you think is going on with the "billowing"?
Thanks, Dave Magrel
ANSWER: Initially, I want to commend you on the excellent job you have done insulating and sealing your crawl space from the living space. While it's a bit of overkill in some areas, that is often a good idea when it comes to moisture prevention, and your RH readings confirm that. Unfortunately, the excellent job of sealing the poly and tarps is part of the reason for the strange billowing of these covers.
The main reason to install plastic sheathing over the floor of a dirt crawl space is to prevent the heated air of the home from warming the cooler soil and drawing moisture from that soil into the living space.
A secondary reason is to prevent soil gasses, such as radon, from entering the home where it can harm the occupants.
Your double-layered air/vapour barrier appears to be accomplishing that task remarkably well. The diligence you have shown in sealing every seam, especially at the floor to wall junction, is preventing small gaps or holes in this membrane.
It's quite common for these to occur during installation, or later, due to storage or traffic on top of the polyethylene sheathing. The secondary layer of plastic tarp is likely preventing any physical damage to the poly, while adding another well-sealed layer of protection against air leakage.
While your high-quality air-sealing methods are preventing air from infiltrating your heated living space, they can do little to prevent air flowing into the soil beneath this area. There is a chance that you have weeping tiles installed around the foundation of this part of the home. If they have interior connections to a sump in the crawl space, this may be where the majority of the air is coming from. You have not mentioned a sump pit, but there still may communication between the soil in the floor of the crawl space and outside weeping tiles even without it.
Even if there is no weeping tile system for the crawl space, air can easily gain access to the sub-membrane soil from outside. The soil outside your home is quite porous and is not much of an impediment to airflow. In recent years, with very dry summer weather, significant gaps are often seen between the shrunken soil and the outside of the foundation. These gaps can allow a larger than normal amount of air infiltration to the soil under the shallow grade beam foundation of this part of your home.
Why this air causes the membrane to billow upwards is a more technical question. The reason for this can be explained by discussing air-pressure differentials. While this sounds very complicated, it's a pretty simple concept.
Air will always migrate from an area of high pressure to one with lower pressure. Because the building envelope of your home may be quite airtight, especially in the crawl space that you have super-sealed, there may be little chance of air infiltration. When you run your bathroom or kitchen exhaust fans, dryer, fireplace or older heating equipment you are drawing air out of the home and mechanically forcing it outside. This may cause a drop in air pressure inside the home, which tries to compensate by drawing in replacement air from outside the building envelope. If this is too tightly sealed, with no fresh-air intake for replacement of the vented air, negative pressure will be maintained. This will cause the higher pressure air outside the home to be drawn into any area where there is a small gap or opening to the exterior environment.
In your case, this air is being drawn into the soil underneath the crawl space membrane, but is not able to penetrate due to your excellent sealing methods. Because the membrane is very light, the high pressure air can cause it to lift up as it seeps into the voids beneath.
While it may seem to be a defect, the only concern with the lifting membrane is that is likely showing us a significant low pressure situation in your home. To prevent the membrane from lifting, covering it with a thick layer of sand or pea gravel may do the trick, but I would also explore installation of a fresh-air intake duct or HRV to help balance the air-pressure differential this is illustrating.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.
trainedeye@iname.com
