Asphalt dampproofing on top of sill for capillary break?
jimmieallison
| Posted in Green Building Techniques on
I am about to apply my dampproofing coating to foundation (probably Henry 101). I’ve noticed that most people stop the coating right above the grade line and then also use various other products as capillary breaks for the sill plate. Why couldn’t I just continue the asphalt coating all the way up to the sill and on top of the sill? I would still use the foam gasket and probably some acoustic sealant for extra air tightness. Is it a bad idea to have the asphalt coating in contact with the anchor bolts and treated sill plate? Thanks!
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Replies
Jimmie,
The reason to stop the coating at grade is mainly aesthetic. If you wish to run it up the outside and over the top there is no harm in doing so, but not much to be gained either. The sill-seal provides a sufficient capillary break for the framing above, and the exterior damp-proofing won't stop moisture migrating up the wall from the bottom and dissipating on the interior face. For that you need a capillary break between the footing and stem-wall above.
Thanks for that information. Unfortunately I didn't install a capillary break between footer and stem-wall because I couldn't find a product rated for that and the inspector also advised against it because he thought it would reduce the connection strength between the footer and stem-wall. I am installing a dimple mat drain board and have a footer drain so I'm not worried about moisture.
Jimmie,
Malcolm was talking about a different source of moisture and its effect on the house than the bulk moisture that is managed by the drain board and footing drains. Those address bulk water that may be sitting against the foundation, draining it away so that you don't flood the basement. The capillary break at the footing/wall interface stops soil moisture that wicks into the footing from rising up through the foundation wall. By applying your coating to the entire foundation and wrapping the top of the wall, any moisture wicking up from the ground can only dry to the inside. Depending on your interior finishes, this vapor emission can cause condensation and mold growth in the walls. In many foundation walls, the inside is either coated with a waterproofing paint or a plastic film, trapping moisture in the wall and that strip of uncoated exterior wall above grade is the only place that drying can happen. OTOH, if you coat the wall inside and out with rugged vapor barrier products, then the wall just stays wet all the time. In most cases, concrete and masonry materials don't really care if they are wet or dry so this may not be a problem. Some masonry materials are more sensitive to freeze/thaw cycling, so a wet wall could be an issue in a cold climate.