Crawlspace Encapsulation Zone 3a
Details: - Single family 2 story modular home in North Carolina. County uses the 2018 NC Residential Building code (IRC 2015 w/ Amendments). - The crawlspace is rectangular in shape and measures ~1460 sq ft. with an average height of 4 ft. (The crawlspace is…
Details: - Single family 2 story modular home in North Carolina. County uses the 2018 NC Residential Building code (IRC 2015 w/ Amendments). - The crawlspace is rectangular in shape and measures ~1460 sq ft. with an average height of 4 ft. (The crawlspace is 2ft on one end and 6 ft on the other). - Landscape slopes ~4° from one end of the house to the other. The crawlspace follows the same slope. - As I'm sure you can guess, it's pretty damp in the crawlspace. HVAC trunk duct regularly sweats, as does the refrigerant lines. Mildew/fungal growth is present on the underside of the fiberboard installed to the bottom of the floor joists. - Some mold growth has been seen around the floor penetration of the HVAC trunk duct and condensation has been seen on nails and screws penetrating the subfloor. The air handler sits inside the conditioned space while the duct work is below the floor in the crawlspace. Questions: NC Code requires the installation of a sump pump. In my research it seems like a drainage system is recommended or required for proper management of water under the plastic. I plan to install dimple underlayment. Is this enough or should I install additional drainage around the perimeter that is routed to the sump? The crawlspace already has a natural low corner that is perfect for the sump install. I'm between using a dehumidifier or supply air (Sections R409.5.1 and 2). I intend to insulate the walls and remove any floor insulation. What are the pros and cons of each option given the insulation plans? Maybe a combination of the two options?
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R409 doesn’t strictly require a sump, it requires the crawlspace floor to drain to one or more low points, each served by daylight drainage OR a sump. Dimple mat is a drainage plane, it won't substitute for drainage. Do you have wall or footing seepage? If so, you can add a sub-liner collection drain, but keep crawlspace drainage separate from roof and foundation-perimeter drains. The calculation on 1,460 sqft requires 49 CFM (1,460/30 = ~49) of supply air, with no return and a backflow damper. I’d still use a dedicated crawlspace dehumidifier as the primary control because HVAC supply is depends on running it, and as a NC resident myself, the humid weather is a monster for this. Wall insulation with the floor batts removed is the correct thermal-envelope strategy, but repair the duct and refrigerant-line vapor barriers and remediate the existing growth first.
I need to do more research regarding the daylight drain. The sump made more sense to me at first glance so that's what my brain went with. The drainage situation is what I'm really struggling with. I'm having a hard time understanding what is needed based on my crawlspace conditions (and planning for 100 year rain events) vs what is fear mongering from the supply stores. We've had some pretty heavy rainfall lately and the ground under the existing vapor barrier is damp, but I haven't seen any standing water or seepage onto the walls. Will definitely continue to keep an eye on this with more storms expected this week. Exterior grade is pretty good though and the downspouts have a drainage system that carries the water far away from the foundation. No perimeter drains to my knowledge, only proper grading.
Based upon your description of the home and the incoming and outgoing grade, I think you're alright. Damp soil beneath the vapor barrier is normal and means the barrier is doing its job. If you aren’t seeing standing water, wall seepage or evidence of water movement after heavy rain, I wouldn’t install a perimeter drain based on fear alone. Grade everything toward the existing low point, then use a gravity drain if the downhill elevation allows it or a sump if it doesn’t. The dimple mat provides a path beneath the liner, but I’d monitor through several prolonged storms before deciding you need anything more elaborate.
Dimple mat's just a capillary break, it won't move water laterally where you need it to. if you've already got a natural low corner i'd bite the bullet and run a perimeter drain to the sump.