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Community and Q&A

Best way to insulate crawl space in new build

Cherylann128 | Posted in General Questions on

Hi. I’ve asked this question before about an existing house, but now I’m building a new house and would like to make sure we get this right. Thank you to everyone who contributes here. Your insight is beyond helpful.

We are building on a barrier island in south Jersey two blocks from the beach. By code, we must have flood vents in our crawl space so it’s not possible to completely seal all air infiltration (although we are using insulated smart vents). The foundation floor has a vapor barrier and fiber glass under the rat slab. The walls are cement block, coated on the outside with waterproofing and covered with one layer of brick.

Unfortunately, we have to run the duct work for the first floor in the crawl space. The current plan is to foam the joists and bays with 3″ of closed cell spray foam and also cover the duct work with spray foam. I’m thinking that foaming the duct work might not be such a good idea after all and that maybe we should just insulate it with R8 bubble wrap.

The other alternative will be to foam the walls and condition the space as best we can. The foam installer says we can spray 3″ of closed cell foam on the walls up to and including the sill plate and rim joists. I’ve read conflicting opinions about this, though. Some say you need to leave gaps for termite inspection, others say that moisture wicking through the foundation walls has no where to go but into the wood where it will cause rot and mold so the sill shouldn’t be covered with foam (I’m pretty certain there is a vapor barrier between the block and sill). The more I read, the more confused I become.

I’m so worried that we’re going to do the wrong thing and get into a big mess. Is there anyone here that can advise me on the best way to proceed. Thanks so much!


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  1. STEPHEN SHEEHY | | #1

    Have you considered an hvac system using ductless minisplits that don't need ductwork? How far along is the project? If still in design, are you using an architect experienced in designing structures subject to periodic flooding? As I understand the issue, you need more than flood vents. You need a design that allows the ocean to flow under the house without damaging it.

  2. Expert Member
    Dana Dorsett | | #2

    In most places it's legal to have floodwater activated vent hatches, and they come in insulated & uninsulated versions. eg:

    Are these what you are using? (They are reasonably air tight.)

    In your area 2" of EPS under a vapor barrier & 2" non-structural rat-slab on the crawlspace floor makes sense, with 4" of EPS on the walls. I'm not sure if rigid fiberglass will really work under a rat slab, but EPS definitely will. If it's a poured concrete foundation wall, using minimalist 2" + 2" insulated concrete form may be quicker & easier.

    In termite zones install a copper clad plastic sill gasket (or copper flashing) between the foundation & the foundation sill. The gasket or flashing needs to extend out over the tops any interior foam. It would force any termites tunneling in the foam to exit the foam to get around the (toxic to termite gut flora) copper layer, thus no inspection strip necessary.

    While 3" of closed cell foam would get you there from an R-value point of view, it is one of the least green options due to the HFC blowing agents used. EPS is blown with comparatively far more benign pentane, most of which is released and recaptured at the manufacturing plant. Sealing & insulating the band joist down to over the top of the interior wall foam with and inch closed cell foam is fine, and sufficent R value for dew point control for R15-R20 of fiber insulation on the interior side of it.

  3. GBA Editor
    Martin Holladay | | #3

    Stephen asked some good questions. It would be great if you were still at the design stage. If your house is already built, we can old solve some, not all, of your problems.

    I recently visited a ten-year-old house in Folly Beach, South Carolina, that was a block from the beach. The first floor was about 12 feet above grade; the foundation consisted of piles, so that a storm surge could rush under the house without affecting the upper floors. Made sense to me.

    You can certainly install spray foam on crawl space walls and the interior side of rim joists. For more information on these issues, see these articles:

    Building an Unvented Crawl Space

    Insulating rim joists

    When it comes to termite regulations, you'll have to ask your local code enforcement official for advice. Termites are a local issue, so we can't give you advice easily.

  4. Cherylann128 | | #4

    Thanks for the quick and informative replies!

    Stephen, I would love to have had radiant heat and mini splits but the spouse strongly objected. Can't win every battle. :) Because of height restrictions we didn't have the room between the first and second floor to run the ducts without lots of soffits so a conditioned crawlspace is the compromise.

    Dana, we are using the insulated smart vents. The drawings specify sill sealer and a layer of asphalt shingle starter which I assume acts as a capillary break between the block and sill. A termite shield is also specified and I will ask which kind was used. I'll talk to the insulation contractor about EPS board. Right now closed cell foam is in the specs.

    Martin, I would love to have had a 12 ft open pier foundation but our height restrictions would have ruled out a second floor with that construction. The house is designed to withstand flood waters but it only sits about 4 feet above grade. The cottage that was on the lot when we bought it was less than a foot above grade and didn't get any water with Sandy so I'm hoping we'll be ok.

    After reading your comments and the articles linked above, it looks like we'll be ok sealing the walls, sill and rim joists. Below are the specs from the construction drawings. If you see anything that looks amiss, feel free to comment and thanks again for taking the time to educate us lay people! It's really appreciated!

    Per NJIRC R408.3-1: Provide rat slab at all crawl areas. Underside of slab to be covered with a continuous vapor barrier. Joints of the vapor barrier shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor barrier shall extend at least 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall. Provide rigid foam insulation below vapor barrier. Provide Crush stone below rigid insulation.
    Insulation at Crawl Space Walls: Provide Quadfoam 2.0 in accordance with the manufacturer's required thickness to achieve R-values complying with NJ energy subcode. Where spray foam shall remain exposed, apply DC 315 Fire Protective Coating.
    Per NJIRC R408.3-2-2.2: At unvented crawl space, provide conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute for each 50 square feet of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with the NJ energy subcode (N.J.A.C. 5:23-3.18)
    Note: Conditioning must be used where masonry porches, terraces, patios, planters or other obstructions prevent air vents from being evenly spaced at significant portions of the crawl wall. CONDITIONING DOES NOT REPLACE USE OF FLOOD VENTS.

  5. charlie_sullivan | | #5

    The specs say rigid foam below the slab, but they don't specify type or thickness. Dana's recommendation is what I would recommend as well.

    If you use spray foam on the walls rather than Dana's recommendation to use EPS, consider specifying a spray foam with low global warming potential. The gas used to blow bubbles in conventional closed-cell spray foam has a 1400X higher global warming effect than CO2. Those are now available from two large spray foam distributors: Demilec and Lapolla

    If you want to try to avoid ducts and avoid minisplits, another option to consider is an air-to-water heat pump, with slim fan coil units in each room.

  6. Brian Knight | | #6

    Um, you are building on a barrier island? Four feet above grade? Based on a highly localized storm of five years ago?

  7. Cherylann128 | | #7

    Thanks again for the replies.

    Building code requires the house be 1' above FEMA Base Flood Elevation. We are 3' above Fema's 9' minimum which puts us 12' above sea level, so it's definitely a risk. The argument against building on barrier islands is a valid one and we may, in the end, lose the house to either the elements or changing regulations, but we'll enjoy it while we can.

    The insulation contractor balked at the use of rigid foam, but the building inspector says no spray foam. He says we would have to remove after a flood. So, I will be pushing for EPS, as Dana recommends. Should a vapor barrier be installed on the walls before the EPS boards?

  8. Expert Member
    Dana Dorsett | | #8

    The foundation insulation does not need a vapor barrier (on either side of it), but it's good to make the foundations as AIR tight as possible with taped seams and can-foamed edges, since summertime temperature inside the crawl space will be below the dew point of the outdoor air a lot of the time. Air leaks during the summer raises the humidity levels in the crawl space, and with it the mold risk. Some amount of ventilation with conditioned space air is usually necessary.

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