Insulating crawl space with sub-grade walls, or spray insulate floors?

Submitted by ThinkSharp t3_10l1syy in DIY

The house we moved into has a crawl space with duct work (wrapped and insulated) and plumbing in it. No insulation. We added a vapor barrier and I had the company run it up the side walls along the CMU. Most walls are against soil, one wall is against air. Only 3 vents around one corner area, not doing much.

Now I want to insulate to warm the floors and save energy. Would it be. Water to complete an encapsulation and insulate the side walls, or have the subfloor spray foamed and finish the vapor barrier and add a dehumidifier?

Whatever I do I don’t want to make it colder in there because of the plumbing.

How is this properly addressed?

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lovallo t1_j5wb8m4 wrote

Knowing if you are in a heating or cooling dominated climate is helpful here.

You want your thermal barrier and air barrier at the same boundary.

If you insulate the floor the space below will certainly get colder. Would the pipes be in the spray foam? that might be safe enough in terms of freezing. Do you have a sense for how cold it gets down there currently in the winter?

If you insulate the crawl space walls then you will want to seal up the vents to the crawl space.
Moisture is a concern, dehumidification can get expensive. An exhaust fan on a humidistat might accomplish what you need at a lower operating expense.

Since you have pipes and ducts down there I would seal the crawlspace and put in mechanical ventilation and or other moisture control.

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ThinkSharp OP t1_j5wckvw wrote

Thank you. I think I’m ASHRE zone 4. Central WV (not the mountains). Winters are not typically severe but can be single digits a few times and we recently had negatives (dry bulb) during this bomb cyclone.

So you recommend if I insulate, do it at the walls mainly because of the moisture issue?

I realize I didn’t answer your whole question. Typical temps in the crawl space during winter are probably 55-60. Long t shirt feels perfect, short sleeve if working. Humidity is typically high. We had to install a vapor barrier and sump pumps. It gets foundation run in from one side. Correcting the swale outside is on my list for things this summer but part of the issue is a cement slab that needs raised. May not be able to handle that. In other words I expect it to always have some water coming in when rainy/snowy. I plan to NOT cover over the boards in that area to let them keep breathing and I intend to place a dehumidifier in that area of highest humidity/source.

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Traveling_Carpenter t1_j5wngkr wrote

You have a couple prescriptive options available in the model code if you go the path of insulating the walls, which is a good choice. Look at R408.3 in the International Residential Code - it spells it out pretty clearly. For a more plain-language perspective explanation, I’d check out the Fine Homebuilding article “crawlspaces that work.” Code available here: https://codes.iccsafe.org/content/IRC2021P1/chapter-4-foundations

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ThinkSharp OP t1_j5yc4jq wrote

Thank you, I’ll read each. That’s helpful because I don’t know where this stuff is already studied and prescribed so this is super helpful. No sense recreating the wheel

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magaoitin t1_j5ulijz wrote

I just debated this exact thing last fall. I really wanted a full encapsulation of my crawl space, but the price was crazy. I only have a 900 sq ft crawlspace and encapsulation came out to $13.50 sq ft. I live in Western Washington (Seattle area), so we do have a decent amount of rainfall and wet conditions most of the year, but with a fully vented crawlspace I have never had any issues with humidification or water in my crawlspace.

I decided against using a closed cell spray foam for the joists, not only because of the price, but also the horror stories I have read about the possible problems with off-gassing that has plagued the industry for years. I probably wouldn't be unlucky enough to be "that one in a million situation, but...I am Murphy's bitch sometimes. One bad install and you are never removing the spray foam to try it again.

I will use closed-cell for tight, hard to reach areas, or for the rim joist bays to get a 100% seal, but not in the open bays.

I got 3 quotes from different companies and ended up with , $900 for a 10 mill vapor barrier, $3,600 for the vapor barrier and R30 insulation in the floor joists, and $12,000 for Vapor barrier, floor insulation, R10 rigid insulation on the stem walls, 100% sealing, and installing a dehumidifier (electrical work was not included as I have dedicated outlets in my crawlspace already).

So $1.00 sq ft for 10 mil Vapor barrier$3.00 sq ft for R30 floor insulation$9.00 sq ft for rigid insulation and full seal (but also includes the cost of a dehumidifier...roughly $1100 is my guess)

https://imgur.com/a/T2LyCsX

I decided not to do the encapsulation and just did the vapor barrier and floor insulation, and then wrapped the exposed plumbing in neoprene insulation after the crawlspace company was done. I am still considering installing the Rigid insulation and sealing the crawlspace up myself this year.

I talked with 3 different crawlspace companies in my area and got the same basic answer, and roughly the same pricing. All three said they almost never do full encapsulation in my area unless there is a water problem in addition to the insulation need. I dont know if this is true, but I got it from 3 different sources that could have had an easy upsell, but they all talked me out of it. It could be with how busy companies were last year, but I am leaning towards believing them.

One thing 2 of them brought up (and I found in my online DIY researching) was if you do a full encapsulation you need to provide conditioned air from the living space down into the crawlspace and let it recirc back up into the house (if you are 100% sealing) to get the benefit of a true air-insulation barrier in the crawlspace. I've redone the floors in my house and have been sealing every crack in the floor prior to this, so now I would most likely need to cut holes somewhere in my floor to provide supply and return air to the crawlspace. Not a big deal, just something I have been reading about. You also need a dehumidifier down in the crawlspace and there is maintenance to think about on that piece of equipment as well as power and probably hiring and electrician on top of the install.

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ThinkSharp OP t1_j5ybf0k wrote

Thanks for the detailed answer. I’ll re read a few times and maybe get some similar quotes if for no other reason than to hear what they would recommend. I’ve heard about and seen insulation that got humid falling out of joists and molding so, I’m cautious about that. But if I can control the humidity first then I’m not afraid of it. I just want to do the “best” option. I don’t want to circ house air through it but I’m not against a dehumidifier to keep it conditioned.

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magaoitin t1_j5yromv wrote

Part of a 100% full encapsulation is you have to install a dehumidifier and pipe the drain line out of the crawlspace. (9 times out of 10 it is aways put in the most inconvenient part of the crawlspace to maximize the draw. Rarely do you ever get lucky enough that it is next to the crawl space access for ease of maintenance). It also depends on how well sealed your floor is to the living space. There are always holes the pathways for air to move unless you have gone over the top with sealing above the subfloor.

If you are worried about humidity building up in a vented crawlspace, you can opt for adding Crawlspace Fan with a Humidistat to the louver locations. Just don't have all of the fans pointing into the crawl space. Either all exhaust the space or you split it 50-50 for intake and exhaust so you maximize air flow across the insulation/joists.

These fans are sized to fit in a standard sized small vent openings you already have, and most are directional.

https://www.amazon.com/crawlspace-fan-humidistat/s?k=crawlspace+fan+with+humidistat

I agree that calling 2-3 crawlspace companies tell them what your concerns are and that you would like a price to just add insulation, and a second price to do a full encapsulation (or whatever your couple of choices are), letting them crawl through the space then ask their opinion and recommendation. If you get 3 separate companies giving you the same basic information you can make an informed decision based on your climate zone and actual subgrade conditions.

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skippingstone t1_j5wr627 wrote

Watch Crawlspace Ninjas on YouTube

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ledge_and_dairy t1_j5ycky8 wrote

Seal it, insulate the walls and then put a small hvac register and return to condition it.

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danauns t1_j5yq8m9 wrote

Encapsulated, sealed crawl spaces are absolutely better in every way compared to all alternatives.

It is a significant amount of work to encapsulate one that wasn't originally designed for that sort of treatment - but it's always absolutely worth the effort.

It is also the sort of thing that can be DIY'd. Contractors tend to run prices up for this sort of work, and rightfully so I'd say - it's uncomfortable, dirty, limited access work in most cases.

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ThinkSharp OP t1_j5zd5re wrote

For sure! I had one quote it with the sump pump work but ended up saying no because of the cost. I had them run 12 mil white vapor barrier up every wall, however, so I have a head start.

So, in terms of floor warmth and energy savings- is a full encap equivalent to insulating just the floors? Cost aside.

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danauns t1_j5znhfi wrote

It's just, different. Apples to oranges really.

Insulating the floor effectively turns the floor into an exterior wall..

Encapsulation keeps the exterior, to the actual outside and elevates the 'envelope' performance of your home, which is typically a massive gain in terms of energy efficiency ~and air quality.

I've never approached this from the perspective of floor temperature, it's always been more of an efficiency and air quality exercise.

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ThinkSharp OP t1_j5zo28v wrote

Fair enough. What’s the proper adhesive for rigid foam against a vapor barrier? (I would assume I could put foam on the vapor barrier I have, wouldn’t have to put it under it and against the CMU?)

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danauns t1_j5zql6p wrote

What's the foundation wall?

I really like the insulfast/ramset - really quick no no fiddling around. Click, bang done.

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ThinkSharp OP t1_j60j8v4 wrote

CMU, no termite powder treatment or anything just standard. LACKS a metal termite shield however so I want to leave a 1/2 gap to monitor for tubes.

The company that installed the vapor barrier up the walls does encaps too. They drilled and used a plastic one-way fastener.

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