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Dear PoP – crawl space insulation

by Prince Of Petworth November 15, 2010 at 2:30 pm 26 Comments

Photo by PoPville flickr user KJinDC

“Dear PoP,

We live an old historic row house that doesn’t have a basement. Instead, it has a very shallow crawl space, with no more than maybe a foot and a half of clearance. We would like to insulate the crawl space, because the cold air seeping up through the floors creates a “stack effect” that means our upstairs bedrooms are substantially warmer than the first floor, making it uncomfortable on both floors no matter what you set the thermostat at. We’ve had several insulation companies tell us that we don’t have enough room to do a normal crawl space encapsulation and insulation, and we don’t want to blow insulation in because we would lose access to the pipes and cables that run under the house. Has anyone else faced such a predicament and have a proposed solution or, better yet, a recommendation for a contractor who can help us out?”

Anyone ever tackled something like this before? I’d also be interested to hear the costs.

  • Jimson Fanse

    You see, this is one instance where child labor laws are now making it more difficult for you to insulate your crawlspace. You could easily teach a baby or a small child how to handle a staple gun, send a few of them into that crawlspace on their backs with some rolls of Owens Corning, and you’d have a comfortable home in the space of a weekend.

  • The experts suggest insulating your crawlspace walls with rigid foam, which makes a lot of sense:


    • la_niña

      I have a similar situation in my house and this is what I did based on a friend’s recommendation:

      1) get some insulation between beams/joists or whatever you are working with.
      2) Over that you can nail the think pink sheets that are R-4 or you can go higher on the R if you get a little thicker
      3) install a vapor barrier – get a roll of thick plastic and lay it on the ground – I did double. I had mine secured with bricks at the edges. This prevents moisture from moving up to the insulation and keeps everything pretty dry.

      This will go a long way to help you – it will not completely solve the problem but will make a big difference.

      the other option is to have insulation blown into the space and then add the moisture barrier.

  • Anonymous

    I’ve insulated, but still have an issue as the poster describes. Anyone ever heard of (ot tried) heating closed-off crawl spaces?

    • EPF

      Spend the cash and put in radiant floor heating.

    • spookiness

      I know of a house I stayed at where the owner said they insulated the crawlspace foundation walls and used the crawlspace as the plenum for the HVAC. I found this out upon asking the owner where the HVAC return was, because I didn’t see one and was curious.

  • I have this exact problem! I’ve been told The crawl space needs to be excavated down to the top of the foundation’s footers or excavated to a 48 inch clearance. When I looked into it, the excavation work was about $3k. While you’re down there insulating you should consider a vapor barrier, basically lining the floor and walls with heavy duty plastic to keep moisture from rising up.

  • Difficult to give the right answer without seeing the place, but this could be helpful:
    Also I think it’s the opposite, the stack effect is causing (in part) the air infiltration in the crawlspace. Finally, if there is room in the crawlspace for a plumber to work on the pipes, there may be room to add some rigid foam insulation, (not saying that 1.5 feet is the most comfortable space to work on).

    • ah

      Yes, you’re going to get “stack effect” without regard to whether the crawl space is insulated. The cold air is not coming from the crawl space, it’s coming from outside and then through the crawl space. So what you need to do is reduce the amount of air flowing into the crawl space by sealing leaks in it.

      The other thing you should look into is balancing your heating. If you have radiators, close the valves partly on the upstairs radiators. If you have forced air, close some of the upstairs vents. Where is the return for your heater? That also has an effects.

      • The NeighborHood Reporter

        Thank goodness someone gave the right answers . Ah ,knows what he is talking about . If you have ever lived in a row house or any house for that matter all heat rises and when you don’t balance the heat upstairs will be hotter than then downstairs common physics

  • Anonymous

    definitely a job for the foam i think. any decent contractor should have brought this up as an option. i have a similar issue (my crawlspace is taller though) and my dad and uncle who been building and renovating houses for 30+ years immediately suggested foam.

    contractors in DC are scum. they seriously suggest the worst, most labor intensive and outdated methods of doing anything. i had a guy give me an estimate to vent my dryer to the outside and he wanted $1200. my dad said it would cost us maybe $200 in material and four hours to do it ourselves.

    i called up 2 contractors recommended on this site about installing central AC and they both wanted to put in $15k+ ducted systems in my 950 sf row house. Mini splits would run me $3k, be just as if not more effective, and i can put them in myself.

    i sympathize for any homeowners who have to find good contractors in DC. if you don’t know what you are doing (and i don’t but luckily my family does) you are going to get screwed, in my experience.

    • joker

      +1 Agree with the DC contractors are scum…

      I own a vacation home in rural VT, had it for a dozen years and I have yet to find a contractor I haven’t been pleased with. This compared to DC where after interviewing dozens of contractors over the years, have found one or two that I “didn’t hate”. In rural locations, its been my experience contractors simply treat your home in the same way they treat yours, versus here they just treat your home as a obstacle to the next one, and another paycheck.

      Standard logic that applies anywhere else is, when the economy is down, Contractors pricing gets competitive as there are a lot of uhngry contractors out there competing for the same work. Not in DC. Nope, its been experience that over the past 2 years, as the economy crashed to levels not seen in my life, Contractors generally jacked up their rates, seemingly trying to make up for an entire years worth of lost revenue in one sale. This is true on both the commercial and residential side.

      I really do feel bad for the folks that may not understand the process, or whats involved because those people, excuse the saying, are getting royally screwed.

  • joker

    My recommendations, and in this order.

    1. Spray foam insulation on the underside of the floor. Still have access to everything you need,fast and works like a charm. Problem…its pretty expensive. Even the cheap stuff makes you do a doubletake at the pricing.

    2. Rigid foam board insulation. Easy to install, cheap, no mess. Problem, won’t completely erradicate the problem, but it will do a decent job.

    • Anonymous

      yes i agree, i posted above and the foam was going to be like $2k or more to do a footprint of like 550-600sf.

      i decided against it, i would have to be in the house for a long time for foam to beat out the old school stuff.

      however my crawlspace is a lot taller than 1.5 feet. i wonder if they can get the board in there and still have access to everything?

  • The rigid insulation boards go on the walls of the crawlspace not under the floor, so the pipes remain accessible. The board joints should be taped. Check out the link I posted above for graphics of how the system works and everything that is involved.

    • Anonymous

      okay so with this method you are essentially conditioning the crawl space, correct?

      it is a good way to go if you can do it but i was lead to believe that a lot of these old houses need to have or do have vented crawlspaces.

      how would this work with that?

      • I don’t know why a vented crawlspace is needed on any house (I would like to know if someone can tell me). Older houses are leakier (and you mentioned the stack effect, hot air moving up because is lighter), and therefore some of the air that you are breathing comes from the crawlspace. Vented crawlspaces have moisture, mold, ground pesticides, etc. You are right that the crawlspace will be conditioned, and dry and clean :D . I hope my answers are helpful!.

        • Anonymous

          oh i’m not the OP, but i think the venting PREVENTS mold, moisture, gases, crap, etc. from building up in your crawlspace. by venting it you make it like any outdoor space like your back yard or something, but also (ideally) keep rodents and junk out so your home’s systems don’t get damaged.

          i agree that conditioned crawlspaces are better (although critters LOVE them), but i think these old houses aren’t really set up for it. I’m not a professional but I think my dad was telling me it would be hard to AC the crawlspace or something. He wanted to condition mine as you suggested but then thought for some reason it wouldn’t work.

          and now i’m really guessing, but I think if you had a crawlspace that was vented (which most older houses were probably designed to have) and decided to seal and condition it (which really is a good idea) you would have to make certain that you did get some air changes between the crawlspace and first floor. if not it might get kinda stale down there.

          this really is a complex topic that can really affect the house and i hope the OP gets some good help.

          • Yes, it is a complex! I see your point about venting when you mentioned the gases. If you check the link I posted above, you’ll see how they deal with that (plastic layer and sometimes a vent pipe for Radon where needed). By venting you make the crawlspace as humid as your backyard and then because of the lower temperature compared to sunny outside, moisture condesates on the floor joists and insulation and then comes the mold. And you are right that the ventilation of the unvented crawlspace is also part of the system.
            Hopefully the OP is reading our conversation!

          • Anonymous

            Okay this is my last post then I’ll shut up. This has been one of the better threads though. No one called anyone stupid or said anything snarky and everyone is being helpful, I like it.

            I 100% agree that conditioned crawl spaces are better logically, in practice, etc.

            I think the reason it didn’t make sense for me to do it was because it would have been too hard to heat and cool. If you are in a 100+ year old 950 sf two bed rowhome with radiators and window units, how will you heat and cool the space? Of course it could be done, but I’m not sure it’s “worth it.”

            New construction, no question.

            People have been venting the crawlspace up until recently anyway, right? It’s not the worst thing in the world, or is it?

            I was told the REAL worst thing in the word is an unvented AND unconditioned crawlspace. Then you’re totally screwed.

        • It would depend on your location, climate zone, dew point, etc. Also depends on the construction of the crawlspace and whether there is a thermal break between crawlspace walls and floor assembly/rim joists. In dc row homes the masonry crawlspace wall acts as a capillary, but in some wood framed structures, it may make sense to vent the crawl space, but it’s rare.

  • Melanie

    I have this problem too! Except i think our crawl space is more like 3′ tall. My father suggested using rolls of the itchy pink insulation, then use rigid boards stapled to the joists to hold up the pink stuff. Two layers, better than one.

  • Read this study and it will be clear as to why it’s a good idea to insulate the crawlspace walls and condition the space: http://www.advancedenergy.org/buildings/knowledge_library/crawl_spaces/pdfs/Closed%20Crawl%20Spaces%20Do%20Double%20Duty.pdf

  • Kelly

    Yeah! I am with anonymous 5:57. Helpful posts and no snarkiness. I might start reading again with some regularity.

    This is a bit off topic, but since some of the comments were about contractors, does anyone know of a decent one to put in a very simple screened porch? Also, will the permit process be terrible?

  • victoria

    So what did people do in the olden days when these houses were originally built? Was there some kind or insulation or were they just cold all the time?

  • Nikki

    I actually had a great experience with my insulation contractor. I recently had my attic insulated, and I have never worked with a contractor who was so detailed and patient. His name is Fabian, and his company is called Metro Insulation. 301-674-5437.


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