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Community and Q&A

Insulating Floor Over Crawlspace

GeoffPritchard | Posted in General Questions on

Hi —

We have a house that was built in the 1990’s using engineered joist systems (BCI/TJI) I-beams and recently had some issues with mice getting at some wiring (we won’t go there right now).  Original under floor insulation was standard R-19 kraft backed bats held in place with wire insulation supports that are now a mess of sagging fiberglass rat (mouse) runs.  We’re planning to redo the insulation under the floor (above a currently vented crawlspace) and are still unclear as to what product(s) to use the give us good insulating value with minimal mouse problems – emphasis on the latter.  I saw that Roxul makes a product that will fit better between the I-joists but didn’t see if and how people were suggesting to install an air/vapor barrier between these batts and the bottom of the subfloor.  In addition, there was some discussion of putting polyiso panels across the bottom of the floor joists.  I don’t know about most other builders or homeowners but I’ve not come across too many crawl space entrances that would allow a 4×8 sheet of rigid insulation to fit through.  This house has a 30×36 entrance at best.

BTW — Zone 5B/6B



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  1. Expert Member
    Michael Maines | | #1

    Geoff, I would suggest using borate-treated dense-packed cellulose. It may or may not repel rodents but it will fill the spaces. Below that I would consider foil-faced polyiso ripped into 2' strips to fit them through your door, then tape all of the seams with foil tape. Or just use plywood, taped, and keep an eye on it. One more approach is to use hardware cloth below a permeable, airtight membrane such as Tyvek or one of the European products.

  2. Expert Member
    NICK KEENAN | | #2

    There is no insulation that is rodent-proof. You need to have a layer of plywood or equivalent to keep the critters out. It needs to be airtight to keep rodents from gnawing at it. The insulation needs to have an air barrier and a vapor barrier between it and the ground.

    Here's a good article by Joe Lstiburek on crawl spaces:

  3. charlie_sullivan | | #3

    Have you considered switching over to an insulated, conditioned crawl space? What climate zone are you in?

    But if you go with insulating the floor, I like Michael Maines' suggestions.

    1. GeoffPritchard | | #4

      Zone 5b/6b

  4. Wannabegreenbuilder | | #5

    Geoff, I am probably not qualified to advise you on what to do in your situation but I build in a primarily rural 6B climate I can tell you what NOT to do. I won’t get into how I know. Rodents are freeloaders and love insulation. I have spent quite a little time trying to get my rodent/insect control barrier correct on various builds. What seems to work for my builder friends in the city, I find lacking for us that live and build near wilderness. Here are my anecdotal findings thus far.
    1) Exterior Polyiso is a rodent magnet and mineral wool not quite as much.
    2) Pest control is never limited to just the building assembly. The homeowner needs to be educated and on board with joining the war to control pests utilizing other means both outside the structure and within it in case the initial line of defense is breached.
    3) We do not have rats in our area but this is more than offset by pack rats (with a cute furry tail) and raccoons that seem to be able to ruin almost any cost effective rodent control barrier I incorporate into my assemblies.
    4). I am not a proponent of any insulation that is installed on the exterior of a building that is below grade. Just don’t.
    5) I love the concept of exterior insulation, however, but use it ONLY above grade for people who can and will do their due diligence to frequently inspect the wildlife/home interface or as I like to explain it “the front lines of the war.” Just because you build it right does not mean it will stay that way as you well know by your experience.
    6) PLAN for failure. That is why we call it a rodent/pest “control” barrier and not a resistant barrier. The battle lines will be breached from time to time so secondary measures are always a good idea and building your control barrier as robust as you can afford is always a good idea.
    7). Do not waste effort and money by replacing what has been already damaged until you have first done you due diligence to uncover rodent the living quarters and beat back the population of rodents. They are prolific breeders and far more industrious than humans.
    8) Don’t be afraid of using Boric Acid around your insulation, within your SEALED building assembly. It is safe if you educate yourself on its application and keep it away from pets and children who might ingest a whole lot of it. Boric Acid is deadly to insects and to some degree messes up the reproductive viability of rodents. I have never figured out if it affects raccoons or not. I don’t have the heart to do the useful testing in a controlled environment to answer that question. They don’t recommend it be ingested by pets but raccoons are survivors of pretty much anything and smart enough to avoid things that are bad for them. They learn fast and can cause incredible damage in a short time. Never underestimate a raccoon. Their relationship with the rodents is somewhat symbiotic. They help each other invade a cozy building assembly.
    9) Finally, be thankful your pest problem is only related to rodents thus far. My brother had a grizzly bear rip two logs out of the corner of his house! Not only did this interrupt the continuity of his control barriers but was extremely difficult and costly to fix. Bears tend to return and do bizarre and costly damage even if they are not rewarded by food and Grizzly bears are federally protected animals that often as not return to the scene of the crime even after being live trapped and relocated. His pest control barrier now consists of unsightly electric fencing! I have not quite got that serious with the war I have with invading raccoons and mice....yet.

    1. GeoffPritchard | | #6

      Thanks Wannabe,

      I am fully aware of the rodent war(s). My recent problem with mice ended up in 6 yr old construction where they found 1 single penetration unsealed going into the crawlspace. I'm guessing 50# of dog food was packed into the 2 or 3 wall cavities they were in. We do live on a farm at an interface with the forest. Wood rats etc tend to stay down near the barns if they're around much at all. It's the deer mice that get into the cars, farm rigs etc. It seems like with one's best efforts, we can keep their numbers below Biblical plague levels.

      Because we need to keep fighting the rodents, I worry about some of the methods being suggested that might make 'checking' for mice difficult if not impossible (covering the joist bottoms with rigid foam or plywood).

      1. Wannabegreenbuilder | | #7

        I use both screen and hardware cloth as a control layer. Not perfect but if your problem is only deer or field mice then it may work to protect whatever you decide to use ultimately. Please report back what you decide on for your other control layers or better if you choose to condition your crawl space in some way.

        1. GeoffPritchard | | #9

          I should mention that while I've not had a grizzly bear assault my house, I did a lot of work with them when I was a graduate student years ago. They are amazingly adaptive and intelligent problem solvers. IMO - if we didn't invent weapons, we wouldn't be running this planet.

          EX- we gave the bears some 55gal plastic barrels to entertain themselves with and noticed that they would stand on the barrels and hold onto the cage fencing for balance. So we wondered if they would be able to use the barrel as a tool to get something. We tossed deer leg onto the wire top of the cage, just out of reach of the bear. Immediately after seeing he couldn't reach the leg, he went to the barrel and moved it close to the leg but not exactly under it. Climbed up. Almost got it. Got down moved it directly under and victory. This all happened in 5 minutes or less.

  5. GeoffPritchard | | #8

    Still considering the conditioned crawl space idea but I have a few more questions regarding the how to.

    1) If I insulate the crawlspace walls with rigid foam board, what R value do I shoot for? The same as underfloor insulation code requires for my zone? Rr19/21.

    2) Since I'm now going to be heating the crawl space that only has a vapor barrier over the soil, won't I be using more energy to get the same level of comfort in my home as the crawl space soil isn't warm?

    3) Do I need to install some sort of air circulation system continuous with the house to pump the conditioned air down and back? Again, this seems like it would be bringing cooler air back into the house space. We don't have any ducting at all as we heated with a woodstove and still do some. We have a Toyo oil stove too. Mini-splits were not a real viable option either --- they wanted over $20K to install.

    1. Expert Member
      NICK KEENAN | | #10

      Code for Zone 5 and 6 is R15 continuous or R19 between studs for walls, R10 for basement floor. In general foam has to be covered with a fireproof covering for safety. Certain polyiso foams have a fire rating and can be installed uncovered, however they can only be used on walls. It's common in crawl spaces to put a thin layer of concrete on the floor.

  6. Expert Member
    BILL WICHERS | | #11

    If you use polyiso, and you CAREFULLY score one side, you can snap a sheet in half while keeping one of the facers intact. This lets you fold the sheet to fit it through an otherwise too-small crawl space opening. Once you're in the crawl space, fold the sheet back out flat, and either tape the cut to make it a "rigid" sheet again, or just fasten it to the wall and let the wall hold the sheet flat.

    I'd try to seal up that crawl space instead of re-insulating the floor. If you ever get rodents in that crawl space again, they'll make a mess of any fiber insulation you use (including mineral wool). Rigid insulation on the walls is less of an issue with rodents, and even if they do chew some of it, it's easier to fix with just some canned foam.

    I had the same problem in my crawl space, my attic, and even some apparently randomly stuffed fiberglass in one spot on my basement ceiling (no idea why that was there, both sides were conditioned). I found mouse poo in most of it. I've replaced most of it so far, with a lot of attention going into rodent proofing things. I've used hardware cloth where ventilation is needed, and metal flashing where it's not. I've also embedded hardware cloth into canned foam where needed. You need a chew-proof metal barrier anywhere they might try to get in.

    I've used a combination of old-school mouse traps and some fully automatic, repeating, all-natural mouse traps that have an intelligent "search and destroy" function. These all-natural mouse traps are more commonly known as cats :-) They are pretty efficient with anything that gets through the other layers of protection.


    1. GeoffPritchard | | #12

      If I choose to insulate the walls of the crawl space, what do I need to do to the floor of the crawl space (currently soil w/ black plastic poorly covering it)? Do I need to insulate the floor? If I don't, will my crawl space be drawing heat from my house due to the soil heat sink? If I do want to insulate the floor, how can I do it so that it will tolerate a person crawling around on it?

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