How do I waterproof and insulate a basement?


We are finishing a concrete block basement as an apartment. What is best way to waterproof then insulate exterior walls that provides maximum floor space? We had a home in Connecticut built in 50’s with block wall, 1x2 furring, no insulation and smallest electrical boxes available. I don’t want that!

  11 answers
  • Ken Erickson Ken Erickson on Mar 20, 2019

    I have used Thompson's Water Seal on concrete. Also, you need to ensure your exterior grade is proper and minimize water damage potential. I would build a 2x4 wall and insulate it. I would consider rigid foam since fiberglass may absorb moisture. You could glue foam to walls to get a complete insulated barrier and then install wall over that.

  • Kelli L. Milligan Kelli L. Milligan on Mar 20, 2019

    First you need to get a water proofing co out to do the water proofing. Then you can frame out the walls as usual with 2x4, run all electrical and plumbing. Add full insulation and drywall.

  • Lifestyles Homes Lifestyles Homes on Mar 20, 2019

    Do your walls weep now? Is there mold in the basement now? Do you have any idea about drain tile or French drains around the exterior? Do your downspouts extend 4’ away from the house? Are your window wells, well protected from moisture? Do you have a anti-back up valve on your floor drain?

    Im trying to determine how much water proofing you need and any possible mold remediation and removal. You can’t leave dead mold there and encapsulating it is not advised.

    As far as insulation is concerned you’ll need expanded foam sheet insulation with the foil face. Adhere it with a Liquid Nails Type product between the studs. Check for low odor.

    Youll need to use treated lumber sole plates. If you don’t flat stud your walls, then you can put in a higher R-value, but TapCons won’t be as easy to use.

    You’ll need “basement board” drywall, which is Georgia Pacific’s tradename for it. I pay about $1.50 more per sheet than regular drywall and it’s $5 a sheet cheaper than “green board” which is mandatory in all bathrooms and garages.

    What is the legal form of fire egress? You can’t use an old fashioned basement window, as egress. Too small and too high up. I had one of my windows cut out, excavated, new drain tile, new large window well with a fire ladder and new high quality large egress window installed.

    It was the cheapest way I could legally add another bedroom to my house, as in I’m saying it was relatively expensive, but 1/10 the price of adding on a bedroom.

    You should also install a high CFM bathroom exhaust fan.

    • John W John W on Mar 20, 2019

      We have a 5’wide double door with glass for egress. You have great suggestions. Thanks

  • William William on Mar 20, 2019

    Don't use Thompson water seal. Needs to be recoated every few years. Great for decks, patios, and driveways. Use Drylock waterproofing paint. Made for your purpose. Absorbs up to one inch into the block making it waterproof. For maximum insulation you will need to fir out the wall, insulate, then drywall. Without any wood firing there is nothing to attach drywall to. Also check your local building code requirements.

  • Seth Seth on Mar 20, 2019


    You will end up having to sacrifice floor space for a proper build out. After you have applied a water resistant coating (nothing you do inside is truly waterproof) you would apply 2" of rigid foam as a thermal break. You can fur the foam in order to hang your drywall or build a traditional stud wall after the rigid foam (makes the electrical and plumbing much easier) and add additional insulation. You will also want to insulate the floor. I am also assuming you plan on insulating the ceiling for sound deadening purposes. You may find you still need to dehumidify depending on what type of HVAC is servicing this space.

  • Lifestyles Homes Lifestyles Homes on Mar 20, 2019

    You must insulate the space between ceiling joists (floor joists above) with R-5 or higher, per Code. That exact insulation saved my house from burning to the ground, BTW.

    Back to egress, if the pair of 5’ doors are directly accessible to the proposed rental space, then it should “fly”.

    You will need to provide reasonable air exchanges and in some states it’s 8% Natural Daylight and 4% Natural Ventilation, per square foot. So, be glad you don’t live there, lol.

    Back to water proofing: the best solution. Is to stop ANY chance of water getting in to begin with- as from the outside.

    Apparently you have water infiltration issues, as you wouldn’t have mentioned it otherwise?

    • See 1 previous
    • Lifestyles Homes Lifestyles Homes on Mar 21, 2019

      Look up your suburb’s Municipal Code on line.

      At a minimum it will have planning and zoning requirements for making the basement a legal rental.

      Its up to the City if they choose to add their interpretation of the National Building Code, which is called the IRC (international Residential Code) and their Energy Efficiency Code.

      The EEC will specify minimum mandatory insulation requirements.

      When you fur-out the walls, you will need to use TapCons to fasten them to the blocks.

      Most basements are flat studded to take advantage of the TapCons (fasteners) and thus the sheet foam insulation is glued in between, 2 layers thick.

  • Setsukograb Setsukograb on Aug 23, 2021

    Don't you think that real professionals in this field should handle such things? If you do something wrong, you can flood your basement. Or the pipe in the basement will start leaking a day after installation. You won't know about it, and you'll come only when the water is already knee-deep there. I know what I'm talking about. It happened in our country house. It's better to order crystal waterproofing to be calm in the safety of the home than to make a mistake yourself and then pay twice as much.

  • John W John W on Aug 25, 2021

    This project was completed last year using dryloc and a stud wall away from block . I dont know why Setukograb posts this answer two days ago which does not answer the question but reads like an promotion to hire him...

  • Deb K Deb K on Sep 05, 2021

    Hello, you could use liquid rubber to waterproof, then use the Styrofoam SM type insulation as it is meant for exteriors

  • John W John W on Sep 05, 2021

    Again, this project is complete and I have no idea how to resolve