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Retrofit sealed basement and crawlspace in 1870 home

bigwillystyle | Posted in General Questions on

I want to create a sealed crawlspace and basement for the known benefits to doing so, and to prevent damage to new HW flooring to be installed on the first floor.  I’m familiar with the process and various methods to achieve a similar result. I’m looking for feedback bc there are some obstacles that make the install a bit tricky as well as new products available.  We’ve been here one year and have a blower door test scheduled for next week. I should add that we’re going to make some improvements this spring/summer to insulation in the attic, but will wait for recommendations from the auditor. About 1/3 of the house is built over crawlspace accessed behind the stairs going down to the basement, it’s a tight squeeze and a small hop up into the crawlspace under the kitchen.  This space connects to the crawlspace under the dining room, the bottom of the joists rest no more than a foot above the dirt and there are tree roots growing through it that look like tumbleweeds.  The rest of the house is over full basement about 7′ all with dirt floor. Boiler is in the basement.  Total footprint of house is 1100sf x 2 story = 2200sf My plan right now is to pull up the subfloor in the dining room in order to access that crawlspace.  Is there any other logical option? To take wisdom from Lstiburek and make it “air tight and vapor tight” the easy and cost effective solution is to use 15 or 20 mil vapor barrier throughout.  I recently stumbled upon this product Delta MS which seemed promising for areas of the floor that might see foot traffic but the vapor perm rating is 0.3 yet they advertise it as vapor impermeable???   Budget will allow insulating at least some of the exterior walls.  Closed cell is roughly $2.2per inch*ft here.  A little additional info, New Englander in NH 03857 built in 1870 that sits on a granite/stone foundation, rare very minor water intrusion observed in the basement that appears fixable by patching a few holes in foundation. Steam heat kept us quite warm through the winter, and over the last 12 months we used 1600 gal fuel oil for heat only.  75% windows have been replaced, remaining original windows have exterior storms. I’m sure you guessed there’s no insulation anywhere other than the couple inches in the attic joist bays.  Thanks everyone for taking the time to read this.

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of issues here. I suggest you do more reading to get a better handle on the range of approaches you might consider. Start with these articles:

    "Fixing a Wet Basement"

    "Building an Unvented Crawl Space"

    "Using a Dimple Mat to Keep a Basement Wall Dry"

    Here are two steps you should consider that you didn't mention:

    1. Lowering the level of the dirt floor in your crawl space and/or basement. This work usually involves an entrenching tool or short-handled shovel, along with several empty drywall buckets and at least two strong teenagers who are willing to accept low wages. Consult an engineer before you undermine the footings.

    2. Insulating the walls of your crawl space and/or basement with closed-cell spray foam.

    1. bigwillystyle | | #2

      Thanks for the info Martin. I want to be cheap, but insulating the foundation walls in the crawlspace seems like a smart choice.

  2. Peter Yost | | #3

    Hi William -

    You mention insulation more than once in your characterization of your building and solutions. Just a reminder that air sealing is even more important than insulating any space, but particularly your attic.

    And as you do work to manage moisture and air flow in your complex spaces, add radon measurement and possible/(your location: likely) mitigation.


    1. bigwillystyle | | #4

      Thanks for the advice Peter. I'm very fortunate to have experts like you actively sharing knowledge on forums like GBA.

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