insulate and moisture manage foundation walls

[msimerson]

Code

• 2012 International Residential Code: R-15

which side of foundation walls?

• exterior
• interior

interior

issues with Interior Foundation Wall Insulation

1. The foundation wall gets cold. Foundation walls must have perfect moisture management (footings, exterior walls, rim joists) to prevent moisture in the concrete walls from impacting other systems.
2. Most interior foam board (except foil faced polyiso) insulation requires a fire barrier inside the insulation
3. It's really hard to:
   • dry wet basement walls that are insulated
   • install capillary breaks in existing systems
   • manage moisture in foundation walls
     • is there a capillary break between rim joists?
       • to prevent moisture in exterior walls from migrating to rim joists
     • is there a capillary break between footings?
       • to prevent ground moisture from wicking up the foundation walls?
     • is there proper drainage for foundation / footings?
       • so water can drain away
4. It's impractical to retrofit capillary breaks in existing foundation walls

Interior insulation options

• Three inches of unfaced XPS (R-15)
• Four inches of unfaced expanded polystyrene (R-15)
• Three inches of ccSPF (R-18)
• Three inches of polyiso (R-18)
  • this option satisfies the fire barrier requirement

interior process

• dig a drainage channel along footings
  • install drain pipe and gravel in channel
• adhere 3" polyiso foam board to walls
  • caulk and seal the bottom edges
  • seal the insulation at the top with spray foam
    • all the way up the rim joists (to at least 2" depth)
• optionally cover polyiso with drywall (fire barrier)

exterior process

• dig trench around the house, down to the footings
  • prepare the concrete surface
• waterproof the building (keep water away)
  • assure footings have proper drainage
    • to french drain, sump or daylight
  • add impermeable layer sloped away (3" in 4') from the building
    • option: clay layer on top
    • option: buried plastic, coarse gravel (for drainage), then fabric, soil
    • mulch 2+ feet from building for insect control
• dampproof the exterior foundation walls (capillary break)
  • option: liquid applied
  • option: membranes
  • down to the footings
• Insulate exterior surface of foundation walls to at least R-15
  • 4" mineral wool board (Roxul Comfortboard, etc..)
    • a green building material
    • repels water and insects
  • 2.5" high-density ccSPF foam
  • Foamglas, Thermacork, etc..
• cover insulation with protection layer from siding to 8" below soil
• insulate rim joists
  • 1 inch of ccSPF (enough to air seal, minimal to provide drying potential to interior)
  • only after exterior surface of rim joists are insulated

References

• INSULATION
• Adding Insulation to Basement Walls
• Basement Insulation
• Understanding Basements
• foundations: moisture resistant construction

[msimerson]

quotes

• Clean Crawls: 3" polyiso around interior perimeter of crawl space: $1,700

[msimerson]

Image archived from Fine Homebuilding

[msimerson]

• depends on insulate and pour concrete crawl space floor #23 (pour concrete crawl space floor)

[msimerson]

• do crawl space #2 seismic upgrades #24 (seismic upgrades) as well before this. aka: depends on crawl space #2 seismic upgrades #24

[msimerson]

I insulated the walls on the exterior. The primary reasons were the difficulty of adding a capillary break between the walls and sill plates, the longest section was dug out coincidentally when constructing the gabion wall on the East side, and leaving the inside uninsulated permits the wall to dry to the interior.

The plan

East wall

The first layer of insulation extends down the the footing. At this stage, a second layer was added extending 8" below grade.

Southwest wall

This wall is dug down to the footing, brushed clean with a broom, brushed vigorously with a wire brush, and ready to be waterproofed.

SW wall waterproofing coat 1 of 2

SW wall insulated

SW wall getting armored

[msimerson]

It's not so easy or fun to apply the Tuff-R (think: elastomeric stucco) in the trench. The trench also gets much narrower after adding in the 4" of insulation. When I got to the SE wall, I smartened up and applied the fiberglass sticky mesh and Tuff-R coating to the insulation before putting the insulation in the trench.