Restoration Guide: Foundations and Insulation

Roger Diez

Editor's Note: This is article 6 of 9 in Chapter 1: Foundations section of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.


Section 1--Overview

There are two ways to maintain thermal comfort in below-ground living spaces: Apply insulation to the interior walls and to the exterior walls. However, recent changes in building codes limit the use of exterior foam plastic insulation because it can allow undetected access by termites and carpenter ants. Check local building codes before undertaking exterior basement insulation as part of a home renovation project.

Section 2--Insulating Below Grade Walls

2.1: Exterior Insulation

Exterior applications can vary from half-wall to full-wall insulation, depending on heating fuel costs and heating degree day (HDD) requirements. Additionally, exterior insulation can protect waterproofing and dampproofing materials and reduce condensation problems. However, building codes in some areas do not allow continuous exterior insulation due to the need to observe possible insect pathways.

2.2: Interior Insulation Covering the Entire Wall

When applying interior insulation to a basement wall, either rigid foam board or batt-type insulation, furring strips are used. These provide for nailing foam board insulation, drywall, or paneling, and accommodate electrical wiring.

2.3: Types of Insulation

Rigid foam board exterior insulation is available in several types. Extruded polystyrene (XPS) board has superior performance to expanded polystyrene (EPS) board, and there are also products treated with borate to discourage infestation by carpenter ants and termites.

Mineral-fiber and fiberglass boards also perform well. Newer developments include grooved XPS boards to allow drainage against foundation walls. EPS and polyisocyanurate boards can be used for interior wall insulation, but the latter should not be used where water is a problem.

In some areas, building codes restrict or prohibit the use of foam products below grade due to the danger of insect infestation. One solution to this is the use of a "vision strip" around the foundation's perimeter to allow inspection for insects. However, this seriously reduces the insulation's effectiveness.

Section 3--Insulating Crawl Spaces

Insulation of crawlspaces helps to protect ducting and plumbing in the crawlspace from freezing. It also helps to stabilize temperatures in living spaces above the crawlspace. Insulation strategy is different for vented and unvented crawlspaces.

3.1: Insulate Vented Crawlspaces

This technique uses batt-type insulation placed between first-floor joists. This is an economical solution using fiberglass or mineral wool batts. It protects pipes and ducts running between the floor joists. However, installation may be difficult if there is limited access.

3.2: Insulate Unvented Crawlspaces

This technique treats the crawlspace the same as a basement, insulating either interior or exterior crawlspace walls, depending on access. Use of rigid foam boards has the additional advantage of reducing moisture, pests and insects, and odors. However, insulating an unvented crawlspace can be more expensive than insulating a vented crawlspace.

Section 4--Insulating Slabs

In a home restoration project, insulating slabs can only be accomplished by adding insulation to the exterior masonry stem wall or to the shallow perimeter grade beams.

  1. Exterior insulation for slabs with stem walls: Insulation should be added to the full depth of the stem wall. This depth varies depending on the depth of the frost line.
  2. Exterior insulation for slabs with grade beams: Insulate to the full depth in areas of moderate to high heating fuel costs.
  3. Insulating basement slabs from inside: Seal the existing slab floor with a vapor barrier, and then fasten pressure-treated wood sleepers to the floor. Add a second vapor barrier on top of the sleepers, then nail plywood or OSB sub-floor to the sleepers and finish with wood or tile flooring.

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