Insulating Existing Homes

Insulating an existing home can drastically reduce energy bills and create greater comfort throughout the house. Where I live in the northeast, it is very common to see houses that are over 100 years old. These houses are generally drafty, cold in the winter, and hot in the summer. But with the state programs available, many of these homes become much tighter and more insulated, and for a lower cost than most anticipate. This article will cover the most common ways to insulate an existing home, creating a more comfortable and efficient home.

Where to Insulate and How

To keep the article brief and to the point we will work through the building envelope and provide bullet points about each area to give you a general idea of what materials can be used, and what to expect if you plan on insulating.

Exterior Walls

The most common way to insulate exterior walls in an existing home is to insulate the wall cavities.

• Insulation Type: Blown Cellulose - made of recycled paper that is fire resistant and packs tightly filling the cavity.

  • Method 1: The insulation company will cut holes in the drywall, usually 16” apart (depends on framing), and fill the wall cavity with cellulose, filling from floor level up to the ceiling.

  • Method 2: The insulation company will remove shingles, cut holes in the sheathing, and fill the wall cavity with cellulose. This is less common through state programs, but does happen on occasion.

A less common way to insulate exterior walls is by removing the siding, and installing foam board before re-siding.

• Insulation Type: Rigid Foam Board - XPS, EPS, Polyiso (these insulation types will be described below)

  • The insulation company will remove the siding, and install rigid foam board against the sheathing, creating continuous insulation on the exterior of the home. This is not covered by state programs, but if you need new siding, this is a great way to insulate. Although the R-value will likely be lower than filling the wall cavity, you create continuous insulation that has no breaks.

Floor cavities

Floor cavities are not a part of most state programs, but can be insulated by an independent insulation company, or by the homeowner. If you plan to insulate your home’s floor cavities, focus on areas over unconditioned space such as the floor over an unconditioned basement/crawlspace, floor over a garage, or over ambient air.

• Insulation Type: Fiberglass Batts or Rockwool Batts

  • These batts fill the floor cavity over unconditioned space, preventing heat transfer between conditioned space and unconditioned space. These types of insulation need to fill the cavity, and you may need to cut the insulation to fill gaps around wires and pipes.

• Insulation Type: Spray Foam

  • This is less common for floor cavities, but the benefits are that the cavity will be completely full, and if closed cell foam is used, it will air seal between conditioned and unconditioned space.

  • The downside of using spray foam, is that wires, pipes, or ductwork between joinsts will be buried in the insulation, making it difficult to fix or remove in the future.

Rim Joist & Foundation Walls

• Insulation Types - Foam Board, Batt insulation, or Spray foam

  • Foam board

    • Foam board is commonly used in in rim joists when the work is done through a state program. Foam board is cut to size, placed in the rim, and canned foam is sprayed to fill the gaps between the foam board and the framing.

    • Most state programs do not insulate foundation walls, but homeowners and professionals use foam board to bring the basement into conditioned space and keep the basement at a more stable temperature.

  • Batt insulation

    • Batts can be used in the rim or at the foundation wall, but this can lead to issues in the future.

    • Basements with moisture issues and batt insulation can develop mold and mildew, so foam board is a better option.

  • Spray foam

    • Closed cell spray foam is an expensive but effective way to insulate both the rim, and foundation walls. It is a high R-value, moisture resistant insulation that is consistent in depth and air seals any gaps to the exterior.

    • Open cell spray foam is often used for the rim, but it is not used for foundation walls unless 1”-2” of closed cell is sprayed first. Like fiberglass batts, open cell is not a vapor barrier and can hold moisture.

Attics

• Insulation Type: Blown Cellulose, Fiberglass Batts, or Spray Foam

  • Blown Cellulose

    • Cellulose can be blown into uninsulated attics, or blown in on top of existing insulation to increase the R-value of the attic.

    • Cellulose is a great option because it fills cavities, it settles around penetrations, and after it settles it creates a blanket like layer that sits over the joists, creating a continuous layer of insulation.

  • Fiberglass Batts

    • Fiberglass batts are commonly used because they are easy to work with, and they can be moved without disturbing dust and creating a mess.

    • If you insulate with fiberglass, it is recommended to install it between the joists, then lay another layer in the oposite direction over the joists, eliminating the thermal break.

  • Spray Foam

    • Spray foam will be installed at the roofline, bringing attic spaces into the thermal envelope.

    • It is common to see closed cell, open cell, or a combination of the two.

    • If you plan on finishing the attic in the future, closed cell can be installed so you can have a high R-value, but still hang drywall against the rafters.

Insulation Types

Foam Board

• Expanded Polystyrene (EPS or Beadboard): Most affordable foam board, good for ground contact (exterior slab insulation), lower R-value per inch compared to XPS or Polyiso.

• Extruded Polystyrene (XPS): Highly moisture resistant, compressive strength, ideal for below-grade foundations and basements because of its moisture resistance.

• Polyisocyanurate (Polyiso/ISO): Up to R-6.5 per inch, often foil-faced to create a radiant barrier, common for walls, less moisture-resistant than XPS.

• Closed Cell Foam - Very expensive, high R-value, moisture resistant

Fiberglass batts

• Pre-cut and made from spun glass fibers and they come in a variety of thicknesses and R-values up to R-49. Fiberglass batts can be faced (with vapor barrier) or unfaced options, and are designed to fit standard 16-inch or 24-inch frame spacing.

Rock Wool

• Rock wool is made from molten rock and recycled slag, with great fire resistance, great acoustic dampening, and is mold-resistant. Rock wool is harder to work with and more expensive than fiberglass, but is great insulation.

Spray Foam

• Spray foam is a liquid polyurethane insulation that expands on application, creating a dense insulating foam. Closed cell is great at sealing gaps and creating airtight barriers, while open-cell is great for soundproofing. These insulation types are expensive, but have a high R-value and do not degrade or settle.