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Everything about Insulating a Basement


Basement insulation involves the application of an insulating layer on a basement’s exterior wall, interior wall, or both. Floors, ceilings, and joist header spaces can and should also be insulated. Different materials can be used for this, the most common being rigid foam board like XPS insulation (also known as extruded polystyrene), spray foam, and fiberglass.

Should I Insulate My Basement?

The short answer is yes.

Basements often include large, uninsulated surface areas below and above grade level, making them a big contributor to a home’s total heat loss. Heat gets lost through penetrations like cracks, windows, and the sill area (the top of the foundation wall). Ontario’s climate is generally severe. With lows of 30 °F not uncommon in Toronto, insulation is vital for making the basement functional, improving the comfortability of other parts of the house, and reducing energy costs.

Other benefits are the prevention of mould and mildew formation, an increase in your home’s resale value, improved air quality, and a better balance of the cooling and heating cycles from one season to the other.

How to Insulate a Basement

You can insulate the basement wall, the ceiling, and the floor. Before you embark on basement wall insulation, ensure that the basement is dry. Water problems such as flooding and persistent leaks must be corrected through such solutions as damp-proofing, waterproofing, insulation from outside, the addition of a drainage system, and excavation.

Another prerequisite is checking for signs of dampness on the finishes and foundation and correcting the same. Signs include mould growth, staining, peeling and blistering paint, spalling, a musty smell, and efflorescence. You can correct minor dampness from the inside by applying a moisture barrier to reduce the movement of moisture from the foundation and major issues from the outside.

Next, check for cracks. If there are active cracks, consult a professional to determine if the basement requires some structural repairs.

You can insulate from inside or outside. Insulating from the outside is the best option, but insulating from the inside is cheaper and may have added practical benefits.

Exterior basement insulation involves excavation, insulation, waterproofing, protective coating and flashing, and drainage system and backfill. Before you begin digging, get a plan on where gas, telephone, water, electricity, and sewage are entering your home – this information is usually provided free of charge from utilities.

How to Insulate Basement Walls in Ontario?

You can insulate the basement wall from the outside or the inside.

Insulating Basement Walls from the Outside

  1. Dig a Trench

Start by digging the trench, going down to the footing level, never below. Make the trench wide enough for work to continue uninhibited. If the soils are not stable, do bracing to prevent collapse. Take preventive action to keep people and animals from falling in. If the drainage system and foundation walls are in good condition, installing insulation to a minimum of 610 mm below grade is sufficient. You can add a horizontal rigid insulation skirt to mitigate frost-related issues.

  1. Prepare the surface

Use a scraper and a wire brush or a pressure washer to clean the surface of the foundation. Inspect the foundation and then repair any major cracks, holes, and damage. After this, seal all penetrations. Replace or smooth old parging and deteriorated surfaces. Check if the drainage tiles are in good condition and repair them if necessary. If there’s no drainage system, install one with the help of a drainage systems specialist. With the help of a contractor, apply waterproofing material from the grade level to the top of the footings and over them and seal all overlaps and penetrations using sheet materials, roll-on compounds, and sprays.

It is recommended you use 2 layers of insulation wherever you have overlapping joints. Use flashing or washers and corrosion-resistant fasteners to hold the insulation in place. The portion of the insulation that is below grade will be held in place by the backfill, but fasteners may also be required to hold it in place during the backfill process.

  1. Apply the insulation

You are now ready to apply the insulation. The most popular options for exterior basement wall insulation are rigid mineral wool boards, polyurethane/polyisocyanurate boards, and high-density Type IV polystyrene boards. The latter is the most ideal for external below-grade applications.

  1. Apply flashing

You need flashing to keep the insulation in place, for a good junction, and to prevent water from getting behind the insulation. The location of the flashing determines how far up the wall the insulation will extend. Use standard Z-flashing at least 2 inches behind the siding if the siding can be pried up or partially removed. If not, use a wood flashing (after the installation) or a metal J-channel (before the installation), with suitable caulking to seal the joint between the flashing.

  1. Cover the exposed insulation

Cover the exposed insulation to protect the insulation from physical damage and sunlight. You can do this using expanded metal lath with cement parging, polymer-modified pargings, or pressure-treated plywood that has stainless-steel fasteners vinyl (other siding matching the house siding). You can then backfill the excavated area.

  1. Finishing

For windows, wrap the insulation around the foundation to meet the frame. Use lath and parging from the top of the insulation to the edge of the frame and caulk the joint between the parging and the frame. Outline doors with a J-channel or an equivalent flashing and extend the doorsill to protect the flashing under the door. Penetrations like electrical conduits and gas lines should also be sealed with a flexible sealant.

Insulating Basement Walls from the Inside

The choice of material for inside basement wall insulation depends on the basement type and condition. You should also consider indications of structural problems such as bulges and cracks, insulation requirements such as RSI (R) value, plumbing and wiring, and finishing details.

The most commonly used materials for inside basement wall insulation are batt or blanket, rigid plastic board, glass fibre loose-fill, and polyurethane spray. Engage a certified installer if using closed-cell polyurethane spray foam.

Rigid plastic board insulation is popular because of its higher RSI value per millimetre compared to batt insulation and because it requires a thinner supporting framework and less basement space. Rigid board insulation application starts with air sealing the old walls, followed by applying the insulating material, and then finishing. This material works best for even, vertical walls as the material is fairly rigid.

Do You Need to Insulate Basement Walls Below Grade?

Below grade basement is the part of the basement that is just below the ground level. Indeed, you can insulate basement walls below grade. This is important because it helps in the house’s thermal performance. It also helps protect the concrete from freezing, which could lead to damage. The decision should be based on the type of wall and the environment, meaning Ontario homes need the insulation because of the sub-zero winter temperatures.

How to Insulate Basement Floor

Most basement floors in Ontario are made of concrete. You should consider basement floor insulation for comfort. The ground beneath the concrete is cool, and heat rises. The lack of heat near the floor leads to cold feet. Concrete basement floors are also susceptible to condensation and, consequently, mould spore growth. The transfer of heat through bare concrete floors also leads to higher heating bills.

So, what basement floor insulation options are available? The choice depends on the headroom (height between the floor and the ceiling) and, of course, the cost. The options available are a fully insulated basement subfloor (ideal for full-height basements), pre-fab subfloor panels where there is height limitation, and electric underfloor heating systems when you want a cozy floor and budget is not an issue.

You must first ensure that the concrete is properly sealed. Despite popular belief, concrete is porous. You should, therefore, start by cleaning the floor and then applying a concrete sealer. Below is the installation procedure for rigid foam boards like the ever-popular XPS rigid foam board.

  • For an insulated subfloor, install a layer of rigid foam board. The foam board should be at least 1 inch thick. The foam board adheres to the concrete with the help of a foam board adhesive.
  • After laying the rigid foam board, seal all the seams on the insulation. Use such tapes as Tyvek to tape and seal along the walls with spray foam.
  • Next, install pressure-treated sleepers, with anything over 3/4-inch pressure-treated decking shown to work well. Use masonry nails to attach the sleepers to basement floor insulation, with foam board adhesive applied along the bottom of the sleepers.
  • Apply a layer of 2/4-inch sub-floor material and screw the material to the sleepers using sub-floor adhesive and stainless steel screws.
  • Finally, install the finish flooring. The sub-floor will be sturdy, and it can support any type of flooring you wish.

How to Insulate Basement Header

The foundation header space, also called joist header space or rim joist space, is the area where the structure of the house rests on the foundation. The area is susceptible to air leakage and is not properly insulated in most Ontario homes. This leads to unwanted dust, drafts, vermin, and pollen. The method of air sealing and insulating the header space depends on the type of joint configuration. The 3 major configurations are:

  • Floor joists resting on a sill plate
  • Floor joists that are partially embedded in the foundation
  • Floor joists that are fully embedded in the foundation

In situations where the insulation of the exterior foundation cannot be extended to cover the full header joist area, air sealing and insulation has to be done from the inside. Where the insulation of the foundation walls is on the interior, ensure the vapour and air barrier are continuous from the wall and header space.

The best insulation is polyurethane spray foam, as it offers airtight sealing and insulation of the space. Ensure the spray form material is a fire-resistant material.

What Type of Insulation to Use for Basement Ceiling

Winters are no joke in Ontario. Other than insulating basement walls and floors, basement ceiling insulation further adds to the comfort and energy efficiency of your home. The best material for basement ceiling insulation is one that helps with moisture control.

Closed-cell spray foam provides a water vapour barrier. The material does not retain moisture, meaning you will not have a mould/mildew problem. It also has the added benefit of being sound absorbent. Spray foam is made of polyurethane and is capable of entering small crevices, after which it expands to completely fill the area.

How to Cover Basement Insulation

An unfinished basement has a less than inviting look to it. You should never leave insulation exposed in the basement, whether it is spray foam, fiberglass roll (batten), or foam board, since each material poses a hazard. An exposed insulating material is a fire risk; it can cause irritating skin issues and respiratory issues due to chemical irritants in the materials.

  • Spray foam: Spray foam is very flammable, and the International Residential Code (IRC) requires that it be covered with a 15-minute thermal barrier. Note that concealed spaces like crawl spaces may be exempt from this requirement.
  • Foam board: Just like spray foam, foam board also requires covering by a 15-minute thermal barrier for fire protection.
  • Fiberglass: Fiberglass is less of a fire risk as it is not flammable. However, fiberglass usually comes in a foil or a paper facing that can burn. You can only leave fiberglass insulation exposed if the paper facing is positioned against the inside wall, if there is no paper facing, or if it comes with a flame-resistant foil facing. In all other cases, use a thermal barrier to cover the fiberglass insulation.

So, how do you know the best insulation for basement? A professional will guide you in the right direction.  At Bernardi Building Supply, we provide the best basement insulation supplies in Toronto, and we will help you with your choice of material.