Basement Insulation Building Science

by Dave Murray, OVRX

Basement insulation, specifically extruded polystyrene (XPS) insulation, is the best basement insulation to use for residential basement construction.

Dow Chemical invented XPS insulation, giving it a closed cell structure that proved to be ideal for use in basement insulation projects. There are no holes or voids that would allow water to penetrate. In fact, Dow’s XPS insulation is so waterproof that it is used to make billets for floating docks. Because it has high compressive strength, it will not sag or flatten like fiberglass insulation and will maintain its R-value over time.

Homeowners in Canada and the northern tier of states in America that have basements have been benefiting from XPS basement insulation on basement walls and floors for over 30 years now. However, the installation of basement insulation has evolved over time. Old ways of installing basement insulation die hard and it is amazing to realize that these outmoded installation techniques still occur today. This is the domain of contractors who resist change even if a better installation system exists.

For example, old-style basement subfloors are still being built using basement insulation between 2″ × 4″ dimensional lumber or “sleepers” installed with screws or nails directly to the basement floor. To complete this traditional basement subfloor system, a polyethylene (PE) vapor barrier is loosely laid over the sleepers. Then plywood is fastened to the sleepers with hundreds of wood screws through the PE vapor barrier, making it the plastic equivalent of “Swiss cheese”. This is required to keep the floor from warping so that it stays tight to the floor.

This basement insulation subfloor system “works” but it is clunky and has a flawed design. It is at least 2" high, which reduces headroom in the basement, a space usually challenged for height. As well, because the vapor barrier is punctured in hundreds of places, it no longer works as a vapor barrier.

The evolution of the basement subfloor system was to first apply rigid XPS basement insulation directly to the concrete floor to create a continuous layer of insulation that was not interrupted by floor-framing studs. Plywood was then installed on top with adhesives and Tapcon concrete screws were fastened directly into the concrete to keep the plywood from warping.

Even this subfloor system was not entirely perfect since it was not friendly for the typical do-it-yourself (DIY) homeowner who did not have the expensive power tools and compressors required to install this basement insulation subfloor system.

Now subfloors with basement insulation have evolved into a system that is easy to install for both contractors and DIYers alike. With the new air gap insulated and modular subfloor panels, such as the floating Barricade subfloor system, which incorporates R3.2 XPS basement insulation bonded to a 2″ × 2″ dimensionally stable oriented strand board with a tongue and groove edge, installation is a snap.

This subfloor system does not have to be fastened to the concrete floor because it “floats” and eliminates the use of 2″ × 4″ dimensional lumber. This saves money on material costs, installation time and virtually eliminates mold by removing cellulose in the wood as a food source.

The modular panels have an airtight gasket at the seams, which ensures a continuous vapor barrier and provides moisture protection to the finished basement floor and the furnishings in the room. The Barricade subfloor system is only 1-1/8" thick, with minimal impact on headroom in the basement.

The method used for basement insulation installation to basement foundation walls was to first tip-up floor assembled dimensional 2″ × 4″ pine lumber studs or even galvanized steel stud framing to the perimeter foundation walls, and then nailing the structure into the ceiling floor joists and the concrete floor. This is an awkward and intimidating task for the average DIYers to perform.

Batt fiberglass in the wall cavities between framing studs is still being used in basement insulation, meaning that only the spaces between the studs are insulated. This insulation absorbs and holds water, reducing insulation performance and leading to mold growth. If the concrete or block walls leak, or even get damp, the lumber and basement insulation will hold the moisture, creating problems with staining, rot and mildew and mold growth.

Rigid XPS basement insulation can be applied to the entire wall, providing a continuous layer of thermal protection and increasing the overall R-value and energy efficiency. XPS basement insulation resists water absorption and does not provide a nutrient source for molds.

Now the application of proven building science has evolved for basement wall insulation with the development of the Barricade system of modular 2″ × 8″ wall panels that integrates XPS basement insulation with an insulation rating up to R12 bonded to an OSB panel that eliminates the need for wall stud framing. This easy-to-install basement insulation system saves on installation time for DIYers and contractors, provides a continuous vapor barrier, provides a barrier to moisture penetration and blocks excessive noise. It is the perfect product for basement wall insulation, combining over 30 years of building technology evolution.

Industry research has determined that the most efficient way to insulate basements is from the exterior. External insulation not only cuts heat losses, but acts as a second barrier against moisture and freeze thaw action. However, owners of older homes who cannot place insulation on the exterior because of economic impracticality can insulate the interior walls to achieve remarkable heat loss savings. There are several ways to install insulation on the interior using XPS basement insulation but the easiest to use is the Barricade insulated wall panel system.

What kind of heat loss savings can be expected from applying basement insulation to the interior of basement walls that did not previously have basement insulation? The results of a thermal computer analysis program determined that using 40 mm (1-1/2") XPS basement insulation, the same as was used on the R12 Barricade wall panel which covers the full height of the interior wall, cuts heat losses by as much as 62%.

It is remarkable what has evolved in basement insulation over the past 30 years of building science technology.

Parameters of thermal computer analysis:

• Fine grain soil around basement foundation
• 40 mm (1-1/2") XPS basement insulation
• Assume no heat loss at box sill and sill plate interface
• 7" 0" interior basement wall height
• Toronto 3,888 HDD C° or 7000 HDD F°
• Montreal 4444 HDD C° or 8000 HDD F°
• Quebec City 5000 HDD C° or 9000 HDD F°
• Edmonton 5555 HDD C° or 10000 HDD F°
• Saskatoon 6111 HDD C° or 11000 HDD F°

This article was provided by OVRX. Visit them online at www.ovrx.com.

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