One of the most common questions I get on energy audits: Should I insulate my basement ceiling? The interior floors feel cold, the house is warm, the basement is cold, insulation slows heat loss so I should insulate my basement ceiling, right (customer looks at me for affirmation)? Actually it’s nowhere near that simple.
The answer surprises most homeowners…insulating between a conditioned basement and your heated house is not a good investment of your money.
What is Happening in the Basement?
When considering basement insulation, you have two choices: Insulate your basement ceiling or your basement walls. It turns out insulating your basement ceiling is not the most effective approach and can cause several unintended problems. Insulating the foundation walls may be more effective and less problematic.
Energy audits almost always show that insulating basement spaces is a great return on your financial investment. Besides the unfortunate weekend warrior who, with visions of 1000 extra square feet of warm cozy living space, finished their entire basement with uninsulated stud walls and drywall, basements are usually storage for extra books, clothes and the spare can of paint or turpentine. In other words, pretty easy to insulate.
Every time I encounter a chilly basement space with finished drywall on studs, inevitably there will be a conversation where I explain that drywall and concrete aren’t the best insulators (Pop Quiz: How do we know this? Because we never pour concrete into our main floor walls to insulate them.)
The Basement Ceiling and Heat Loss
The first thing is understanding the building shell. Or at least understanding what it is (understanding HOW building shells work is the multi-billion dollar question for several industries).
Simply enough, the building shell is the control layer that separates the interior heat, water vapor and air from the outside. The walls and roof that keep heat in and the rain out. In the basement, the foundation wall is the thermal boundary of the building shell. It’s just a really lousy thermal boundary.
This leads to what I call:
The Conundrum of Foundations and the Building Shell: (cue Hans Zimmer score). Concrete (or stone, rubble, brick, etc.) foundations are the threshold of the building envelope, not the main floor of your house. If your house is heated to 70 F, your basement is 55 F and the surrounding air is 10F, follow the laws of thermodynamics. Heat will flow from the main section into the basement and out into the surrounding air and ground.
I’ve had a couple emails about the 55 F temperature of the basement, suggesting it is mostly because of heat radiating from the main house. Not quite true. The ambient temperature of the ground maintains a big chunk of the basement temperature.
Imagine a four sided square. The top is 70 F, the bottom is 55 F, the top of the sides is 10 F increasing to 55 F by the bottom. If you average all sides, you get slightly below 50 F. The top and bottom are adding heat to the average and the cold top of the sides is subtracting a big chunk. End note
Concrete does a great job at holding your house up. Couldn’t ask for a better material to just sit there and support several tons of house. However, concrete foundations are piss poor at another functions of the building envelope, controlling heat and moisture. So how does a giant stone sponge with no insulating value work as part of the building shell? Generally, very poorly…making them a good energy savings opportunity (with many caveats).
What if…I Insulate The Basement Ceiling?
Knowing this, let’s insulate the basement ceiling of a house at 22 Hypothetical Rd. Don’t worry, it’s not your house. We’ve decided on unfaced 12” fiberglass batts and hired quality contractors. The works is completed and, yes, the floor is warmer.
Everything is great until winter starts. You notice your heating bill hasn’t improved that much. You’ve never had standing water but recently the basement has smelled mustier. Mold has started cropping up on underside of the floor joists. Ten years of maintenance-free plumbing ends this winter when the pipes freeze twice. Finally after installing a gorgeous new natural gas sealed combustion boiler (with on demand hot water!), the nightmare scenario occurs: you wander into the basement to find 4 inches of water from the burst pipe. So what happened?
Insulation slows the transfer of heat. Prior to insulating the basement ceiling, warmth is radiating from the heated first floor into the cooler basement space. Adding insulation cuts off this heat transfer. Compounding that is the sealed combustion installation. The older giant cast iron boiler was a huge thermal mass radiating heat into the basement space. The two changes would lower the basement temperature by several degrees.
The drop in temperature has several repercussions; colder temps could move solid surfaces below the dew point, causing condensation. Since the magic formula for mold is moisture + plant based sugars (meaning wood or paper), condensation can be the autobahn of mold growth.
Hot and cold water pipes experience more heat loss because of the cooler air temps. If the pipes are uninsulated, long pipe loops can dip below freezing during extreme temperature lows. Do you have a basement work shop? If you use the basement space for work or projects, you’d need to add a heat source, negating your energy savings.
The final irony is that you’re not saving much money. Heat loss is a function of temperature differences. The heated first floor is around 70 F while your basement is 55 F. The basement heat is moving from 55F to whatever the outside air temperature is (we’ll say 10 F). We want to insulate the path of greatest heat loss and that is through the foundation.
Options for Insulating the Basement
So it might be worthwhile to add insulation over your exposed foundation walls. Before insulating, the basement moisture should be well controlled with exterior and interior systems (french drains, gutters and contoured landscaping outside, sub-slab ventilation, perimeter drains and vapor barriers inside).
You should opt for a water resistant insulation such as closed-cell spray foam or XPS foam board. Avoid using fibrous insulations like fiberglass, rock wool or cellulose which could trap moisture, especially if installed with an interior vapor barrier. Be sure you check with your local building department as some insulations that are flammable – such as closed cell foam – must be covered with drywall or other approved material to prevent rapid fire/flame spread.
This article details the questions and concerns with basement insulation in greater depth.
For more details on the exact thermal and hydro dynamics, Joe Lstiburek of Building Science helps with ‘Understanding Basements‘.
A brief how to guide on insulating a basement wall.