Should I Insulate My Basement Ceiling?

by Erik North on May 18, 2011

should I insulate my basement ceiling

 

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.

Important note:
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.

Other Reading

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.

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{ 30 comments… read them below or add one }

Tony October 12, 2012 at 3:27 pm

Hi
I am in the process of renovating a pub which has an underground cellar, just wondering weather to insulate the ceiling, as the cellar has a chiller to cool the barrels, more to keep the cool air in but cool air sinks !

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Erik North November 19, 2012 at 10:03 pm

Tom,

Sorry about the delay…busy election and auditing season. My approach is to control the environment so that equipment can do its job independant of building factors. If the basement needs to be cool and the main pub section warm, you need to control the environment through insulation so your mechanical systems can do their jobs properly.

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Jay Lytle November 24, 2012 at 12:33 pm

Thanks for the info I live in a ten year old house and the basement ceiling is insulated and the floors are cold. I’m going to remove the insulation and install radient heat to solve this problem

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Erik North November 24, 2012 at 12:47 pm

Jay,

Thanks for the comment…If you are installing radiant floor heat, make sure the sills are insulated with a closed cell foam product (either spray or XPS foam with appropriate ignition barrier).

Many times I see radiant floor heating systems with a radiant foil layer or foil faced polyiso underneath and no insulation on the sides. With infrared, you can see the hot water tubes right through the sills on the exterior.

Erik

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jerry cusac December 16, 2012 at 10:30 am

I am considering insulating my cinder block walls in my basement along with the floor, from what Im reading I guess I will leave ceiling uninsulated. I am also thinking about heating the floor probably electrically. We have lived in this house for 28 years and have never had any water at all in the space. Any ideas or suggestion you can give would be greatly appreciated. Also planning to build 2×4 walls inside block walls, and adding fiberglass insulation with plastic vapor barrier, using pressure treated lumber. Again thanks for any ideas or suggestions.

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Erik North December 27, 2012 at 1:17 am

Jerry,

Thanks for the comment…getting back to you after the holidays.

Check out the guide to insulating basement walls link above. I have an almost identical installation in my home basement. 2″ XPS foam board against (in my case) board formed concrete, framed up with 2×4 studs with a composite board under the bottom plate as a capillary break.

If you choose to go the 2×4/fiberglass route, just make sure you don’t put the plastic vapor barrier over the fiberglass. Having the kraft paper and wood studs encapsulated next to the stone sponge that is a foundation is not such a great idea.

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Christian February 2, 2013 at 12:43 pm

Erik,

Thanks for the info; I was going to cut into the plaster (yes, plaster) ceiling in my 5′-tall basement of my 129-year-old home cross-wise to the joists so that I could blow loose-fill insulation into the cavities, which have minimal pipes, circuitry, etc.

The house is here in “north-east” Wisconsin (not really in the state’s north east: think how Lewiston/Auburn is considered “Central Maine,” when it really isn’t, geographically…), and the basement is field stone with concrete slab for the floor. There is some water leakage every Spring during thaw-and-rain season, but it’s one stream & goes directly to a drain.

Anyway, the basement isn’t really usable, except by the kids during warmer weather as a play-space; still, wintertime brings crazy cold first floors; it feels more like a draft emanating from the basement/outside (and it’s a LOT colder here than back in Maine / New England, though less snow falls here). The second & third floors aren’t as cold, however, leading me to believe that the walls are insulated fine – including the 1st floor – but the heat loss is from there.

Any tips on framing / insulating / walling a field-stone cellar?

Thanks,

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Erik North February 2, 2013 at 3:16 pm

Christian,

Thanks for the comment. Yes, not everyone has nice clean concrete walls to insulate. My aunt lives in Wisconsin and I went to Univ of Min, so I certainly appreciate how brutally cold it can get in your neck of the woods.

Here’s a link to a Building Science article on insulating rubble foundations, including comprehensive moisture management.

http://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations/

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Christian February 2, 2013 at 12:47 pm

Sorry: “there” in the last sentence to the second-to-last paragraph meant “from the cellar,” as in, “…the heat loss is from the cellar.”

PS: have some lobstah for me; $22.00 a pound or so out here, and it’s all pre-cooked or flash frozen.

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Gary McQuay April 14, 2013 at 10:18 am

Erik,

Great information. I’ve also read the “How to insulate basement walls” article.

We are about to refinish our entire basement. It’s a split-foyer home on a slite hill, so part every basement wall is below grade. Walls are formed concrete. Builder installed fiberglass insulation to walls with shiny barrier surface facing basement interior. Every wall in basement will be framed and dry-walled. We are also adding a suspended ceiling. We installed an internal french drain system 2 years ago with two sump pits/pumps as well as doing things like ensuring gutter water is channeled from house. We also have propane back-up generator to power sumps in extended poweroutage. I have painted part of the basement walls with two coats of masonry water-proofing paint.

My plan is now to insulate the walls vice the ceiling. But I have a two-fold question for you regarding the wall insulation: (1) should I finish adding water-proofing paint to rest of wall, and (2) if so, do I need to add the DOW extruded insulation layer between the wall and framing? My thinking is that the water-proofing paint would serve the same purpose as the DOW insulation.

Thanks,

Gary

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Erik North July 7, 2013 at 1:53 am

Gary,

Just realized I hadn’t responded to your question.

I would add the water proofing and the XPS foam layer, taped at the seams. A closed cell foam layer from the sill down to the footer. One, your basement insulation is not only preventing moisture from entering but should also prevent humid interior air from contacting the cool concrete interior wall and condensing.

Two, XPS foam board wouldn’t adhere flush even with a very modern formed concrete foundation. That means some of the surface area wouldn’t be in contact with the vapor barrier with the condensing and potential inward vapor diffusion that entails.

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William A Miller July 5, 2013 at 5:52 pm

Dear Erik, I read your article with great interest. My question is , one side of my basement is about 50% out of the ground(the way it was landscaped to take advantage of the view), and should I insulate the ceiling near the wall that is out of the ground or should I make sure to insulate the wall on this side. I have like a French double door to get stuff in and out of the basement on this wall as well. I was thinking about some sort of air lock apparatus for this area. Any ideas would be greatly appreciated. Sincerely, William A Miller

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Erik North July 7, 2013 at 2:19 pm

William,

I would still insulate the wall. The thermal boundary of a building should be continuous. Insulating part of the ceiling wouldn’t be particularly effective.

With the double french doors (always difficult to make tight and efficient), I’ve had some success building a well insulated exterior mud room around the door. Another alternative, if the entrance is not often used in the winter, is building an insulated foam insert to fit. Cut it to fit (perhaps two boards if particularly wide) and cover with a fabric sheet for better aesthetics. Seal around the edges and remove once spring comes around and you need to get the lawnmower out.

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Barb July 20, 2013 at 7:37 am

This website has s lot of good information so thanks. however I have been searching and searching for information on whether or not we should insulate our basement floor. We arr currently building a house in Ohio and we have insulated our basement walls but would it be beneficial to insulate the basement floor or would it be a waste of money? It would cost roughly $1300 so we want to make the right decision. I would appreciate your feedback. Thanks!

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Erik North July 21, 2013 at 4:18 pm

Barb,

Ooh, great question. I’ll have to write a post on this topic.

My feeling as an energy guy is that it ALWAYS makes sense. Your house will be with us for decades, maybe centuries. The insulation will more than pay for itself, keeping the money otherwise spent on heating fuel in your (and future owners) pockets.

My very rough back of the envelope shows that if you heat with anything but natural gas then yes, it makes financial sense. If you heat with gas then the payback would be a bit longer (see how I have to wrench myself in knots to say not to insulate).

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Joann July 22, 2013 at 10:00 am

Hi Erik,
Great articles you have written. We live in Mass., house is 27 yrs old. The entire basement is underground, no exterior egress and two at grade windows with little ventilation. Moisture and a musty smell have always been noticeable. There is fiberglass (pink) insulation in the ceiling, paper side up. In some areas, this insulation is dark. Should the basement ceiling insulation be removed and the concrete foundation walls be insulated instead? The walls were Drylock’ed a few years back and this greatly reduced the summertime humidity. Now we just run fans and not the dehumidifier. In winter, the basement is just cold. Thank you for all of your information.

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Erik North July 24, 2013 at 8:56 am

Joann,

Thanks for writing. Sounds like a textbook case for insulating the basement walls.

The question I always pose to contractors and homeowners who want to insulate the basement ceiling is: Did you insulate the interior walls of the stairwell? Because unless that stairwell is air tight and insulated, you have a 80 square foot hole in your insulation.

Erik

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Joann July 25, 2013 at 2:22 pm

Erik, Thank you for writing back. Just to clarify my interpretation of your recommendation: you would insulate the basement walls ~ and ~ remove the fiberglass ceiling bat insulation.
Thanks again!

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Erik North July 25, 2013 at 3:59 pm

Yes, and remove the fiberglass. It just wouldn’t be serving any point.

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Mike July 23, 2013 at 1:23 pm

Erik,

Great article and well written, as a fellow auditor, your explanations will help me better educate future clients. A couple of questions for you:

1. Upon hearing the concern of cold floors. I still often argue in favor of insulating basement/foundation walls rather than the ceiling. Even after describing all of the benefits, it always seems to come back to the comfort question, “But that won’t keep my floor warm will it?” In your experience, do you find that insulating the basement walls (as opposed to the ceiling) raises the temperature of the floor, since the delta T between the spaces is now less. Being a young guy myself, I’ve yet to gain a lot of hands on experience. Most of my knowledge comes from research and theory, so this kind of input is useful.

2. Also, I often come across homes with fiberglass batts already installed in the ceiling (poorly I might add) and contemplate the cost-effectiveness of insulating the foundation walls when there is already insulation above, mostly to bring mechanical equipment within the thermal envelope. I’m curious what your thoughts are here.

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Mike July 23, 2013 at 1:25 pm

I should expand on question ’1′. When I say “raise the temperature of the floor” I mean to say to an acceptable comfort level.

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Erik North July 24, 2013 at 10:18 am

Mike,

Thanks and good questions. I can answer from personal experience with my home and second hand from customers that, yes, it will make the floor warmer. In my house, we had cold spots everywhere. Once the basement was insulated, the cold spots largely disappeared.

However, the cool basement is only one factor. Using my house as an example, we have a drafty back door (better since I weatherstripped and added a storm door) next to our tile kitchen floor. Any time I want, I can pull out the thermal camera and see the cold air streaking across the floor.

Ceramic tile is such a good conductor, that cold winter air sucks any heat energy right out of the tiles.

As for 2, like I mentioned in the previous post, what’s the point of those batts if the stairwell isn’t insulated equally well.

To answer an implied question, how would you get the floor warm if you insulated the basement ceiling? Install batt insulation in the joist bays with a 2″ air gap. Then install rigid foam board insulation with all the seams taped like this illustration (credit to Building Science).

Edit: Oh, yeah…this illustration doesn’t meet current building code. From the IECC: “402.6 Floors. Floor insulation shall be installed to maintain permanent contact with the underside of the subfloor decking.” Something to keep in mind though most code officers are reasonable.

This is all from Building Science’s article: http://www.buildingscience.com/documents/insights/bsi-064-bobby-darin-thermal-performance/

A warm floor

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Mike July 24, 2013 at 11:37 am

Thanks Erik,

Gotta love BSC and all their info!

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greg July 27, 2013 at 7:42 am

We live in northern wisconsin and our basement is not heated, temps reach freezing. To improve heat retention in living area, would it be a good idea to insulate basement ceiling with spray foam. would hope to insulate walls later but they have many, many leaks and it seems overwhelming to me now.

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Erik North July 28, 2013 at 12:55 pm

Greg,

Thanks for writing. Yeah, Wisconsin can get cold. My aunt has a farm on the St. Croix river that they’ve converted to livable space over the years but man it was cold in the winter.

You could insulate the ceiling. I’m not saying it doesn’t work at all just that attacking the basement walls is far superior.

You can spray foam just make sure you entirely encapsulate the basement ceiling joists. Sometimes the applications are much heavier in the bays than on the joists when it really should be evenly applied.

You might consider an approach like I mentioned a few posts up, using batts then adding taped foam board. This could be removed later if you decided to insulate the basement walls where a foam application is pretty permanent.

Also, if insulating the ceiling leaves your pipes outside the thermal enveloped, they need to be well insulated otherwise you will create a problem.

Erik

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Toddd September 19, 2013 at 4:58 pm

Eric,

I think you have the right idea but you seem to be misguided in your knowledge of thermodynamics:

“follow the laws of thermodynamics. Heat will flow from the main section into the basement and out into the surrounding air and ground.”

Heat isn’t going to flow to the basement. If that were truly how thermodynamics worked a hot air balloon would never get off the ground.

Hot air rises, cold air doesn’t. That’s why the most important part of a house to insulate is the attic, it caps off the heat loss, just like a hot air balloon.

You can think of your house as a hot air balloon. And that’s why you don’t insulate the basement ceiling. Hot air in the basement will rise, the cold air will stay in the basement. You’re not accomplishing anything by holding down cold air…that’s not going anywhere anyway.

If you assume hot air goes down to the basement, it would make sense to insulate the ceiling, but that’s not the case.

Insulate the walls of the basement to keep the basement warm as cold walls will cool the heated basement. Also, if you open a door or air seeps out the top of the house, that hot air will escape to be replaced by the cold air from the basement.

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Erik North September 19, 2013 at 6:59 pm

Todd,

Thanks for writing.

You’ve only mentioned one manner in which heat energy moves. Heat moves through three means (http://en.wikipedia.org/wiki/Heat_transfer): radiation (like the sun), convection (movement of heat within a fluid such as gas or liquid) and conduction (transfer of heat between two medium).

Stand twenty feet away from a raging bonfire and you can easily feel the heat. That is heat radiation. Radiation does not go ‘up’; it moves in all directions. When solar radiation emits from the sun, it goes in all directions not floating upward.

Warm air floating upward is an example of convection. Grabbing a hot pot handle and getting a burn would be an example of heat conduction. The heat is moving from the hot pan to your tender hand.

Thankfully, as everyone knows, heat goes up. Anyone can safely pick up a scalding hot pan by grabbing the underside. The heat will go up and your hand will remain unburnt. Or not.

OK, now on to particulars…
“Heat isn’t going to flow to the basement. If that were truly how thermodynamics worked a hot air balloon would never get off the ground.”

Sure it does. Suppose the interior air of the house were 70 F, the basement at 50 F and the exterior was 10 F. The heat would move from the house’s interior air to the flooring by conduction, through the flooring by conduction and into the basement space with a combination of radiation and conduction. The basement heat will then move through the foundation wall and out by the radiation/conduction combo (there’s a gradient here with the heat loss growing less severe as the exterior ground buffers the concrete).

At the same time, warm air will be moving up through the building enclosure, escaping through flaws in the attic air barrier. This will create a positive pressure in the upper floors, pushing warm air out and a negative pressure (suction) at the bottom, pulling cold air in.

This is a phenomena called stack effect.

http://www.greenbuildingadvisor.com/stack-effect-when-buildings-act-chimneys

I have to wrap this up but fix the attic air barrier (stops convective warm air escaping), attic insulation (slows conductive and radiant heat), seal the basement (stops induction of cold exterior air) and insulate the basement walls (slows conductive and radiant heat).

You’ve got the right idea about warm air movement but skipped the other ways heat moves.

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John September 28, 2013 at 12:53 pm

Hi Erik,
I just sent an email to enorth@FreeEnergyMaine.com with a question about crawl space insulation. I appreciate your website and I would very much appreciate your insight. Thanks!

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Bill Gray October 11, 2013 at 7:22 pm

Our basement has a room/closet underneath the front porch. The porch is brick/concrete block walls covered by a concrete slab. Condensation forms in this room in June/July/August in Tennessee with the high humidity. We removed the insulated ceiling below the concrete because it was soaked. We have been using a dehumidifier to keep the room dry, but wonder if there may be a better solution. Can you help us understand what is happenning? Thanks.

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Phillip Norman December 12, 2013 at 11:37 am

I found this conversation by search interest in heating first-floor floors of a home with basement or conditioned crawl space, by simple, rugged resistance heating serviceable from the space below. Insulation under these heaters then makes sense. The goal is to just raise any floor surfaces that matter, to a temperature warmer than that achievable by warming room air. Say, to 80°F. To whatever it takes to make feet comfortable without over-heating the air and the walls. Such basement or crawl space should have conditioned air. Couple it well to house air circulation.

Does any manufacturer now offer such floor heaters?

If this makes sense, let us speak up, together.

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