Reader Questions – Basement Ceiling Insulation and Moisture

by Erik North on March 21, 2013

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I have been trying to educate myself so that I may finish my house. I have a garage/basement that is above ground. Above that is the finished house. I want to fire safe my house, i.e.. separate the garage from the main house using 5/8 drywall in the ceiling. What I am not clear about, is what type of insulation to use, ( if any at all) so that the ceiling space does not trap moisture and become a mold source.

Secondly, should I proceed to insulate the basement walls, is it prudent to first seal the walls and floor with a concrete waterproofing such as Foundation Armor?

I have enjoyed your writings on the subject, and want to Thank You in advance for any advise you can give me.


—– Original Message —–

From: Erik North
Subject: Re: Garage Ceiling Insulation


Sorry about the delay. I’ve been off my feet with a broken leg for the better part of a month and am just now catching up.

Thanks for writing. I get questions about mold a lot and there are some basic things to understand.

Mold needs a few conditions to grow. It needs food (as in plant cellulose like paper or wood), a moderate temperature (over 32 F) and high, sustained levels of moisture (over 50% RH and likely well over).

Mold has become a prominent concern over the last couple decades for a few reasons. One, the building materials we use, like OSB, plywood or engineered I-joists are highly processed wood products. They are chopped, mulched, chipped and formed into their final shape. Mold gets its teeth into these processed wood products much easier than a solid pine board.

Second, we engage in a lot more ‘water sports’ indoors than in the past. Hot showers for everyone, hot tubs, aquariums, indoor plants, etc. All these add a great deal to a house’s water load. Third, the building materials we use produce a tighter building envelope, meaning less drying potential.

Now granted, this doesn’t answer your question…it was more a chance for me to put down some thoughts about mold. The question to ask is: Is there so much moisture and humidity in your garage/finished room over the garage area that mold could be a concern? Is the humidity consistently over 60%?

In terms of insulation, if you are truly paranoid about mold growth, close cell spray foam would solve that issue. It would be entirely mold and moisture resistant. Otherwise, dense packing cellulose and finishing with drywall would work fine. Modern cellulose is treated with boric acid (or a comparable flame retardant anti-pest/anti-fungal agent) and the plywood flooring of the finished room would be sufficiently vapor permeable to allow some drying.

A more likely concern (but much less known) would that of carbon monoxide. After parking, car exhausts emit carbon monoxide for hours afterward. Make sure your drywall finish is a continuous air barrier, taping all seams and sealing around any penetrations. Also, have a carbon monoxide tester installed in the room over the garage.

On your second question, it is usually a good idea to water seal any exposed concrete. I recommend two applications of Drylok UGL. The only time this can be a problem is if there is enormous external water forcing its way inside. It would bubble up underneath the water seal.


—– Original Message —–

Subject: Re: Garage Ceiling Insulation


Thanks very much for your reply. Breaking a leg is no fun. Wish you speedy recovery. Of course you may use this on your blog.

I found your comments about mold enlightening. So to better understand my application, I would like to give you more detail:

The home is built on a mountain ridge in Tennessee. Living space is raised 9 feet above unfinished basement with a concrete block enclosed walls with door and garage door openings. This space is an above ground basement. It has a fiber mixed concrete floor, 5000 psi, 4 inch minimum thickness, over heavy plastic sheet, 6 inches above grade. The ceiling above the concrete is manufactured I joist, 9-1/2 inches high with lots of plumbing and electrical wiring. This floor/ceiling consist of 3/4 inch Advan Tech subfloor, asphalt felt 15#, and 3/4 inch hardwood except in bathrooms where tile is installed. There is no active air seal on the sill plate or band joist from the interior. The sill plate has a sheet metal termite isolation between it and the concrete wall. Exterior walls are Advan Tech, house wrap and Hardy Board covering.

Peculiar to this location is high humidity. Winter temperature can get in the teens. Summer temperatures up to 100 degrees. The concrete floor is always very cold summer and winter. In the summer, basement doors must be keep closed to prevent sweating on the floors.

I am looking to solve two problems. First I want to fire isolate the basement space from above (using 5/8 drywall). As you mentioned, sealing the sheetrock will also reduce carbon monoxide exposure. That is where the question of insulating the floor/ceiling came about. Second I want eliminate air leaks and moisture in basement. Presently the dehumidifier controls a lot of moisture and runs a lot.

You recommended close cell spray foam for this application. Can this be done successfully with closed cell sheet foam attached to subfloor, or is spraying so much the superior method.? Or… would it be better to not insulate the floor/ceiling, just apply sheet rock and at a later date apply sheet foam to the basement walls and band board. Finally, if I do insulate the basement walls, should I insulate the basement floor.

I know I have lots of questions. There does not seem to be an industry consensus on this issue; hence my inquiry. Thanks again for your time. Your suggestions will be honored.


—– Original Message —–

From: Erik North
Subject: Re: Garage Ceiling Insulation


First, I mistook this as a stand alone garage/finished room situation from the title. Now that you’ve detailed it, I realize it’s a more straightforward basement insulation/moisture mitigation where the garage is located on the basement level. My mistake.

Here are two good reference articles. The first details building an XPS insulated basement wall. In the second, it’s a similar approach with spray foam and butyl plastic as a floor cover.

The idea is to basically create a styrofoam cup lining the inside of your basement. Conceptually, you’re using that styrofoam cup to keep the moisture out of the house. It involves a few steps, starting from the exterior and working in.

- Make sure the exterior drains very well. No areas around the house that pool water, no window wells that accumulate, gutters with 8+ foot water sluices directing water away from the foundation.

Everything on the exterior should drain away from the house.

- Install very good drainage below the flooring…Make sure that if you have water infiltration or moisture, that it will drain and dry.

- Apply 2 layers of Drylok UGL (again, just the brand I use) to all exposed concrete.

- Install 2″ XPS foam board on the floor and tape all seams.

- If you have a board formed or mold formed concrete foundation walls, XPS board can be used to insulate the walls. Otherwise, use closed cell spray foam. This would be applied from the sills to the foot of the walls.

- Install subfloor

- Frame up the exterior walls, either wood or steel studs. Add drywall to studs as an ignition barrier.

- Finish however you’d like.

I included a screen shot (for the email response) that pretty closely illustrates this approach. The only difference being that they re-poured a foundation floor.

Regarding the ceiling, now that I realize it’s a straight basement, I would just drywall but only after dealing with the moisture issues.

Hope that gives some conceptual guidance to insulating and addressing the moisture issues.


—– Original Message —–


Thanks for your help. That is what I wanted to know.


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{ 2 comments… read them below or add one }

Larry August 16, 2013 at 3:22 pm

I would like to know more about carbon monoxide emissions from cars that are not running. I can find plenty of confirmation that parked cars emit CO while idling, but not once they have been turned off. Are you perhaps thinking of evaporative emissions of VOCs from fuel?
BTW, thanks for your blog. I find it very interesting and helpful.


Elizabeth February 15, 2014 at 7:35 am

Hi Erik,

I recently moved to western ma from cape cod and am adjusting to the staggering fuel costs of heating an old historic home. The house has a very leaky field stone basement and we are going to address some of the water issues with better drainage outside, but given the grade of the land around the house, I don’t think it will ever be completely moisture free so I suspect insulating the walls is out of the question. It already has very raggedy blanket insulation stalled upside with the vapor barrier facing he basement floor and it’s falling down in several places. So, I’d like to tear it down and start over. We just had a ne energy efficient boiler installed so it’s pretty cool there, but no pipe issues even with the extreme cold we’ve hadn’t his winter, what type of insulation would you recommend for a limited skill do it yourselfer?



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