underlayment for garage ceiling

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Forum topic by tws posted 10-28-2013 07:05 PM 3738 views 1 time favorited 15 replies Add to Favorites Watch
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19 posts in 2037 days

10-28-2013 07:05 PM


i am finally going to insulate my 24×24 foot garage. i have 24” centers. these are trusses that are self supporting – pls bear with me as i am not a contractor and don’t know all the correct vernacular.

i’ve found the right insulation – fiberglass – that will give me an r factor of 38.

i hope to do this job myself with one helper the insulation is easy but it’s the ceiling material that vexes me.

i don’t want to use sheetrock with all the expense especially labor with taping and finishing etc. plus i don’t want that weight.

i want to use as little weight as possible. these trusses i’ve been told do not support a lot. at the moment i have only a few hundred pounds of stowage in the attic. the garage came equipped with one of those spring loaded folding down ladder/stair thingys. currently except for an 8’ wide 3/4” flooring of plywood (top side of trusses) all the trusses are exposed. i have so far chosen 5.0mm 4×8 sheets of underlayment from homedepot as the ceiling material… it’s not too expensive and i’ve been told it will easily support the glass insulation. granted i’ll have an extra expense painting the ceiling white once it’s installed. more cost…

iam asking for advice on materials. so far it seems the moisture proof underlayment seems the best choice especially at $15 per sheet. i am unsure about the best fastening procedure. the pdf declares screws to be the best or ring shanked nails. pneumatic staple guns are not recommended.

if any one has experience with this type of construction your advice would be welcomed.

i can produce pictures of my garage if needed.



15 replies so far

View crank49's profile


4030 posts in 2964 days

#1 posted 10-28-2013 07:49 PM

I have a friend who used that white coated hardboard material they sell as shower liners.
Didn’t have to paint.
Put it up with screws with washers to spread the load.
Worked great.
His shop is 23’ x 23’, trussed attic with 6’-6” stand up storage, 8 ft wide. Sounds very similar.

View tws's profile


19 posts in 2037 days

#2 posted 10-28-2013 08:13 PM

white coated hardboard? i’ll look it up maybe lowes or homedepot

Thanks! crank49

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19 posts in 2037 days

#3 posted 10-28-2013 09:13 PM

just found a solution! and it will be fire proof – corrugated steel sheeting.

if anyone has applied and accomplished this pls chime in.

thanks for your replies…

View SCOTSMAN's profile


5849 posts in 3578 days

#4 posted 10-28-2013 09:20 PM

dont use steel sheeting unless you are in an area where weather does not allow condensation.I bought an expensive steel shed/garage single car and use it just to store timber etc.I already have commented on the rain from within from condensation funnily enough my sons and dear friend are today starting with building an overocoat of bubble wrap onto it to stop it from dripping so please consider this well before spending as I would hate to see you have problems in the very near future.

-- excuse my typing as I have a form of parkinsons disease

View tws's profile


19 posts in 2037 days

#5 posted 10-28-2013 09:27 PM

great point! scotsman. thanks… i’ll give it careful consideration…
one reason i’m doing this long over due insulation/ceiling is because i want to be able to work in the garage year round. it’s fairly dry here in southern oregon and the garage is well built and ventilated. i plan on using light gage galvanized corrugated steel sheeting. which i hope is rust resistant.

i’ll be able to heat and maybe even cool in the summer with city gas. the fixture is actually mounted on the exterior garage wall so plumbing in heating should be easy.

View LakeLover's profile


283 posts in 1933 days

#6 posted 10-28-2013 09:46 PM

I would use the steel over wood. Wood is likely against a code issue. Also steel is more of a vapor barrier.

R38 should be nice and warm/col as needed.

View LakeLover's profile


283 posts in 1933 days

#7 posted 10-28-2013 09:57 PM


Living here in the Canadain Praries we build with condensation in mind.

Many large metal building will rain inside. I have seen way to much of it. They are a pain to insulate properly.

They are inexpensive to put up as a basic shelter but they need ventilation and no water sources ie damp soil. dripping cars/trucks. Propane heat makes tons of moisture, which adds to the problem.

I am presently completeing a big reno on half an old farm house. There was so many mistakes made the place was rotting. So to get an energy grant we had to have a pre reno blower door test.

We will have a post reno blower test. So far I have fixed/sealed about the same area as an entry door. Upped the insulation ceiling to basement. Vapor sealed, replaced a bunch of windows.

Lots of work buy this will last a long time. Just waiting on a new steel roof to be put on. No more cold feet in this place.

View redSLED's profile


790 posts in 1886 days

#8 posted 10-29-2013 01:57 AM

Cost-wise, getting something cheaper than $10 per 4×8 sheet (1/2” drywall) to cover your ceiling is close to impossible. Drywall is indeed an upfront labour pain for a garage or workshop plus you’d have to botch it up after just to bolt/hang stuff to the ceiling if you want to.

Fire code issues aside (I won’t make any claims here), the hardboard choice might work (also called ‘masonite’ at some stores) but even at 1/4” thick the hardboard may bow down between the trusses over time. Alternately, you could use something like corrugated fiberglass panels (very rigid along their lengths, light and easy to install) and they are often available up to 12’ lengths at various suppliers – but then you’ll have to shell out 2x-3x the cost per sq ft of drywall for that. And this would match the cost of 1/4” plywood anyway which would likely be preferrable to put on the ceiling – again, possible fire code issues aside.

One trick with stapling (or nailing) to get better fastening along trusses/studs is to staple/nail over plastic band strapping (available at industrial supply outfits) on top of your vapour barrier. In your case with 24” centres, I’d use the thickest gauge staple possible, at least 1” long, and every foot along the lengths of band strapping. This would be very adequate to hold up your R38 fiberglass insulation. Also if it were me, because of the 24” between trusses, I’d go with 6 mil thick poly instead of the 5 mil for less sagging and more strength. These are just my ideas, not recommendations. Please note plastic band strapping generic photo below for your viewing pleasure.

-- Perfection is the difference between too much and not enough.

View fuigb's profile


481 posts in 2951 days

#9 posted 10-29-2013 02:13 AM

+1 on the band strapping. Exactly what I used, but mine was free: bundles of papers from my kid’s newspaper route came in these, as did all kinds of stuff in the warehouse at work. With the proper staples they won’t come down in your lifetime.

-- - Crud. Go tell your mother that I need a Band-aid.

View tws's profile


19 posts in 2037 days

#10 posted 10-29-2013 04:44 AM

wow! absolutely excellent recommendations.. i’ll definitely follow this advice. i’ve viewed a youtube video of some pro’s applying the corning fiberglass.. they stuff it up into the the trusses and whack away with some hammer stapler device… hopefully i can rent one of these…

i’ll locate the strapping and use adequate staples…

thanks again for this superb forum.


View GFYS's profile


711 posts in 3464 days

#11 posted 10-29-2013 05:06 AM

you could go with osb if the price point is right. I would still put 6 mill poly up first

View Woodbum's profile


812 posts in 3059 days

#12 posted 10-29-2013 12:41 PM

Your trusses should be designed to hold drywall @ 24” OC. Check your local codes, but I think that most places might require sheetrock/drywall. The fire rating is much better, As was stated before, I would stay away from metal for your ceiling material. Any of the lightweight coated masonite materials such as Marlite etc. will more than likely sag over time at 24” OC when hung on a horizontal surface. Then your ceiling will have waves, plus it will not support much (any?) weight between the ceiling joists. For a real solid permanent solution, check the load bearing capacity of your trusses with the mfg., bite the cost bullet and then IF POSSIBLE, use 5/8 fire code drywall. I did this in my entire house after a rebuild due to tornado damage. The cost difference in the material and the hanging labor is minor, and the taping and finishing labor is the same. Long term, you will be much happier. Just my HO from personal experience.

-- "Now I'm just another old guy wearing funny clothes"

View IrreverentJack's profile


727 posts in 2836 days

#13 posted 10-29-2013 09:48 PM

If you are insulating to r38 there won’t be a condensation issue. Condensation is caused by temperature differential. Warm humid air contacting the underside/inside of a cold/cool metal roof will cause it to “rain” inside. In this case the metal ceiling will be the same temperature as the the air inside – or warmer (heat rises) due to the insulation. Metal liner panel is a good option IMO as it is priced right, meets any fire code, and needs no taping or finishing. It might be louder when you run your power tools but it won’t sag in a few years like hardboard (masonite), white bath panel, or even lauan will on 24” centers. -Jack

View Shawn Masterson's profile

Shawn Masterson

1325 posts in 1942 days

#14 posted 10-30-2013 12:06 AM

Everyone stop and take a deep breath. Trying to be cheap up front always cost more later. At a price of $15 a sheet I could have it hung and taped professionally without getting my hands dirty.

First woodbum is right trusses are rated for drywall, but 1/2” is for 16” on center (OC), and 5/8” is for 24” OC. 1/2” drywall will sag in time on 24” centers. No one says you must finish drywall, it just looks better, and gives it a fire rating. I personally used OSB, at the time I did mine, it was almost as cheap as drywall.

Second and this is just my opinion. Blown in insulation is better than batts. It fills all the nooks. Weather it is fiberglass or cellulose is more of a preference. In my area I can get the machine for free at any of the home stores in town for free ($250 refundable deposit) when you buy the insulation there. It shouldn’t take more than a few hours. On a safety note it is a 2 man job (one to feed and one to blow), and I strongly recommend a good mask and goggles. You may be surprised how cost effective it is to hire it out. I hired out the last house I did because by the time I got the insulation, blew it in, and returned the machine it was almost the same price as doing it myself.

Third you absolutely must put in draft chutes. They allow for proper circulation from the eaves to the ridge. If you don’t have proper ventilation then you will cook your shingles and possible cause a mold issue.

fourth enjoy the project and research it so you can be happy with it for many years to come.

View tws's profile


19 posts in 2037 days

#15 posted 10-30-2013 09:14 PM

thanks… i’ve got good draft chutes.

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