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Dovetail Greene
Dovetail Greene, General Contractor
Category: Home Improvement
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Experience:  Licensed Building Contractor & Certified Building Designer
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I had a garage built 8 years ago in So. California. The roof

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I had a garage built 8 years ago in So. California. The roof is flat to accommodate a patio on top. The roof is thus sealed from exterior moisture. Inside, the garage is finished with a dry wall ceiling. Between the drywall ceiling and the roof are 2X12s. Inside this space, I have 8 inches of insulation, and nailed to the underside of the roof is a moisture barrier. I noticed discoloration last week, and to my horror, found the space between the roof and ceiling to be completely wet. It is now obvious to me that moist air slowly gathered in this space, and over time, condensation occurred over and over again. The insulation and water barrier just made things worse. The areas between the 2X12s aren't vented at all. There is no fan, etc. It's just a terrible design. Must I tear out the ceiling and all insulation to let the area dry out? Can I cut out holes and attempt to save the ceiling using a humidifier? More importantly, can I ever have a finished ceiling in the garage even if I vent each space between the 2X12s to the exterior? Will be tough as 2X12s border the edges of the garage ceiling spaces. To be clear, the design is such that the 2X12s create separate spaces between them. It isn't one big area, so fans don't work.


My name's Kel.


I have a few questions for you before I answer.


Is your garage heated and air conditioned?

Are there a lot of windows in it?

Is it in prolonged sunlight or shade during the day?

Customer: replied 4 years ago.

It is neither heated nor air conditioned. However, two walls are cinder block because these two sides are against earth (the garage is partially subterranean. That, and 7 motorcycles probably contributed to the moisture.


Only one window, kept closed. One door, kept closed, and one double garage door, kept closed. The garage is like a vault. No air flow. No venting throughout.


More sun than shade most of the day. Our temps don't very much year round 60-85 degrees. Moisture was trapped, and ultimately, the heating and cooling of the roof caused condensation big time. No mold yet.

Is there a polyethylene vapor barrier between the insulation and drywall?

Was poly put down before the floor pour?

Are the top of the concrete blocks sealed or are the voids visible?

Customer: replied 4 years ago.

The vapor barrier consists of sheets of material approximately a quarter inch thick that state they are for blocking out moisture. They were cut and place between each 2X12. The layers of the ceiling are as follows: drywall ceiling/ above that, 8 inches "cotton candy" like insulation/ above that, the vapor barrier facing upward/ above that, the 5/8" plywood ceiling-floor material. It appears the water primarily condensed between the plywood (which has the water sealant on top) and the vapor barrier..nowhere for the moisture to go. I see signs that it got so prevalent, the water trickled over the vapor barrier and ran down the sides of the 2X12s.

Floor: I think no poly. I had a ton of lime come up through the grout in the tile floor the first two years--telling me again, a probable moisture source. Garage floor is poured concrete on dirt.


Concrete blocks were sealed, they end above ground, and a french drain was installed at the base to boot. However, the ground adjacent to the garage is irrigated, so the dirt against the concrete block walls is always wet/damp

Final question.

The material for blocking out moisture -- do you know what it is?

After it was installed -- were all the edges caulked?

Customer: replied 4 years ago.

Yes, the material that was cut to fit between the 2X12s and placed next to the plywood ceiling is "Insulfoam" "A premier insulfoam product USA" 20 year thermal warranty. "Excellent resistence to moisture for insulating performance".


No. The Insulfoam was just stuck in between the 2X12s. Not caulked nor sealed in any manner. So, instead of keep the plywood created the perfect storm for condensation. I get it...but am looking forward to your confirmation, and then more importantly, the fix and what I do in the future. The garage is very'd be a shame to have an exposed ceiling with no heat insulation...but perhaps that is my only option since it will be so hard to vent if a new ceiling is installed.


You'll have to come at this from several directions.

Moisture management.

Vapor retarder.



Either the landscaping needs to be changed, so you don't need to irrigate or the whole wall needs to be dampproofed on the outside.


Make sure any rain runoff is captured in gutters and directed at least five feet away from the foundation.


I can't think of a way to save the ceiling. It's got too many flaws.

Before demo you could try this --

You could hire a restoration contractor to drill holes at each end of the 2x12 bays and ventilate it to dry.

The contractor can use a moisture meter to indicate when it's all dry without pulling all down. After drying the holes can be patched

You could use a vapor retarder paint on the ceiling in lieu of a poly vapor retarder.


BUT you MUST vent the roof.

There are any number of ways that could be accomplished. Cut a slot in the exterior horizontal element perpendicular to the 2x12's, add screening to keep bugs out and then a rain diverter. Will need to do both ends so the air flows. That will also cool down the space on a hot day.


You'll also have to seal the top of the wall blocks so moisture that's wicked won't be released into the space and add to the moisture load. That could be done treated plywood or treated boards.


Does this make sense?

have I answered your questions?

Customer: replied 4 years ago.

Yeah, I agree--with what little I know! Venting at each end is possible, but the problem I see is that the spaces are crammed full of insulation. The spaces can't vent with all this in the middle, right? And, to dry the ceiling out, I need to open it all up and pull out all the insulation and vapor barrier material anyway. The garage is 25 feet wide, so venting each end with insulation in the space won't vent it--or probably ever even dry it out adequately. If you agree--then you also agree I need to tear the drywall ceiling down, pull down the insulation, dry the ceiling out, and then either (1) reinstall the drywall ceiling but leave the spaces empty and vented at both ends, or (2) not reinstall the drywall ceiling, paint the plywood and 2X12s white with a a vapor resistant paint, and call it a day. Yes?

The key word is crammed. Insulation shoulder never be crammed. It's no longer insulation then. One of the deep flaws is having the vapor retarder toward the roof. That's fine in your area when the space insulated is air conditioned, bu in the this case we have to worry about the moisture load and source.


If the bays are crammed then you'll have to remove it all and start again.

You can buy baffles to keep the insulation away from the underside of the deck and maintain air flow.


If there's NO sign of mold on the insulation that can be reused. The batts can be split length wise so they're thinner.


There's no need to paint the plywood and 2x12's.


You could reuse the Insulfoam.

If the there's a craft paper or aluminum vapor retarder on the insulation -- it needs to be removed.


The sequence would be demo, dry framing, inspect for mold, clean with bleach, install venting, install venting baffles, install fiberglass insulation, install board insulation, install 4 mil poly vapor retarder, caulk any penetrations like wiring, install drywall, prime and paint drywall.


Reduce moisture load around garage especially where it's built into the hillside.


You could just remove it all and paint the framing. If the garage is attached to the house it will increase the air conditioning load with the it being hotter and the heating load with it being cooler.

Dovetail Greene and other Home Improvement Specialists are ready to help you
Hello Paul. This's Kel checking in to see how the garage roof projects going. . .