garage insulation/ventilation

**Martin N.** · December 29, 2009, 03:25 PM

Re: garage insulation/ventilation

MARK-

I guess you could say that your Garage is TOO tight. It's like parking your car inside a Zip-Lock bag. When we build houses here in Northern Illinois we insulate and vapor barrier just as you did, but with the exception of NO vapor barrier on the ceilings. This allows alittle "breathing" of the home. I would venture to guess that once your split A/C-Heating system is installed and running your problem will be nil. For the mean time, any type of venting (such as cracking a window open) with a SLOW moving fan should suffice in eliminating the moisture conditions.

Marty

**Paul J.** · December 29, 2009, 03:52 PM

Re: garage insulation/ventilation

Mark:

Don't do anything. Your problem (if it's really a problem) will go away once the AC system is in. The water will eventually evaporate anyway, and should not cause a problem if it only happens occasionally. If you think that this is really a problem, the dehumidifier will take care of it or you can leave a door open for a little while, depending on humidity and temperature. Don't put any unnecessary holes in the roof or walls.

We don't insulate ceilings in the mid Atlantic because, quite frankly, it's not required by code. Moisture rising through insultation into an attic space will dissipate in the air space before it can condense on the inside of the roof sheathing. This is unlike a wall where the moisture will condense on the back of the sheathing because there is no air space between the insulation and the cold surface.

I see problems with insulated ceilings without moisture barriers that have decking laid over the joists against the insulation. Moisture condenses on the underside of the decking just like a wall. Most people don't notice this because they never check. You would not have to worry about this since you installed a moisture barrier against the warm side.

Paul

**Gary C.** · December 29, 2009, 04:16 PM

Re: garage insulation/ventilation

Mark, FYI - last year or early this year there were some good discussions on garage insulation - try a search. California like Texas is a warm climate which does not require nor is recommended a vapor barrier on the warm side like up north does. All that's required is fiberglass insulation with a paper backing towards the inside wall. Warm weather climates also require outside "skin venting" to circulate the air from the bottom of the outside wall cavity be it siding or brick exiting into the attic via convection. This removes the moisture from the wall cavity and greatly reduces the opportunity for mold and mildew. In Texas alot of us run dehumidifiers year round in addition to heat and air. California's building code requirements should be available at a nearby library and will contain insulation and ventilation information. Also Residential Building Code books are available at bookstores and would have this info as well. Good luck and don't make it so tight it can't breath. JMTCW, Gary....

**Paul J.** · December 29, 2009, 05:09 PM

Re: garage insulation/ventilation

Mark:

Before all of these well meaning people give you advice, I got my old general contractor's hat out of the attic and looked up your codes. Like almost all States, they are online.

According to your 07 California Energy Code, Title 24, Part 6, Subchapter 7, Section (g):

"In Climate Zones 14 and 16...(which probably includes you)...a vapor barrier shall be installed on the conditioned space side of all insulation in all exterior walls, unvented attics and unvented crawl spaces to protect insulation from condensation."

Since your attic space is vented, you don't need a vapor barrier in the ceiling (but it's a good idea).

The purpose of the vapor barrier is to prevent an environment for mold growth and to prevent moisture from contacting and potentially rotting structural wood members.

As Gary said, some of my comments are in the discussion in the archives earlier this year (or late last year, I forget). As for your question, please see my original post.

Paul

**Roger P.** · December 29, 2009, 05:11 PM

Re: garage insulation/ventilation

Mark:
In my opinion, the advice you have been given is all very good. I am an architect in South Florida where we have a hot, humid climate. My advice would be to dry off your car once you bring it in the garage so you don't have so much moisture in a relatively small space which lacks ventilation. Is the 2 tons of air conditioning for a portion of the house including the garage, or for the garage only? That is a lot of A/C for only 600+ square feet of garage. Air conditioning will dehumidify the air without the need of a separate dehumidifier. However, if the unit is sized too large for a space, it won't run long enough before cycling off and therefore will not do its job as a dehumidifier. Most likely, it would also create condensation at the A/C supply vent(s). Of course, this is the situation when you require cooling (for me, that's nearly year-round). Split units typically have electric heat strips installed for heating. This will help dry the air when heat is required, but again, I wouldn't park a dripping wet car in a garage without a quick dry off at a minimum. Just like cooling, too much heating isn't good either as it can dry out rubber and other materials on our cars. Mechanical ventilation is always good, but you then lose the ability to control temperature as you need to replace the air that you exhaust (typically from the cold or hot outside). I hope this information is helpful.

Happy New Year,
Roger (50141)

**Frank C.** · December 29, 2009, 05:13 PM

Re: garage insulation/ventilation

I have followed the garage advice on this site, and it is always good. I do not know if it has been mentioned here before, but I am also a member of the garage journal, which also has great tips and pictures of garages and workshops.http://www.garagejournal.com/forum/?&

Frank

**Gary C.** · December 29, 2009, 05:39 PM

Re: garage insulation/ventilation

Mark, both Paul and Roger have provided sound information. All I can say is when I built my first shop I installed a visqueen vapor barrier on the warm side on top of the fiberglass wall and ceiling insulation with paper backing. Did not use any external wall skin vents. Attic was vented with eve vents and turbines. This shop was too tight and held condensation. A dehumidifier was required to keep out the moisture. My current shop has exterior wall skin vents in the brick every 3 feet and has a gap going into the attic for convection flow. Continuous soffit vents with a roof ridge vent and radiant barrier foil on the roof decking. Walls are 6 inch fiberglass with paper facing on the warm side. Walls are sheathed with plywood and covered with DuPont Tyvek moisture barrier wrap. Ceiling insulation is R39 fiberglass. Both 16x12 garage doors are insulated. Inside humidity stays about 40-45% year round. Don't have to run A/C until August when it's 100 plus outside. Heater maybe run for a couple of weeks in late Jan or early Feb. Summer inside temp normally 70-80 until August and winter temp 60 until late Jan or early Feb. Just sharing my experiences with shop building. Gary....

**Mark P.** · December 29, 2009, 08:13 PM

Re: garage insulation/ventilation

The 24,000 BTU unit was sized to provide adequate cooling. I am not sure if it heats that much too. The A/C calculators say I need a 20,000 BTU A/C unit.

This 24,000 BTU unit is only for the garage. My garage is rarely in the shade, faces south east, and we have a lot of 100 F days from May through September. I also have a cathedral ceiling in part of the garage (see attached pictures) This cathedral ceiling was just built and I will soon be able to install a 4 post lift once my garage door is changed over to a high lift garage with a side opener. I had the structural engineer that worked on my house 12 years ago design all the truss modifications to make this possible. It was pretty straight forward. I have a PDF of the truss changes if anyone has a 2 or 3 car garage and is considering this.

The sole vapor barrier is the Kraft paper facing the warm side on the walls and ceiling. The rest of the house is built this way too.

I'll try cracking the garage door open or I can try leaving my attic access door open. Wiping the car down sounds like a good idea. We get less than 15" of annual rain so it isn't too much of a problem.

Thanks for all your help.

Attached Files

**Ray C.** · December 29, 2009, 08:34 PM

Re: garage insulation/ventilation

There are many factors needed to be explored when dealing with moisture and vapor barriers. Is your garage in a woods, built in a hollow or low area, or is the ground in the area of your garage typically wet or damp. Did you have a plastic barier placed under the concrete floor. Manny, manny years past I researched a mosisture problem in a housing develoment in PA, all the problems listed above were previlant in this developement. In some of the homes we removed the vapor barier in the ceiling and installed a continious ridge-eve soffit vent system. In homes that had major mositure problems we did the same and added power roof vents with humidistats to control the moisture in the homes. Gable vents are a very poor way to vent an attic.

Good luck with yoru project!

Ray

**James G.** · December 29, 2009, 10:49 PM

Re: garage insulation/ventilation

Mark, just wondering how old is the concrete floor? A fresh slab will hold a lot of moisture. Also slab temp verses air temp could cause condensation on the slab. As far as your attic venting you should have a ridge vent to work with your soffit vent. Your exhaust venting should at least equal your intake venting. I am a carpenter here in Ct. and have not installed gable vents in a new house or garage since the early eighties. If you do install a ridge vent close off your gable vents.

**Tom L.** · December 30, 2009, 08:30 AM

Re: garage insulation/ventilation

Mark, I happen to agree with Roger on the A/C unit size. I am a refrigeration/ A/C contractor in the northeast and 1 ton/ 400 sqft. is a "rule of thimb" and of course there are factors that influence loads beyond that rule.

Typically A/C units should be slightly undersized for best dehumidification. This sounds backwards but think of it this way; A/C controllers, thermostats, only care about temperature not humidity. If the unit is oversized it will run a short time and cycle off on temperature very quickly. If it only runs a short time, it only dehumidifies a short time leaving a space with a high relative humidity. Conversly, a slightly undersized unit will run more often and provide greater dehumidification while still being able to cool adequately.

What type and brand unit did you buy? The reason I ask is I am a fan of Mistsubishi (sorry guys

) mini-splits using inverter technology. These units have a variable speed compressor and fan motors that deliver cooling in proportion to demand. If there is low demand the compressor and fans slow down this allows the unit to run longer which results in better dehumidification.

If you purchased a unit using inverter technology oversizing is not a problem, the unit will slow itself down according to the load, as low as 30% of capacity. Some units also have an infared sensor that see's warm spots in a room and directs air to those area's, when I told my wife this she didn't believe me until she felt cool air on her neak while cooking one day. They really work. Here's a link: http://www.mehvac.com/Products/subCa...CategoryID=139

Oh, and one other thing. Depending on the model you purchase Obama provided a tax credit of up to $1500.

Hope this helps, and as others said, get the A/C running and your problem will be corrected. The funny part is, I ordered one of these units for my 600sqft garage, but a heat pump for my application, last night. Merry christmas to me!! Have fun!!

**Kenny C.** · December 30, 2009, 09:36 AM

Re: garage insulation/ventilation

Mark
I am a HVAC contractor here in California and do a fair amount of residential work. The info you have been receiving regarding insulation and air conditioning sizing and how it pertains to dehumidification is all pretty good however no one has touched on a very important point VENTILATION. You have succeeded in creating a very tight envelope actually too tight. Remember this is a garage and not the living space of the home (no matter how much you want to live in there). Garages NEED ventilation and by California building code you need low venting to the outside air. The reason for this is that in a garage you have that thing you like to drive and it contains a quantity of this stuff called gasoline and that stuff tends to leak out of the car. Gasoline vapor is heavier than air and puddles in the garage floor area, eventually it reaches that other thing in that space you were worried about "The Water Heater". When that happens you get something we call Ignition! This situation makes the wife very unhappy as she scrambles to save her favorite china from the flames; also your fiberglass corvette tends to melt. Generally these are seen as very undesirable events. By the way although you may not be seeing an immediate problem you also need the air inlet in the garage for combustion air for your water heater and for make up air for the combustion vent. If you properly size the vent surface area in relation to floor space you will solve your condensation issue and have a safer garage. Yes you may experience a small draft or lose of heating and cooling efficiency but it is a garage and needs to be treated as such even if you do bring a pillow in and sleep with your car.
Oh and if not already set up this way your water heater needs to be on a platform at least 18" in height, again this is to raise the flame off the floor and potential gas vapor, once again a California building code requirement and just plain common sense no matter what State you live in.
Regards
Kenny

**Mark P.** · December 30, 2009, 02:22 PM

Re: garage insulation/ventilation

Lynn - I bought a Fujitsu ASU24R1. I only paid $500. It was used in a server room for under a year and is in nice shape. A friend is a HVAC guy and ran all the wires and hoses. We just need to mount it after the drywall is textured and painted.

Thanks,

Mark

**Mark P.** · December 30, 2009, 02:30 PM

Re: garage insulation/ventilation

Kenny - I had two vents about a foot from the garage floor that were about 12" X 7". I blocked both of them off when I installed the drywall and insulation. That is all the venting I had before the remadel project and the space was completely drywalled. The attic space has soffit vents which are still there and I added more when I put in the cathedral ceiling to double the amount of soffit vents.

If I restore both of those two 12" X 7" vents then I should I be OK ? Will I lose much in heating and A/C efficiency ?

My water heater is already on a platform 18" off the ground.

Thanks,

Mark

garage insulation/ventilation

garage insulation/ventilation

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment

Comment