Insulating ceiling of garage??

[rb69]

I have a garage that has a loft in it & would like to get some insulation in the upper part.
Stick built garage w/ 2x6's used for the ceiling of the garage , which would be the floor of the loft & is insulated & drywalled on the ceiling side. They used 2x6's for the roof trusses.
I know I can't insulate the area between the roof trusses and the roof sheeting.

Any ideas how to insulate the upper level & still be able to use the loft for storage??

thanks

 

[Steve in Mi]

I guess I don't understand this comment;

I know I can't insulate the area between the roof trusses and the roof sheeting.

Why can't you?

 

[Franz©]

Because the underside of the roof boards need to ventilate or the roof will rot.

Foam board is a possibility if installed along the bottoms of the rafters and a ridge ansd sophet vents are installed.

The 2 x 6 floorjoyst of the upper level do limit storage capability though.

 

[strizzy]

Why not use some baffles?

 

[Steve in Mi]

Because the underside of the roof boards need to ventilate or the roof will rot.

Dah, I was on another tought track and didn't clear my head. Yes I agree Franz...

I also agree with this;
"The 2 x 6 floorjoyst of the upper level do limit storage capability though." at least for any appreciable unsupported span.

 

[timgr]

Because the underside of the roof boards need to ventilate or the roof will rot.

Only if you don't have a moisture barrier between the occupied space and the insulation and roof. Your local code may specify a cold roof (ie cold air space between insulation and underside of the roof). However, my research shows that a warm roof, applied directly to the underside of the rafters above a continuous vapor barrier, will not degrade the sheathing or shingles, and can provide superior insulation for a given amount of materials and effort.

A successful warm roof depends on a near-perfect vapor barrier though. Any air leaks will let moisture into the space between the vapor barrier and the roof sheathing, and there will be condensation (not good).

 

[Steve in Mi]

Only if you don't have a moisture barrier between the occupied space and the insulation and roof. Your local code may specify a cold roof (ie cold air space between insulation and underside of the roof). However, my research shows that a warm roof, applied directly to the underside of the rafters above a continuous vapor barrier, will not degrade the sheathing or shingles, and can provide superior insulation for a given amount of materials and effort.

A successful warm roof depends on a near-perfect vapor barrier though. Any air leaks will let moisture into the space between the vapor barrier and the roof sheathing, and there will be condensation (not good).

Okay, okay. Don't do this at home, unless you take full responsibility for your actions. I've tended to hedge on this point in the past because there are a couple of different points of view to contend with, both of which have been set forth here already. I'm going to explain further but not attempt to make the decision for anyone else. Not shown in my photo above is some detail that tends to support both the warm roof and cold roof approach. Yes, I have both types in my upstairs loft (aka, woodshop). The knee walls are what you describe as warm roof (ply, tar paper and shingles) and the flatter portion of the roof is cold (vented stringers and metal). Here is a picture that should help.

Still not shown is an inch of Dow rigid foam to the underside of the trusses (2" X 6" 's on 16" 's with R-19 tucked between) and a near perfect 4 or 6 mil Poly barrier throughout. I built my shop in 1981 (~26 yrs.) and it is still solid but I'm aware that there is/has been some trapped moisture in the ply right at the transition of the two systems (top of the knee wall). It was a trade-off when I built to have the look of shingles to better blend with the roof of my ranch home. If I need to re-shingle/re-roof (probably not in my lifetime) I will probably change out the shingles for metal on the knee walls. The metal upper portion (Brown side up) is faded to the point that galvanized metal is showing thru. Anyway that's my story and I'm sticking to it. Thanks timgr for giving it a name "warm roof".

 

[timgr]

From my understanding, the terms hot roof, cold roof and warm roof are in common usage.

The cold roof is the vented roof we usually expect and that is called for by code (usually - maybe not for garages). A warm roof puts the insulation between the occupied space and the roof sheathing with no ventilation between, and the vapor barrier facing the occupied space. A hot roof puts the (rigid foam) insulation on top of the roof sheathing, with the vapor barrier between the insulation and the sheathing, and the roofing on top.

Glad the post was of interest.

 

[flesburg]

I am no expert, but I have 2x6 rafters as part of the trusses in my garage. I have a ridge vent and louvers in the gutter "edge" overhang. I was told I could use R13 kraft backed bats (with the kraft facing the inside) between my trusses in the attic of my garage. There will be about a 2 inch gap between in insulation and the plywood roof sheeting. I think this will be a "cold roof".

Does this make sense to you experts, or should I not use R13 fiberglass insulation in this instance?

 

[carlosh]

Ohh jeese, the garage roof of mine has no vapor barrier at all, just drywall then the insuliation, then the roof sheathing.

How to I retrofit or keet it from rotting out???

Sorry to hijack....

 

[Steve in Mi]

I am no expert, but I have 2x6 rafters as part of the trusses in my garage. I have a ridge vent and louvers in the gutter "edge" overhang. I was told I could use R13 kraft backed bats (with the kraft facing the inside) between my trusses in the attic of my garage. There will be about a 2 inch gap between in insulation and the plywood roof sheeting. I think this will be a "cold roof".

Does this make sense to you experts, or should I not use R13 fiberglass insulation in this instance?

Here is how I would finish it up (my thoughts, not really advice). Use 1" X 3/4" strips tacked on each side of the space between rafters to give a one inch spacing for air movement soffet to ridge vent. Install 1" thick rigid foam against these strips and tight side to side in the cavity. Consider laying a small bead of DC Mastic 111 along all edges and **** joints if the foam is loose. If you use Dow rigid foam it will be ~ R= 6/inch, that will fill 2" of the 5.5" , 2" X 6" deep cavity. Next add the R= 13 fiberglas @ ~ 3.5" for a total R= !9 and poly moisture barrier on the warm side (inside) to protect the insulation from saturating from moisture inside the building.

BTW, the strips could be ripped from rigid foam, perhaps cheaper than wood and no rot.

 

[timgr]

Let me see if I can help, since I stirred this up.

First I'm an engineer, but not a pro in the trades, so I'm just offering what I've learned in the research I did to insulate my garage. The conventional roof, aka a cold roof, doesn't need a plastic or bituminous vapor barrier like the warm or hot roof do. The vents and space between the insulation and roof are supposed to carry away any moisture that passes through the gaps in the interior ceilings and insulation. Drywall is a pretty good vapor barrier actually, so you don't really need an additional vapor barrier between the interior and the insulation if the roof is ventilated. If the space between the insulation and the sheathing isn't vented, you could have moisture condensing there - without vents, you need something like a 4 mil plastic barrier between the conditioned space and the insulation to prevent moisture infiltration. And it needs to be well-sealed ... near perfect.

The cold roof was developed because modern structure are tight enough that moisture gets trapped inside the structure and condenses on the cold underside of the roof sheathing if something isn't done to prevent it. This condensation problem is typical of houses with flat interior roofs, where the bat insulation is laid on the attic floor, and the attic is cold. If I were going to make a cold cathedral ceiling like flesburg suggested, I'd use a 4 mil poly barrier on top of the kraft paper, or else drywall. The drywall has an added benefit of being fire retardant.

hth!

 

[Franz©]

Let me say it plain and simple, I see no tremendous need for a vapor barrier in a garage situation. Garage environments simply do NOT generate the ambient internal humidity a house environment generates.

Another simple reality, comparing a stressskin panel with enclosed foam to a built in place roof is a fools errand. You just can't duplicate the result in a field built assembly.
Even sprayed on foams fail to acheive the R value per inch of stresskin panels. They do however offer the added strength of the foam reenforcing the skin and superior connection between skin and structure.

I also do NOT like the conventional vapor barrier in a metal roof or sidewall situation. Metal is a very rapid heat conductor, and will condense any available vapor. In a metal skin situation, be it roof or wall, I firmly believe in a good membrane, 6 mil poly or equal, between the metal and the insulation, especially if the insulation is fiberglass, be it blown or bat. Fiberglass wicks moisture very effectively. It's used to soak up oil spills on water because fiberglass has a greater afinity for oil than water. A fiberglass bat against a metal skin will soak up water until it becomes saturated and it won't give the water up easily.

This can be easily proven by weighing a new roll of fiberglass and then leaving the fibrous edge against a damp floor for a week and reweighing it. Then hang the roll in a manner that completely surrounds the roll with dry air and weigh it a week later.


By adding the poly membrane between the skin and the fibrous type insulation, the metal skin can do what it will do with any humidity, condense it, and the poly will prevent the insulation from coming into contact with the condensate. There will still be some air movement between the skin and the poly, and this will reduce the amount of condensate stored there. The poly also provides less overall condensation in the inside of the building skin because it is a poor conductor of heat. Any airspace between the skin and the poly becomes an insulation layer.

The same principal of dressing in layers with multiple coats applys to a wall.

In any roof, a layer of poly will save the roofboards and insulation when there is a minor roof leake. Damn cheap insurance in my book.

 

[Steve in Mi]

Drywall is a pretty good vapor barrier actually, so you don't really need an additional vapor barrier between the interior and the insulation if the roof is ventilated.

No NO absolutely not going to happen. Drywall is not a vapor barrier or a moisture barrier. Lay a sheet of drywall in a puddle of water and watch it - the proof is there to see.

 

[Steve in Mi]

Let me say it plain and simple, I see no tremendous need for a vapor barrier in a garage situation. Garage environments simply do NOT generate the ambient internal humidity a house environment generates.

Tonge in cheek, when was the last time you washed your car in the house or let the ice and snow melt off of it without giving off moisture, lots of moisture.

 

[timgr]

Ok, Steve, what I read was "If you have a tight house with lots of insulation, you must seal up air leaks from the house into the roof. This doesn't mean you have to use a polyethylene vapor barrier. Carefully installed, drywall does just fine at stopping air leaks." This is from a Taunton article - no poly, drywall only, specifically in reference to a cold roof. What you write makes more sense to me though - I had not considered drywall as a barrier prior to reading this.

 

[timgr]

Tonge in cheek, when was the last time you washed your car in the house or let the ice and snow melt off of it without giving off moisture, lots of moisture.

Car exhaust contains a lot of moisture too.

 

[Franz©]

Tonge in cheek, when was the last time you washed your car in the house or let the ice and snow melt off of it without giving off moisture, lots of moisture.

Last time I washed my car is something I can't even remember. I don't believe in spoiling cars & trucks by washing them, they get in the habit and they get pissy if you break the cycle.

Washing a car might deposit a lot of liquid water onto a garage floor, and most of that will run down the drain as liquid, not become water vapor.

Ice and snow melt to liquid water, and rarely become water vapor, although a minimal amount will vaporize from a heated slab radiant system.

In a house, people exhale many gallons of liquid water in vapor form every day. Plants give off tremendous volume of water vapor, and cooking a pot of spagetti or anything else gives off vapor too. Entirely different situation.

Right now the humidity in this room is 40%, and the outside ambient humidity is 5%. The humidity in the garage is also 5%. When the garage door opens probably 60% of the volume of air in the garage will exchange with fresh outside air in under a minute. Even allowing for the humidity added to the garage from starting the car and driving it out, most of that will flush via the open garage door rapidly.

I suggest sticking a humidity meter in the garage and observing the actual situation.

 

[Steve in Mi]

Franz you don't have a heated garage do you? Maybe you don't need to heat where you are, don't know. Here in Michigan I find it desireable to heat 24/7 ~6 months of the year so I can putzy in my shop at will.

Humidity numbers without the temperature at which the humidity was measured doesn't mean a whole lot.

I see no tremendous need for a vapor barrier in a garage situation.

Define tremendous. Never mind just let me say (as you have said yourself) "poly is cheap" and if it keeps my insulation dry it does a GOOD thing and keeps me comfy and more of my heating $$$ in my pocket, which keeps me happy.

 

[carlosh]

ok, so I am feeling slightly better that I do not have to rip down part of the ceiling in the garage. Some background:
I have an attached expanded gargage that went from 20 x 20 to 26 x30. It looks like the front of the gargage was moved 20 feet closer to the street and the roof there is a cold roof, no venting as it is attached to the front side of the house. They used 2x 10's 12 inches on center with soffet vent at the bottom, but nothing at the top.

I put my bass boat in the garage that is wet and has some water in it, and at times I can see condensation on the garage door windows.

Am I still ok or should I be concerend that I am goign to rot out 17foot of roof sheathing......

 

[carlosh]

ok fellas, Am I in a sinking ship here? Any and all help will be appreciated

 

[dipper]

I plan on putting baffles in my ceiling rafters, then installing either r19 or r30 faced (will depend on pricing). The baffles provide adequate airflow from soffit up to the ridge vent.

 

[Blue Seducer]

I plan on putting baffles in my ceiling rafters, then installing either r19 or r30 faced (will depend on pricing). The baffles provide adequate airflow from soffit up to the ridge vent.

Thats how it's done here. Humidity usually 80 to 100%

 

[JMURiz]

I plan on putting baffles in my ceiling rafters, then installing either r19 or r30 faced (will depend on pricing). The baffles provide adequate airflow from soffit up to the ridge vent.

That's what I plan on doing, only I'll just use some of the R30c stuff so it'll fit nicely into a 2x10 w/o the need for baffles.

Off-Topic, dipper, what R-value are you using on the side walls?

 

[Ebbsspeed]

In a house, people exhale many gallons of liquid water in vapor form every day. Plants give off tremendous volume of water vapor, and cooking a pot of spagetti or anything else gives off vapor too. Entirely different situation.

Of the three listed above, the only one not applicable in my garage is the spaghetti cooking. But I do have a coffee pot out there. All of our outdoor plants spend the winter in the garage, and if I recall I continue to breathe when I am in the garage. Otherwise I wouldn't have all of that black **** in my nose from grinding & welding.

 

[bluesman2a]

if I recall I continue to breathe when I am in the garage. Otherwise I wouldn't have all of that black **** in my nose from grinding & welding.

This is commonly referred to as DRB or Dirt Road Boogers...

 

[kickassgarage]

I used a styrofoam product called DuraVent. It comes perforated down the middle so you can use it with 16"OC and 24"OC rafters. There are several different types of baffles that can be used to provide the breathing space between roof and insulation. My place is set up with ridge vents and soffit vents. I plan on stuffing insulation in the ceilings and then covering with drywall. I live in Georgia, so there is no telling what weather we will have....But one thing is for sure: We will have humidity.

You can see the duravent installed in the following link:

http://www.kickassgarage.com/images/Kick *** Garage HVAC Vent Bedroom Side.JPG

 

[rancherbill]

Here's my 2 cents worth.

Insulation is GOOD. http://www.garagejournal.com/forum/images/smilies/smile.gif
Insulating 2x6 will allow you only to put in R12 (2x4 insulation. this is way too low. I found this recommendation for insulation. I know it is residential not for a garage, but, it makes the point that insulation is good.

http://www.naima.org/pages/resources/library/order/BI487.HTML


I would nail a 2x4 onto the existing 2x6 to give you a cavity of 9" (3.5" + 5.5") and then put some R20 (6") into the space. it will give you lots of room for ventilation.

Do you have a continuous ridge vent? You need somewhere for the air to get out easily. Vents are for moisture and also for summer cooling. If you don't have a continuous ridge vent then nail a 2x4 across the peak of your roof to make an UPSIDE-DOWN A and put a vent in the end wall.

Moisture barriers are a great thing. Don't forget to put air barriers around all the electrical fixtures / outlets.
http://www.garagejournal.com/forum/images/smilies/beer.gif

Bill

http://www.naima.org/pages/resources/library/order/BI402.HTML
http://www.naima.org/pages/resources/library/order/BI473.HTML

 

[Deacon]

You would think I am pimping this material and solution but it will work very well. Spray in closed cell foam against the roof sheathing forms a total vapor barrier and NO air to condense. Net over the bottom of the rafters and blow in insulation. Same application in the floor joists. In a living environment you will need some type of air exchange/dehumidifier. I use HRV, heat recovery ventilation by Carrier. In a garage they are typically not as tight and there is most likely enough air leakage to make up the fresh air. And as soon as you open a garage door you will exchange air. If you have a gas heat source it will also draw moisture from the air during heat or air conditioning.

Foaming process, foam is approximately 2" in thickness to leave about 3-4" for insulation.

I don't have a shot of the vaulted ceilings but the same applies as the walls.

Blowing in insulation behind netting. The FG in the ceiling is below the foam since there was 12" and there is living space above...done primarily for sound barrier.

Finished insulation, again batts because of the depth of the rafters in this room.

Hope this provides yet another solution.

Deacon