No ventilation ?

[Moprar Racer]

Hello, great site and first post.
I have a 40x20 garage, that has a wall electric ac and heater combo.
The rafters have r-19 batts insulation and one roof vent.
We have moisture on the insulation which at times will drip to the ground.
And now mold has formed.

The roof has no ice and water shield just shingles on the sheathing.
We need to re roof anyway, so i was going to put it down, but was wondering if i should add addition roof vents.

How do i get the moisture out?
Do i add more roof vents?
Do i finish off the ceiling and insulate like a home attic?


Thanks alot...

 

[jvitez]

1. What type of construction, wood stud walls, cinder block, pole barn, metal building?
2. How old?
3. What type of roof: plywood/OSB, or individual boards?
4. Do you have soffits? Are they ventilated?
5. Do you use an unvented gas burning appliance for heat in addition to the electric unit (propane or kerosene heaters of any type)?
6. Vapour barrier under the insulation?
7. Where are you located?
8. Building heated all the time or just intermittently?

 

[Moprar Racer]

1. Wood stud construction.
2. 15yrs. old
3. Plywood roof.
4. Soffits vents on eave about 4 on front and rear.
5.Electric heater.
6. No vapor barrier.
7. Illinois, northen
8. Heated in winter, cooled in summer.
Its the issue in summer months. when really hot out and the air is on the insulation is soaked. I think the heat just stays up and cant get out.

 

[danski0224]

Is the insulation covered and touching (but not compressed) building materials on all 6 sides?

If the insulation is exposed, air moves through it and then has no insulating value. Moisture lowers it further.

 

[Moprar Racer]

Yes insulation is exposed. I am doing a new roof this weekend and going to install roof vents (6) I am thinking of finishing off the ceiling and adding blown in insulation and a pull down stair to access it.

The attic is open right now and not sure on to drywall and just use 4x8 osb on top of the ceiling rafters to finish it.

 

[pauls340]

I had the exact problem caused from stapling the insulation to the ceiling trusses. As soon as I removed it and just stuffed it between the floor joists, the sweating went away...thank you City Inspector.

 

[Moprar Racer]

Mine are stapled also.Is you garage ceiling opened with just insulation in between the rafters?

Any vents in the roof?

 

[Doxhog]

How about putting on a ridge vent?

 

[Rich H.]

Calculate the attic square footage, then use either rule of thumb:

Without vapor barrier or retarder, 1 sq foot of soffit and roof vent for every 150 square foot of attic

With vapor barrier or retarder, 1 sq foot of soffit and roof vent for every 300 square feet of attic

Split the vent area 50/50 between soffit and roof. Does not have to be exact science, just try to get in the ballpark. With only one roof vent it's way off from where it should be. A little extra vent area is way better than not enough, so go conservative/on the high side.

Ridge vent contradicts all of the above, as far as the area calcs....the idea is the same though.

Make darn sure the soffit vents don't get clogged up...use rafter baffles to keep them open.
All the roof vents in the world won't work unless the soffits are open and can act as an intake.

Also make darn sure the rafter insulation overlaps the top plate of the wall completely! If you leave a gap at the top plate you'll get moisture inside the wall instead of in the attic, and there will be mold in the walls.

Kraft paper is considered a vapor retarder, so if it is a stapled assembly you do have a little something there to slow down the movement. Some sort of panels over the insulation painted with several coats of latex paint is an even better vapor retarder and would be preferred. Painted drywall is supposed to be a better vapor retarder than osb, and it does have some r value + noise mitigation benefits but I understand why some would not want it, it can be a real PITA.

 

[Moprar Racer]

Do i run the soffit baffles under the insulation? (Attached to the decking/shething)

So take down the insulation, run the baffles from soffit all the way to ridge vent then put the insulation over the baffles, Each rafter space should have the baffle, and then a continuous soffit under the eave?

 

[ForceFed70]

One roof vent is definately not enough. You should have at least 3 of them.

I would try to add a gable vent as well.

 

[Rich H.]

Sounds like you want to do a ridge vent, then?

You only really need 1 to 2 inches of "breathing room" from soffit to ridge. The only purpose of the rafter baffles is to make sure the soffits stay open and don't get clogged up with insulation.

The baffles don't need to be installed from bottom all the way up to the ridge.... You only need them in the areas in close proximity to the insulation.

Check out this link and click around, it will explain it better than I can.

http://www.energystar.gov/index.cfm?c=diy.diy_attic_ventilation

As far as the soffit vents go, you won't do any harm by going either continuous, or for instance every other rafter bay.

 

[Falcon67]

I would try to add a gable vent as well.

Not if you are using soffit vents - it will kill off the air flow.

 

[ForceFed70]

Not if you are using soffit vents - it will kill off the air flow.

Can't see how additional venting would hurt airflow.