Another insulation/ventilation thread (Raleigh NC)

[ttownthomas]

I know there are a ton of threads about this topic but I have looked and looked and cant seem to figure out this answer. I have an unfinished attached garage and I have a ventilation question.

I live in Central NC (Raleigh) Zone 8a. The garage was built in 2005 (I moved in a year ago). It has uninsulated, OSB exterior walls, 2x4 studs on 16" centers and engineered trusses with 24" spacing. Its an attached garage that was an addition, it looks to have either a 4:12 or a 5:12 pitch. It is 22' x 24' and the soffits overhang 2 feet. It has a plastic 2" wide continuous soffit vent that is functional. There are no gable vents, box vents or ridge vents. I am looking to insulate the walls (r15 batts) and ceiling (batts between the trusses) and then put up 5/8 sheetrock on the walls and on the ceiling under the trusses. (sheetrock to minimize the likelihood I burn the place down while welding) and I will eventually add some heat source or perhaps a mini split.

The question is: I would rather not deal with cutting in ridge ventilation or box vents, and only one gable end is exposed and to able to have a gable vent installed. Can I get the proper amount of airflow to avoid moisture problems if I only add gable ventilation on one side? Or do I also need a box vent on the other side? The attached gable end is under the house roof gable end overhang. Or is there some other way to solve this ventilation issue?

Chris
Raleigh NC

 

[billconner]

I believe you would be fine with one large table vent. Exactly what I did. I used a hinged one for required attic access. s8zed it to be 40% of 1/300 of plan área. Don't oversize it or not much.

ps: size is ner free area

 

[OccupantRJ]

I live in Goldsboro. You could add one properly sized gable vent along with a thermostatically controlled fan blowing out the vent. My shop @1095 sq ft will be at 55-56 degrees recently inside when I go out there on a 23 degree morning with no heat on. It has one gable vent along with soffit venting, insulation in the walls, but none above the ceiling, and none on the metal garage door. The door radiates heat inside because it faces South, and when I have the heat on and the thermostat set at 65 degrees the heat rise from the door will outpace the heat pump setting by several degrees. I have considered insulating the door, but not sure if it would be all that beneficial overall considering I only heat it when using the shop. In the summer I try to have the garage door open unless it is scorching, then I shut the door and run the ac just enough to dehumidify things a bit.

 

[ttownthomas]

Is there a table or a formula someplace for sizing gable vents?

 

[Notgrownup]

Like RJ said, 1 thermal controlled end gable fan would work, I would like to have some kind of passive vent.. I have 24x24 batt insulated walls, blown ceilings and a mini split. I’m 1 hour east of you. Mine is ridge vented.

 

[oldmxracer]

Is there a table or a formula someplace for sizing gable vents?

Roof Ventilation Calculator

Let Owens Corning Roofing help you calculate exactly how much ventilation you will need for a healthy and balanced attic, with our 4-step ventilation calculator.

www.owenscorning.com

 

[billconner]

Is there a table or a formula someplace for sizing gable vents?

I suggested above what's in building code for a 60% low and 40% high system - total net free area venting 1/300 of plan area.

I don't trust manufacture's calculators to much; they tend to favor buying more product.

A hard problem will be calculating the net free area of existing soffits.

 

[ttownthomas]

My soffit vents flow 9 square inches per 8 linear feet....which I think is quite a bit more than 150:1, Right? I have 21 feet of soffit ventilation per side. 9 inches / 12 = .75 feet per 8 feet. .75 feet x (42/8)=3.93sq feet. Attic is 22x24=528 Square feet. 528/150=3.52 sq feet, which I think means I have more than double the recommended soffit ventilation? And I think that means I should have 6.55 total square feet of ventilation to match a 60/40 system. <-3.93 / 60% = 6.55. That leaves 6.55 - 3.93 = 2.62 feet of upper ventilation needed???

I researched ridge vents. dosen't seem too complex after all.

 

[never enuf time]

Ridge vent isn't too hard to do. See if the osb is below the very top of the peak. If not will need to cut 1 1/2 below that.
The plastic ridge vent goes on easy. Roofing gun would be helpful nailing cap on