Contractor Talk - Professional Construction and Remodeling Forum banner
1 - 12 of 12 Posts

·
Registered
Joined
·
143 Posts
Discussion Starter · #1 ·
Am updating an old house that the homeowner gutted some years ago. Storey and a half, with 2x8 rafters, and the ceiling following the rafters for about 4' from the exterior walls, then 2x4 ceiling joists with a cold attic space. The attic was previously ventilated with 2 gable vents, and no soffit venting.

Replacing the HVAC required putting second floor supply ducts in the attic, in insulated duct wrapped with insulation. Replacing unvented soffits with perf aluminum, rotted fascia with new, and new asphalt shingles. Gable vents will be blocked.

The original plan was to install ventilation baffles over the fg batts in the section of the attic where the ceiling is tight to the rafters, and install peak vents, in order to have ventilation from soffit to peak. Problem is, the insulation is so tight that I can only squeeze some hardboard in between the roof sheathing and the insulation in about half the rafter bays (working from the eaves with fascia removed). Vent space is created where roofing nails through sheathing hold the hardboard away from the sheathing, about 1/2".

Question for you, if you understood the mess above, will there be enough attic ventilation with baffles installed on about half the bays, or should I install vent intakes under the shingles just above where the ceiling joists meet the rafters, creating a short circuit and leaving the roof below that level unvented?
 

·
Registered
Joined
·
90 Posts
we had a similar situation. our insulation sub convinced the client & architect to go with a hybrid 2" closed-cell foam with firespread. lesson learned on our end is that the 2" closed-cell qualifies as vapor barrier which, according to our architect, makes venting through the soffits unnecessary. had we room i would have used the venting boards and kept the soffit vents active, but we nixed venting all together in this case.
 

·
Registered
Joined
·
143 Posts
Discussion Starter · #3 ·
Not removing drywall on interior, so can't do much with the insulation unless I remove the roof sheathing entirely. So, sloped area insulated with fg batt, and attic has batts topped with cellulose.

I don't have a problem installing mid-slope vents, as I think it will provide good ventilation for the upper attic, but I think it will short-circuit the soffit vents, and leave the sloped area essentially unvented. I am looking for some feedback; maybe I'm wrong about the short-circuit.

I thought I was wrong about something last week - turned out I was mistaken about that; I'm never wrong.
 

·
Registered
Joined
·
90 Posts
you're not wrong...i think you knew that.


any bay not fully vented from soffit to ridge presents moisture issues. my gut is suggest decorative attic vents on opposing gable ends and declare victory?
 

·
Registered
Joined
·
330 Posts
I think your method will work .

I have vented many a roof that previously had ice dams a foot tall with icicles stretching to the ground and now not even a popsicle I so enjoy driving by these places year after year .

Again I think your approach will work not the way we would design it new, but will accomplish your goal of no ice dams
 

·
Registered
Joined
·
2,565 Posts
Question for you, if you understood the mess above, will there be enough attic ventilation with baffles installed on about half the bays, or should I install vent intakes under the shingles just above where the ceiling joists meet the rafters, creating a short circuit and leaving the roof below that level unvented?
http://www.contractortalk.com/report.php?p=824198



This formula should help here.
Proper ventilation is neccessary for roofing warrantys as well as moisture and ice dam problems.




Calculating how much venting your attic needs is relatively simple. All you need to know is the area of the attic floor. Include the garage, if you have one, and the soffited overhang because heat gets trapped above them, too. A common rule of thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square feet of attic floor space. Let’s look at an example. Say you have an 1,800 square foot home with a garage that measures 20 feet by 22 feet. This will yield a total area of 2,240 square feet. You then divide this number by 300.


2,240 ÷300 @ 7.5This tells us that we need 7.5 square feet of ventilation for the attic. Most attic vents are measured by square inches so we need to convert the 7.5 square feet to square inches. This is done by some simple multiplication. 1 square foot is equal to 144 square inches, so we multiply 7.5 by 144.7.5 x 144 = 1,080 So we need 1,080 square inches of Net Free Vent Area. Divide this by two and we see that we need 540 square inches of intake ventilation and 540 square inches of exhaust ventilation. There is always a lot of concern for what the best type of ventilation is. You have already read that you need both intake ventilation and exhaust ventilation installed at an approximate one to one (1:1) ratio. Now remember that the idea behind this is for maximum air circulation. Installing more than 1 square foot of ventilation per 300 square feet of attic floor space will not hurt anything – it’s a general guideline and code requirement in some areas. Most roofing professionals will agree that the best type of ventilation is continuous soffit and ridge ventilation. If a continuous exhaust vent and an equal or slightly greater amount of intake vent is installed, then the attic will be ventilated for its entire length.
 

·
Registered
Joined
·
2,565 Posts
Not removing drywall on interior, so can't do much with the insulation unless I remove the roof sheathing entirely. So, sloped area insulated with fg batt, and attic has batts topped with cellulose.

I don't have a problem installing mid-slope vents, as I think it will provide good ventilation for the upper attic, but I think it will short-circuit the soffit vents, and leave the sloped area essentially unvented. I am looking for some feedback; maybe I'm wrong about the short-circuit.

I thought I was wrong about something last week - turned out I was mistaken about that; I'm never wrong.

  1. Intake and exhaust ventilation should be installed at an approximate one to one (1:1) ratio. More at the eaves is better if it can be attained.
  2. The 1/300 rule - 1 square foot of attic ventilation (Net Free Vent Area) per 300 square feet of attic floor space.
  3. No attic vents should be installed between the intake and exhaust vents.
  4. There should be at least three feet (3') of vertical distance between the intake vnets and the exhaust vents.
 

·
Registered
Joined
·
143 Posts
Discussion Starter · #10 ·
Um - my bad. Didn't mention that I had pushed up some 3/4" strips in some of the very tight vent spaces. I will go back and open them all up more.

The calculations make it clear - go back, open up the vent spaces more, and no midroof vents.

Tom - I certainly didn't think of the hardboard as a condensing surface. Cardboard baffles are installed at the eaves on every new house, so I expected a similar result. I can't install the hardboard tight between the rafters anyhow, so I am expecting that if there is airflow above the hardboard, there will be some air movement within the batts as well, since a few inches at each side of the hardboard will be exposed.

Wheeler - gable vents not an option, as one gable is blocked by an addition.
 

·
Registered
Joined
·
17,271 Posts
not saying it would be,just could be

besides keeping the roof surface cool another reason for ventilation is to help remove moist warm air that could come thru the fiberglass

i realise this is not your situation but sometimes you hear of continuous insulation baffles installed in each bay from soffit to ridge in cathedral ceilings which to me doesn't address this moisture migration and could potentially cause it to condense on the underside of the baffles
 

·
Registered
Joined
·
143 Posts
Discussion Starter · #12 ·
Actually, I just completed a cathedral ceiling with that detail. The baffles were not required, as we had 16" depth in the parallel chord trusses, and 12" batts, but I wanted to avoid excessive wind wash through the batts - the building site is windy and exposed, and damn cold.

I installed 2' cardboard baffles from bottom to within a foot of the peak, but lapped the baffles with a small opening between each. The hard draft upward should be mainly confined between sheathing and cardboard, but the gaps at 2' should draw some air (and its water vapour) out of the batts.
 
1 - 12 of 12 Posts
Top