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Cathedral ceiling ventilation problem

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Mr Rudnicki,

I am on my 3rd roof in 6 years. 1st roof had shingles only they started to curl after about 2 years. 2nd roof had architectural shingles and new plywood 3 years later the plywood started to lift due to moisture. I just put my 3rd roof on with new plywood and architectural shingles again.

I am having this problem with the back of my house only. The front of the house has architectural shingles and original slotted wood instead of plywood. The problem side is the cathedral ceiling for about 30 feet to the attic where I have a 5 foot in height open space into about 20 foot wide in length of house. On the catheral side I have insulation with vapor barrier down I have channels in there for air.

I have a power fan, no gable vent, no ridge vent and I have the soffets open only on the cathedral side, I went to the attic 2 days ago and had a layer of frost covering everything. Also my attic floor is insulated I have no insulation on the roof in the attic. I have a gas insert in my fireplace and gas heat in the house could running both at the same time present a problem and should the soffet vents in the front of the house be open?  Any help would be appreciated I am at my wits end!!
asked in Attic Area by richardb (120 points)
edited by darekrudy
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1 Answer

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Hi Richard,
Darek is perfectly fine smiley, "Mr" makes me feel even older than I am...
 
An important part of proper attic ventilation is its balancing. I can’t figure out from your description if the front attic section (where there’s no problem) and the cathedral ceiling attic (5’tall x 20’ long x how wide?) are connected in any way.
  1. If the attics are connected, are there any vents in the front attic (what is the floor size of that front attic) or everything you have (for both) is a single power vent and soffit vents along the cathedral section?
     
  2. What type of controls there are on your power vent? Heat thermostat or heat / humidistat combo?
     
  3. Power vent rating in CFM’s? So you know if it is adequate to ventilate an entire attic.
     
  4. Cathedral ceiling soffit vents: are they open under every rafter bay? Installed doesn’t always mean that they are open unless you did it yourself.
     
  5. Cathedral ceiling baffles (air channels): are they installed in every rafter bay / 30 feet of open air flow?
     
  6. Is the frost visible on roof decking surface in that 5’ tall section or inside the baffles as well?
     
  7. Where are you located (climate zone)?
     
  8. Are your appliances vented through the chimney or side wall / direct vent / located under the soffit?
     
  9. Any other vents (bathrooms, kitchen, etc.) accidentally discharging into the attic?
Let me know so we can find a solution to your problems.
Just click “comment” below. 
answered by darekrudy (21,730 points)
At the top of the Cathedral ceiling the front and rear attic is connected 5 tall 29 wide 40 long. when I say there is no problem with the front section I mean the shingles and boards have never lifted but the whole attic is covered in frost.

The attics are connected it's one big open space the vents in the front are not open the floor size of the front attic is 20 wide and 40 long.  Soffet vvents are open along the cathedral section.

Controls on power vent are humidistat combo. Power Vent rating is good for 2600 square feet.

Cathedral soffet vents are open under every rafter bay I did this myself.

Cathedral ceiling baffels are installed in ever rafter bay from soffet vent to open attic space I did this also.

Frost is visable on roof decking surface and inside baffels.

Climate zone is East Coast New England

Appliances are vented thru the chimney the cloths dryer is vented thru the side of the house.

No vents accidentally discharging into the attic.

Thank You,

Richard

all applai

Hello Richard,

We need to start from the fundamentals of attic ventilation. Based on your attic’s power vent potential it has probably around 1800 CFM’s. There is simple formula to calculate required intake area for a power vent:

1800 CFM / 300 = 6 square feet of intake net free area needed

Because net free air flow specification are always listed in inches we need to multiply 6 x 144 = 864 square inches of intake net free area needed.

Since I favor continuous soffit vents the most, I’ll use AirVent’s® product as an example (I’m not their affiliatesmiley). It has 2-3/4” x 96” dimensions and each foot provides 9 sq inches of net free area.
864 / 9 = 96 linear feet of this particular vent (http://www.airvent.com/homeowner/products/intake-soffit.shtml)

For the attic ventilation to function properly those vents should be distributed equally along all the soffits, not only in your cathedral ceiling area. The air has to flow continuously from each soffit to the top and outside the attic before the vapor has a chance to condensate on decking surface.

However, that continuous air flow might be a problem in your configuration. Power vents perform best (at least based on my observations) when installed at a peak of a cone / pyramid shaped roof, where the ridge line is very short. This creates chimney-like effect and directs all the air towards the power vent opening and the sensors.  

With a 40’ long ridge line and a single power vent cutout somwhere along this line there may not be enough passive air flow area in that upper section of your attic to create a draft and pull moisture saturated air all the way up to activate humidistat sensor. The power vent would have to run constantly (or most of the time) to provide recommended amount of air exchanges for such a big attic.

This is what I would do: start from opening the soffits around the house and try adjusting the humidistat so it activates the power vent at a lower humidity level. You can also install (personally I’ve never seen that installed but it seems like a good idea) a fan that operates continuously at a low speed inducing air circulation through the attic - http://www.tjernlund.com/retail/Roof-Vent-Booster.htm.

If all of the above fails, install a ridge vent on your roof. Although experts don’t recommend mixing power vents with any other types of passive roof ventilation products I wouldn’t remove it. With heavy snow accumulations on top of the roof this might be the only way for the humid air to escape from your attic.

I hope this will help and I’d appreciate if you let me know about your progress in correcting this problem. Maybe this can help somebody else as well. Thank you and let me know if you have more questions.

Darek
 

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