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-   -   venting an odd low pitch cathedral (http://www.diychatroom.com/f9/venting-odd-low-pitch-cathedral-8242/)

jfree 05-04-2007 09:13 PM

venting an odd low pitch cathedral
 
2 Attachment(s)
I live in upstate NY and have been asked to remedy a low pitched cathedral roof section that has noticable signs of water damage at the ridge where it meets a steeply pitched section, on the other side. The wall board tape is peeling, at the ridge and considerable staining shows, etc. The owner says that coming out of winter when there is a warm up, the water starts dripping. I suppose that moisture is entering the roof cavity through the can lights and freezing. The cathedral is less than 2:12 and meets a 14:12, on the other side. The low pitch is torch down with shingles on the high pitch. The high pitch is vented properly with baffles all the way to the ridge. The cathedral is insulated with 12" bats and has a vapor barrier between insulation and sheetrock, but no air space for venting. The ridge is not vented on the cathedral side, but has soffit venting. There are also can lights that are probably leaking heat and moisture, not helping the matter. If I gut the ceiling and insulation, pull the can lights, provide air space with styrofaom baffles to the ridge, add vapor barrier,add soffit vents, how do I then ventilate this tricky low slope situation at the ridge? There is currently a ridge vent situation that goes from the steep slope over the top to the low slope, but there is no way for the air to get into it from the low slope, no air space cut into the ridge on the low pitch. I think the the ridge vent might also leak where it comes over onto the 2:12.

Ed the Roofer 05-04-2007 09:33 PM

If you could post some photos of the areas in question, I may be able to offer you some additional advise.

For the rest of your provided solutions so far, you are heading in the right direction.

Remember, the soffit, or air intake ventilation system nust be continuous and provide the same or greater amount of NFVA, (Net Free Venting Area), as that of the exhaust ventilation system or product.

I use a product called "Smart Vent", manufactured by DCI Products Inc., and it provides 9 square inches of intake ventilation per lineal foot installed. If this is done equally on both sides of the ridge, which can provide 18 square inches of exhaust ventilation per lineal foot, then you have a "Balanced" ventilation system.

Ed

jfree 05-05-2007 08:14 AM

Thanks Ed

I will upload some photos tonight. The problem here is the nearly flat section may let water in through a ridge vent. Right now someone has one going from a very steeply pitched shingled side to a nearly flat side. I don't think the flat side is vented at the ridge. I reached my hand inside the ceiling through a can light and felt the ridge beam to roof sheathing connection and it feels like there is no opening. Are there ways to vent very low slopes without power venting each bay. Someone has recommended 2# closed cell foam in the entire roof system, to disallow all vapor, I am not sure about this.

AaronB 05-05-2007 09:22 AM

WHy would you need to vent a low slope, except for moisture getting through to your various blding materials?

If there is no attic space, what are you trying to accomplish?

Ed the Roofer 05-05-2007 10:57 AM

How large of an attic space are you talking about?

Include photos of all of your areas of concern please.

Ed

jfree 05-05-2007 04:56 PM

2 Attachment(s)
Here are a couple of photos I took today. The side of concern is the Cathedral ceiling, with no attic space, just a 10 rafter cavity, 24" on center. I cut several large holes in the sheetrock on the 1:12 shed roof side and found alot of moisture between the vapor barrier and the fiberglass insulation, doing damage to the sheetrock, etc. The top of the rafter bay is dry under the roof sheathing except in one place where I lifted the ridge cap and found a section where water is coming in, shown in the photo, obviously a problem to be fixed. But I also think there is alot of condensation build up where hot meets cold. I am wondering whether it is best to try and vent this roof, by drilling holes in the ridge on the low pitch side to let air into the steep pitch side to escape so moisture doesn't condense on the cool surface, or focus more on sealing this roof up, eliminating can lights, soffit vents, etc. Some one has recommeded close cell spray foam as a means to do this. What do you think? What do roofers do ordinarily on flat or near flat roof sections? Are flat roofs usually sealed up? Also this house is on a creek which might also be adding to the moisture problems. THis area is approx 12x20.

Ed the Roofer 05-05-2007 08:16 PM

Firstly, are you sure that the soffit venting is unclogged and actually providing the intake ventilation to the insulation baffled chambers, allowing for a free flowage of unimpeded airflow?

Secondly, the metal ridge vent looked like it was the cause of moisture infiltration on the top side of the decking.

I use the Shingle Vent II, manufactured by Air Vent Corporation. As a secondary protection against moisture entering into the vent slot in unusual circumstances or applications, I install an aluminum J-channel on the interior of the ridge vent in the hollowed space inside. the long side of the J-channel is secured to the roof decking and the felt paper or Ice and Water shield is applied up to the right angle bend of the J. This prohibits moisture from getting blown in under the exterior baffle of the ridge vent product, especially on low slope tie-ins.

Somehow connecting the opposing side attic and vaulted bay air spaces to create a funneling effect to flush out the moisture laden air in the interior would be advisable, and I feel you have enough insight to determine the best way to achieve this on your own.

Elimination of the canned light heat sources also will eliminate the "holes" in the cieling structure, which promotes the moisture infilatration due to interior Relative Humidity. Also, possibly running a de-humidifier in the vaulted sections of the home may thwart the excessive content in the first place.

Ed

jfree 05-06-2007 09:10 AM

Thanks for the reply, Both sides of this roof are cathedrals, no attic space. What do you think about the closed cell foam idea.

AaronB 05-06-2007 11:29 AM

Are you sure this is not roof leakage? Looks like it to me.

jfree 05-06-2007 08:33 PM

Thanks for your interest,

This roof section is about 22 feet long and it is only taking in water in this location where you see me lifting the ridge and pulling back the roof membrane. The rest of the roof ridge seems dry looking from the outside. This area is wet as you can see and this has to be stopped. Any ideas as to how to seal this up, add a vent cap that at least vents the other side. From the inside I took down sections of drywall that were not in the area that is taking in exterior moisture and they were very wet as well, but only right on top of the vapor barrier and the first inch of fiberglass batt insulation. The top of the rafter bays are dry. There are also can lights that I will eliminate. My problem is to decide whether to vent the roof or not. If it was easy to vent I would just go ahead and do it. But I am a little concerned about drilling holes through the ridge to allow air from the flat section to go over into the ridge vent on the steep section. Can I cut a slot on the 1:12 pitch to let air escape into a ridge vent? I could not find any vents that were for pitches this shallow. I will seal the interior up completely by removing can lights and filling any drilled holes in the tops of the walls, adding a new vapor barrier,etc. I would love to find some solution that is firstly watertight from the outside, and prevents moisture buildup. The owner is willing to pay to do the right thing. I will be consulting with roofing contractors, but I thought I would inform myself as to what would be needed first. Again, what about spraying the cavities full of closed cell foam? Soy based, thus eliminating all air flow, eliminating the need for a vapor barrier. one problem with foam is that the cavity is 10" deep and I would not need to spray it full, would I need to use batts for what was left or leave a void between the foam and the sheetrock? Anyway, lots of questions I know, sorry to ramble, thanks again, trying to do the right thing!

AaronB 05-06-2007 10:47 PM

Are you sure the water leakage isnt runni9ng down the ceiling?

That ridge detail is surely the leak point. This roof should not be vented, and the front roof face should not be vented in this manner due to the existence of the flat roof.

I was thinking you werent a roofer by the questions...did not know if you had mentioned it.

AaronB 05-06-2007 10:49 PM

Spray a foam sandwich....

1" of closed cell, all but one inch of open cell, and one more inch of closed cell, and trim. this will give you your vapor retgarder top and bottom. 1/2# is an airseal at 5", but not a vapor barrier.

R-35 plus.

AaronB 05-06-2007 10:51 PM

and it will save you money opposed to closed cell, and closed cell does not have a fire rating at more then 4" thick.

You should be fine with the way I described, but check the local codes.


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