cathederal ceiling

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  #1  
Old 03-06-08, 06:20 AM
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cathederal ceiling

I need to insulate a new cathederal ceiling. I have a shed style roof with no venting on the top. The rafters are 2 X 6 and will have the soffit vented. We have pretty cold winters. I want a bullet prove way to do this so as to have no moisture problems. How about drilling the rafters along the top plate and venting through the side? thanks
 
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  #2  
Old 03-06-08, 07:34 AM
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We have a cathedral ceiling with no venting anywhere and a prior owner put in 1" foam board covered with wood paneling. It did cut down on heat transfer. We also have no moisture problems. We still have about 2-2.5" of rafter showing. The foam board is nailed to the roof.
 
  #3  
Old 03-06-08, 03:56 PM
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Drilling holes in rafters in order to faciliate airflow is a waste of time.

When you say you have a cathedral ceiling with 2x6 rafters, I hope you are not saying that the roof is on one side of the 2x6 and the drywall ceiling is on the other side of the 2x6. If so, you won't have adequate insulation thickness and you're asking for problems with frost / melting cycles.

A closed attic with foam board insulation (seal all edges to prevent air infiltration) or professionally installed spray foam would probably be a better solution if your rafters lack enough depth to get plenty of r-value up there.
 
  #4  
Old 03-06-08, 07:52 PM
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yes it is

The roof will be on one side and the drywall on the other. My builder says all you need is a air gap above the insulation. I know that is not true. I know he has researched the situation with other builders. He is now going to install a vent up top where the new shed style roof meets the house the entire width. I do agree on the spray insulation. I will have to decide tonight on the vent being he wants to shingle tomorrow. I am going to call for a price on spraying it as soon as I can. Thanks for your inputs...Jim
 
  #5  
Old 03-06-08, 08:09 PM
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When framing a cathedral with a free standing ridge or against an existing wall, (shed) the ridgeboard must be held "low", so that ventilation flows around the end (ridge cut) of the rafter and parallel down to the soffit vents. Butting the rafters to a ridge without holding the ridge low creates numerous separate channels. You want all to be in a "common air space". Once the ridge is dropped and the rafter tips stick above the ridge you have now created a way to leave the plywood short by 2.5 inches and run strip vent. They make a special venta-ridge for this application. (no leaks) So air enters the soffit vents and follows the rafter bays up and flows around the ends also, just like a real attic. Follow the rest of the rules when you insulate between, and leave that airway above whatever insulation you choose. You cant blow air into a pop bottle! Sounds like your builders vent strip at the top is right on the money! Thats the way we do them here in Sugar Valley!

Good Luck!

INDII
 
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Old 03-06-08, 08:12 PM
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Yeah, I'd have to disagree with your carpenter on this one. While in many cases, it's good to have an air space, if it means you can only get R-13 fiberglass in there... or compress batts of R-21 (which results in less than R-21 since fiberglass should not be compressed) that will be a very UNDERinsulated roof. The only way to get a good R-value is to forget the venting, spray the rafters completely full with Icynene and go with a closed roof.

I' was in one house where water was running out of a cathedral ceilings roof like a garden hose. And they thought they had properly insulated and vented the space! Guess not!
 
  #7  
Old 03-06-08, 08:16 PM
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Forgot one thing. In our area an R-30 is required in a cathedral and baffels above. FIR DOWN YOUR RAFTERS! That foam idea is a good one and the only way your going to achieve a good R-value. Good Luck again!

INDII
 
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Old 03-08-08, 06:49 PM
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Exclamation Why vent at all?

Is XSleeper not right? If spray foam is used or dense pack cellulose, then why is there a need to vent? If code requires a vent space due to the historical problems with fiber glass, then the contractor or insulating subcontractor should sit down with the building inspector to explain the need not to vent.
 
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Old 03-09-08, 03:47 AM
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I think where the carpenter is missing the boat in venting, is you have to vent both ends of the cathedral, ridge and soffit to allow proper air flow over the insulation, using air baffles over the insulation. Here, again, I agree with XSleeper, R13 is about all you will get in a 2x6, so it will be minimal at best. If you spray, venting will be a moot point, as the air will have no where to go, nor a way to get there.
 
  #10  
Old 03-09-08, 07:38 AM
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Spray foam insulation is a major product used in the air sealing industry. There are quite of few different kinds of foam. Some foams even have a fire rating. Regardless of the type of foam they all have a “Perm Rating”. This indicates water vapor transference per square inch of this material in time. This explicitly implies that all foams will allow water vapor to permeate it, regardless of the type of foam.

When it comes to cathedral ceilings many people argue that air leakage is the cause for the problems associated with moisture. They believe that spray foam being an excellent air barrier eliminates these problems. If you haven’t figured this out, I oppose this view. To be fair they argue that the amount of water vapor permeation with foam is very small and this amount can easily be absorb by other components of the roofing system without any adverse effects.

There are several ways moisture is transferred within a structure. To name a few, equilibrium, capillary suction and diffusion. The factors that influence this transmission are temperature and pressure. While one can argue each aspect independently, what has more of an impact is when these mechanisms and factors interact. For example, let’s say the water vapor permeability is at 18% Relative Humidity (RH%) at 70 degrees Fahrenheit on the warm side of this cathedral ceiling. Since RH% increases as the temperature drops if the temperature of the roof decking is cold the RH% would be 100%RH and condensation would be formed.

While one can argue that the roof decking without any adverse effect can easily absorb this amount of condensation, what is clearly overlooked or even ignored with their argument is the water vapor permeability of the roofing material and the probability that this moisture flow will be continuous during the heating season.

These articles we read concerning this application with foam are either from manufacturers and/or people who support this application. I clearly oppose it.

There are several ways to ventilate a cathedral ceiling and in my opinion the best way is to install a “Cold Roof”. However, you do have other options.
 
  #11  
Old 03-09-08, 09:09 AM
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thanks all

I appreciate all you feedback. As it stands, I now have an opening at the top of the roof about 2 inches wide. It's covered up with basically a ridge vent with the opening on down side and air baffles will be used. I am wondering what the store bought looks like...thanks again...
 
  #12  
Old 03-12-08, 04:15 PM
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same deal?

i dont want to hijack a thread but i have the same deal...2x6...shingles and plywood on top dry wall will be on bottom..ridge is vented and eaves are vented..builder says instll those "pink foam thingys" used in other attics..right from the eave to the peak...then cover and fill everything with spray foam.....i know nothing of building homes...will this work...i do understand that i need foam to get t eh r value..but is this the proper way of venting?

the vents will be under the plywood..then filled with foam..drywall will be tight against the foam (or close anyway)

thanks
 
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