Replies

1. 
   Rob_Susz_ | Mar 22, 2001 06:06am | #1

   *
   Why do you want to vent? What problem are you trying to fix?

   -Rob

   P.S. - I am positive that your house, because it is a cape, needs some work to improve it's performance, but please tell me the symptoms first.

1. 
   Charlie_S. | Mar 23, 2001 02:05pm | #2

   *The house is in a heavy snow area. There is no way to maintain a cold roof over the sloped ceiling since air from the soffit vents can get up to the gable end vents. The fiberglass-packed rafter bays block air flow, but also keep the roof warm enough to met some of the snow resulting in ice dams at the eaves. the homeowner doesn't want to remove the sloped ceiling to install plastic air channels along the roof sheathing.

1. 
   Rob_Susz_ | Mar 31, 2001 04:22am | #3

   *The problem is convection. Too much hot air leaking out and the glass doesn't do much to stop it. A common retrofit for this is to tube fill the rafter bays with cellulose, and do air sealing work anywhere you can on the inside.I am willing to bet alot of money that this cape has TONS of air sealing issues. New or old - doesn't matter. Has a blower door test been done? Without a blower door test the "tightness" of the house cannot be estimated.If this cape didn't have tons of sir sealing issues, it wouldn't make ice.So, is there a continuous air barrier between the first and second floor? Are the access doors to the kneewall attics gasketed, insulated, and airtight? Are the chimney chases capped and blocked as far as airflow is concerned?Venting a roof is insurance that people buy because they don't know what the hell they are doing. If you are thinking at this point that you want to debate the validity of venting, please do some research first. Air sealed attics and roofs don't need venting, period.-Rob

1. 
   Ron_Rosa | Apr 02, 2001 04:21am | #4

   *Question? Would you install venting on your home if your attic and roof where air tight? Before you answer that question , the cost of venting the average home is is about .25% of the homes cost. Small price to pay for a good insurance policy eeh?

1. 
   Bill_Conner | Apr 05, 2001 06:48am | #5

   *Big price if you consider that the venting often lets percipitation into the house, may lessen insulation effectiveness by the air movement; and may reduce the value of the house because it's unattractive.

2. 
   Rob_Susz_ | Apr 06, 2001 06:30am | #6

   *I agree with Bill and Disagree with Ron.The best way to assure an airtight roof assembly is to move the thermal envelope out to the roof. This eliminates all the problems with chases, interior walls, drilled holes in top plates, stairs, etc.In any case I have imagined, it will add cost to vent it. With little or no benefit and an increase in cost of any kind, it seems to be a loser all around.-RobP.S. - the only hope you have for making a cape's roof absolutely airtight is from the oustide with Nail Base Insulation. This is expensive, but has the added benefit of being worth it.

3. 
   Tommy_B. | May 03, 2001 02:34am | #7

   *Hey Rob/Bill,Here in central PA, common practice is to insulate the kneewalls and the first floor ceiling. An attempt to air seal the walls is often made by installing foam on the backside. Is this a viable alternative?The reason I ask is that I am in the process of finishing a second floor cape addition to a ranch house. At present the first floor ceiling is insulated with probably r30 fg and maybe 8 inches 10 inches of blown fg over top of that. I was contemplating installing r30 fg batts between the rafters from ridge to plateline. I'm not sure that solves the air sealing issue though.What is the current conventional wisdom for this scenario?Tom

4. 
   Bill_Conner | May 03, 2001 05:18am | #8

   *I'm not clear on the exact plan but consider that you want a complete and continous pressure barrier around the living area and in contact with a thermal barrier - insulation. Insulation not in contact with the pressure barrier is much less effective. Fibreglass especially looses almost all of its value when not tightly enclosed on all sides.

2. 
   Charlie_S. | May 03, 2001 05:18am | #9

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   We're looking for the best way to vent an existing cape's sloped rafter bays between the knee walls and the collar ties. The bays are filled with fiberglass. We can access both ends of the bays - but it ain't easy.