Attic Insulation Project Advice


  #1  
Old 04-21-05, 06:04 AM
Durnik
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Attic Insulation Project Advice

To all,

about to start on a project to insulate my attic and would appreciate some feedback on my plan (and any corrections)

Situation:

13 year old house with a very large attic (~100' x ~40'). Attic has 2"x8" (16" on center) rafters meeting at a ridge board at top (not a truss system as I believe they're called) with 2"x12" cross braces about every 5 feet (about 9' up). Floor is 3/4" plywood with fiberglass batt insulation underneath it to protect the lower floors and is rated to support a future finished load. Full soffit vents (not blocked) and one large electric temp controlled fan to exhaust air. Problem is that the attic gets extremely hot (so hot some plastic items stored up there have melted/deformed and the fan runs continously) and you can feel the heat radiating from the ceiling from the floor below during the summer. Since I want to finish it at some point in time I don't want to just add insulation to the floor, I want to insualte the entire roof.

Plan:

1) Install the plastic channels (from the home stores) between the rafters to carry air from the soffits up and keep the roof sheathing well ventilated
2) Install a ridge vent to exhuast the air from the channels
3) Install fire rated backing/paper fiberglass batt insulation between the rafters. R-Value as high as I can get it (R19 or better) or build out the rafters if I need a deeper cavity than the 2"x8" will hold. Paper would face the attic since it would be the finished side at some point and would stop ~4 inches from the ridge vent to give the vent access to the channels air.
4) Install a vapor barrier (4mil plastic or better) and stop it 4" from the ridge pole to allow air to escape via the ridge vent
5) Get a permit before I start any of this since they're required in my area

Would appreciate any feedback (good or bad) since I'm going from as much information as I can glean from the forums

My thanks,

Bob H
 
  #2  
Old 04-21-05, 05:24 PM
R
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http://wsuonline.weber.edu/course.cm...ent_Index.html

This is a fairly easy to read lecture a Ventilation.

What it does not discuss well is the dominant heat transfer mechanism during the summer. Which is Radiant. This is what makes it so hot in your attic during the summer. In this situation there are two major factors to consider. First, the hotter an object gets, the more heat that object will radiate. Second, insulation retards heat flow by retention. Conductive heat flow is what insulation retards.

Ventilation uses the third type of heat transfer mechanism, Convection, to reduce the attic temperature during the summer. Since the attic fan was on constantly and the attic temperature was still high, it explicitly implies that the introduction of heat (BTU's) to the attic is greater than the ability of the fan to extract the heat (BTU's) from the attic. What compounds this is the characteristic of insulation to retain heat. This is why most attic temperatures are usually higher than the temperature outside during summer.

So insulating the underside of your roof, even with adequate ventilaton. will probably make your attic warmer than cooler. Though the insulation in your attic floor keeps the inside of your house cool by retarding heat flow, it is one of the reasons why the temperature in your attic is high. Remember, insulation retards heat flow by retaining heat.

To lower the temperature in your attic you must address the source of the heat and the mechanism it uses, which is radiant. The best way to address this is tree shading but in most cases this is impractical. The next would be light colored roofing shingles. The next would be increasing the amount of vents along with an attic fan.
 
  #3  
Old 04-22-05, 06:34 AM
Durnik
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Radiant Heat

Resercon,

think I understand the radiant heat transfer. But (ignorance mode on), since it's the roof itself which is generating the heat (via the absorbtion of the sun's rays), would not insulating the back surface of the roof reduce the amout of heat that's allowed to radiate into the attic? I have neighbors which have finished attics and they do not have the same problems I have. Only difference is that they have rafter insulation between the roof and the ceiling drywall?

Bob H
 
  #4  
Old 04-22-05, 01:45 PM
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Insulation does not stop heat flow, it slows it down.

The difference is an unfinished attic is an unconditioned space and a finished attic is a conditioned space. Heating and Cooling systems are sized to replace or extract the amount of BTU's Lost or Gained per hour. In your situation, let's assume your home requires a 3 ton air conditioning unit to cool your home. What this means is that based on a Heat Loss/Gain calculation your home can have on average 36,000 BTU's of heat gain per hour. And a 3 ton unit is capable to extract 36,000 BTU's of heat from the house per hour.

But what happens if you insulate the attic like a finished attic but you do not condition it? Let's put it another way. Have you ever left your home during the summer and left all the windows and doors closed? What was the temperature inside the house when you got home? How long did it take you to cool the house to your desired temperature? Imagine if you left the house under those conditions all the time and never used air conditioning. And you would have simulated a finished attic that is unconditioned.

In other words, the "Thermal Boundary" separates a conditioned space from an unconditioned. Applying insulation between two unconditioned spaces, namely the outside and the unconditioned attic, will increase the radiant heat transfer because the insulation characteristic of retaining heat and the lack of cooling equipment to extract the heat gain (btu/hr.)
 
 

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