Need help with rafter insulation

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Old 02-03-16, 07:42 PM
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Need help with rafter insulation

Ok to start...I already have blow in insulation.

I live in Kingman, AZ where we don't quite get Phoenix heat, but it still gets damn hot. Usually 10-15 degrees lower than PHX. We get humidity during monsoon season for 2-3 months. Rest of the year is pretty dry other than rain, but then the ambient humidity is minimal.

House is a single story stick home, 2 years old, asphalt 3 tab roof on top of OSB. Quality stucco, with insulated outter walls, 3 natural vents in attic. Pretty simple construction. All things considered it's pretty efficient. With somewhat conservative settings, september 2014 I used 512 Kwatts. (was staying with girlfriend a lot this last summer so had AC minimized).

Ok, onto rafter insulation. Would pushing fiberglass batting between the rafters be a proper way to keep roof heat from entering the attic? I see online where many people sprayfoam the attic, or use white styrofoam insulation to create a gap that vents to???? My problem is the only access is the hole in the hallway of the house. It's big enough to crawl into, but not really big enough for sheets of that white styrofoam. And after reading some horror stories of spray in foam (chemical smells)....I'll just pass thanks.

As for the batting, I guess I'm worried about either 1. humidity getting between the batting and OSB and molding....2. Too much heat building up and destroying the OSB/Roof/etc.

Thanks in advance
 
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Old 02-03-16, 08:05 PM
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Your blown in insulation should be sufficient, and adding insulation between the rafters would be a bad idea. If anything, you could increase ventilation with a power vent that is hooked up to a thermostat, if your goal is to keep the attic cooler to help with the AC. But the power you use running the vent fan may negate any savings from the AC running less. Power vents can also pull air out of the house faster if it's not sealed well. It would extend the life of the shingles, though. Not sure what you are trying to accomplish, or why you think your 2 yr old home needs any improvement.
 
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Old 02-03-16, 08:16 PM
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Your current insulation is what protects the house from the heat and the ventilation removes any moisture that seeps through before it can cause any damage. Batt insulation against the bottom of the roof would complicate the issue with little or no benefit.

Not sure what you pay for electricity, but 512 KW isn't that bad.

You said "3 natural vents in attic" what are they and where are they?

Bud
 
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Old 02-03-16, 09:11 PM
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gable vents

The main body of the house (30 ft span) runs east and west. There is a gable vent at the east end. On the west end are 40ft. span rafters that run north/south with a gable vent on each end. But during the summer we don't get a lot of wind, so does it vent? yes, but with little help from nature. I understand how those powered attic vents don't help that much.

A note: Half of the 40ft. span is car port that built as part of the house. In other words the natural attic is shared with the car port. I've always wondered how this affects my heating/cooling.

Anyhow, that 512 KW was with pretty moderate settings observing peak hours strictly (78 degrees till 8pm- 72 degrees after). Was hoping to crank it up a little more right when we get home (4pm) and still not get clobbered on the bill. Yeah I know it's not that bad...but still......
 
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Old 02-04-16, 06:47 AM
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Natural ventilation needs low vents to supply air for the high vents. You didn't mention soffit vents under the eaves and I'm not sure how the car port fits in. I don't think it would be a good location to be supplying air into the attic.

Bud
 
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Old 02-04-16, 07:24 AM
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I can do soffit vents. That as I'm assuming I'm just drilling the holes and adding screen. Inexpensive too!
 
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Old 02-04-16, 07:59 AM
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Well, yes and no. The calculation for venting is 1 ft² of NFA (net free vent area) for every 300 ft² of attic floor. Double that result is the attic floor is well air sealed with a vapor barrier. Some paints qualify for the VB. If your attic has 1,500 ft², your 1/300 number is 5 ft² or 720 in². That area is then divided half for the high vents and half for the low.

Now, your suggestion of drilling the holes and adding screens, if you are thinking of pop-in vents, some have only 20% NFA. A 3" pop-in vent may have less than 1.5 in² of vent area. 360 divided by 1.5 would suggest 240 vents. That's why I said yes and no.

But, all of that may not be how yours calculates out. More details about what you currently have for soffit material, length of soffits, and access to add baffles to provide an air path past the insulation.

The point here is, for natural ventilation to have any chance of controlling some of that attic heat, you have to have a reasonable amount of well placed vent area.

Many threads here on attic ventilation.

Bud
 
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Old 02-04-16, 09:49 AM
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Regular insulation will do nothing for radiant heat. You need a radiant barrier installed. This will reflect the radiant heat back out the roof.
 
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