Insulation questions

Old 10-27-04, 04:49 PM
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Insulation questions

I've got a 1937 wood framed house in the San Francisco Bay area. The insulation in the low hip roof attic sections is old and covered with debris from when the previous owner put on a new roof. Looking at the thickness, I'd say it's probably around an R11 value (it doesn't come to the top of the ceiling joists), so I'd like to add additional insulation to keep the bedrooms warmer at night. Should I remove the roofing debris prior to laying the fiberglass batts down? Would I be better off just putting reflective barriers on the undersides of the rafters?

Also, I've noticed that there are no vents in the eaves of the house. Just some gable vents and roof vents...should I add some vents down in the eaves to improve the air flow through the attic?

Old 10-30-04, 04:37 PM
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Attic insulation

In San Francisco, the ceiling requires R-30 and walls R-11. Additional batts of unfaced (no vapor retarder) insulation can be rolled out on top of old. Remove debris first.

Building codes specify the minimum amount of attic ventilation needed in a new home to prevent winter moisture buildup, but your summer needs are much greater. Also, older homes were often built with inadequate attic ventilation–at least by today's standards–and may need to be retrofitted with proper attic ventilation. Check with your local building inspector.

A good attic ventilation system is designed for summer needs. It includes two types of vents: intake vents are placed along the soffit to allow fresh air into the attic, and exhaust vents are installed in the upper third of the roof to allow attic air to escape. The object is to create a continuous "wash" of air along the underside of the roof sheathing. The rule of thumb in the summer is that you should provide enough ventilation to completely change the air in your attic every six minutes.

There are many types of attic vents available today. There are static vents, power vents, ridge vents, turbine vents, soffit / cornice vents, gable vents, starter vents, and cupola vents. These all come in a wide variety of sizes, styles and shapes. Some will ventilate better than others depending on the roof configuration, attic size, climate, etc.

Calculating how much venting your attic needs is relatively simple. All you need to know is the area of the attic floor. Include the garage, if you have one, and the soffited overhang because heat gets trapped above them, too. A common rule of thumb is the 1/300 rule, which means 1 square foot of net free vent area per 300 square feet of attic floor space.

FHA Guidelines recommend the 1/300 rule. Some building codes require the 1/150 rule. The Home Ventilating Institute (HVI) recommmends a 60/40 ratio where 60% of the total ventilation is for intake and the remaining 40% is for exhaust. Some roofing material manufacturers will not honor their warranty unless attic ventilation has been installed.

Check with your local building inspector re: ventilation requirements and recommended vent systems in your area.
Old 11-01-04, 09:04 AM
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Dont forget to make sure you put the foam rafter vents in. So air can get up into the attic there. Over the insulation from the vents out in the over hang. We still like to work at 1sq ft for every 150sq ft of attic 1/2 in and 1/2 out for vents there. Might look into a power vent fan on a tstat there on a gable or roof vent. like said clean it up and add more. Could go cellulose and blow it in . lowes and Home depot have the blowers.

Old 11-01-04, 09:20 AM
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Thanks so much for the advice. Did a little more investigating this weekend which is generating more questions.

There are actually two over the west side and one over the east. Both have a fairly low hip roof (3 ft high max at the peak) and they each cover about 700 sq. ft.

The west side has NO insulation whatsoever, but a ton of old roofing debris. How do I get that out of there? The access is through a closet ceiling and there's no way a person can sit in there. We'd need to crawl in and back out. From the access point to the far end of the attic is about 30 ft. Any suggestions on debris removal?

Also, if I use loose fill insulation, how do I install a vapor barrier?


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