I have a 1 1/2 story with an attic peak that I want to insulate with blown insulation. The rafters bird’s eye sits directly on the brick, so there is no soffit vents. It is brick on block, and there is no eave vent. I will add a ridge vent in the spring. I have 4″ or 5″ inches of 1950s wool insulation in the rafters. My question is whether I need to try to add vent baffles to the attic rafters, or can I push the old insulation into the rafters, and start over on the flat part with roll insulation and cover with blown insulation. There is not a lot of room to move around, so I am trying to make it easy to crawl up there and blow in fiberglass. The 2 bedrooms upstairs are 5 – 6 degrees colder than the first floor, so I need to do something. Thanks in advance.
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If you don't want to condition the attic space, the easiest thing to do is to just blow some celulose over what's there. Is there a floor in this space? If so, or if you want one, you might consider sistering on some floor joists to the existing joists to raise the floor level enough so you can get 10"-12" insulation depth. If you wanna vent the space, it couldn't hurt, and may help keep the house cooler in the summer, but it's not as big an issue as it would be if you insulated between the rafters.
Mike Hennessy
Pittsburgh, PA
"I will add a ridge vent in the spring. I have 4" or 5" inches of 1950s wool insulation in the rafters. My question is whether I need to try to add vent baffles to the attic rafters, or can I push the old insulation into the rafters, and start over on the flat part with roll insulation and cover with blown insulation. There is not a lot of room to move around, so I am trying to make it easy to crawl up there and blow in fiberglass."
Don't install a ridge vent without soffit vents or you'll create a negative pressure in the attic which will draw moist, heated air from the conditioned space below and likely cause condensation problems (and greater heat loss).
Don't push insulation tight to the roof deck. Even without ventilation, an air gap between insulation and deck can reduce the likelihood of condensation and allow drying. Use vent baffles to maintain that air space.
And, for heaven's sake, don't blow in fiberglass. It's as close to worthless as an insulation can be. Use cellulose - it's much denser, will resist air flow, and has a higher R-value per inch. It's also not carcinogenic like fiberglass, is far more fire resistive, and won't encourage rodents to nest.
Solar & Super-Insulated Healthy Homes
"has a higher R-value per inch. It's also not carcinogenic like fiberglass, is far more fire resistive,"You gotta work hard to prove all that.
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What Riversong said. If you do decide to vent, one option would be to add some button vents thru the roof right above the insulation and more of the same near the ridge or ridge vent. If you create outlet, you need inlet to balance it.
http://www.quittintime.com/ View Image
"The rafters bird's eye sits directly on the brick"
Bird's mouth seat cut?
I agree you should not add a ridge vent without balancing the income to the outgo or you may need a bail out.
Chopped FG and cells have nearly equal R-value, except that in some instances, the FG can have more infiltration. A vented attic is one of those instances, but as long as it stays unvented, the performance will be equivalent.
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Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
"Chopped FG and cells have nearly equal R-value, except that in some instances, the FG can have more infiltration. A vented attic is one of those instances, but as long as it stays unvented, the performance will be equivalent."
Not even close. Loose-fill fiberglass has average R-2.95/inch (as little as 2.2), while loose-fill cellulose has an average R-3.4/inch (minimum 3).
In addition, tests by US DOE demonstrated that the R-value of fiberglass diminishes with a delta-T of 25° or more, and at sub-zero outdoor temperatures (maximum delta-T) the R-value was less than 50% of rated value.
The opposite occurs with cellulose. It's R-value increases with delta-T to as much as 13% above rated value.
Also, there is no comparison in fire resistance of the two materials:
In 1994 the Research Council of Canada (NRCC) reported that fiberglass decreased the fire resistance of insulated walls while cellulose produced a 22% to 55% increase in fire resistance.
Adding cellulose insulation to wood frame walls increases their fire resistance rating by 15 minutes. Southwest Research Institute Project # 01-5920-611 tested and approved a one hour fire rated load bearing wall assembly, consisting of a single layer of 1/2" type X sheetrock on each side of a 2 x 4 stud wall filled with cellulose insulation.
Borate-treated cellulose has been third-party certified as a firestop more effective than solid wood.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
"Loose-fill fiberglass has average R-2.95/inch (as little as 2.2), while loose-fill cellulose has an average R-3.4/inch (minimum 3)."It takes a reference to begin to prove something - not just you saying it."the R-value of fiberglass diminishes with a delta-T of 25° or more, and at sub-zero outdoor temperatures"That would be with batts, right? I don't believe blown has been tested that way."In 1994 the Research Council of Canada (NRCC) reported that fiberglass decreased the fire resistance of insulated walls while cellulose produced a 22% to 55% increase in fire resistance. "Again - batts provide better chimney effect and convection in the wall cavity. We are talking about attics. FG does not burn. I have SEEN cellulose burn three times in my life. So that little label saying it is fire treated doesn't fill me with confidence.
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It takes a reference to begin to prove something - not just you saying it.
Only if you are predisposed not to believe it when I offer very specific data, while you made only crude claims.
My data is from http://en.wikipedia.org/wiki/R-value_(insulation)
"the R-value of fiberglass diminishes with a delta-T of 25° or more, and at sub-zero outdoor temperatures"
That would be with batts, right? I don't believe blown has been tested that way.
The test results from the U.S. Department of Energy verify that Fiberglas R-values are compromised when a 25° Fahrenheit temperature difference exists between the inside (70°) and the outside (45°). When the test conditions reached an outside temperature of -18°, the R-values of the fiberglass insulation (both blown in and batts) decreased to less than 50% of the manufacturer’s stated R-value.
Cellulose R-values, on the other hand, went up. The R-value of Cellulose insulation began to rise when a 25° Fahrenheit temperature difference existed between the inside (70°) and the outside (45°). When the test conditions reached the outside temperature of -18°, the R-values of the Cellulose Insulation continued to rise above the manufacturer’s stated installed R-value. It was determined that cellulose insulation improves in R-value any time a temperature difference of 25° or more exists
U.S. Department of Energy Test Results
Loose Fill Fiberglass
Metering Chamber (F)
Climate Temperature (F)
R-Value
70
44.6
17.8
70
32
16.1
70
26.8
14.1
70
8.6
12
70
-4.0
10.6
70
-18.4
9.2
<!----><!----> <!---->
Loose Fill Cellulose (Settled)
Metering Chamber (F)
Climate Temperature (F)
R-Value
70
40
18
70
20
18.8
70
8
19.4
70
-5
19.6
70
-18
20.3
We are talking about attics. FG does not burn. I have SEEN cellulose burn three times in my life. So that little label saying it is fire treated doesn't fill me with confidence.
The asphaltic adhesive and kraft paper facing is highly flammable and fiberglass melts at temperatures significantly below that of a typical house fire, resulting in open cavities and rapid flame spread.
Like any material, the quality of manufacture makes all the difference. Early cellulose didn't have fire retardants. Today's cellulose with borates (not ammonium sulfate, which some manufacturers use) is so flame-resistant that you can hold a blow torch to a lump in your hand and not only won't it burn but you won't feel the heat.
You anecdotal "evidence" means nothing to me and should mean nothing to you if you haven't done a complete forensic analysis of the source and fuel for the fires you claim to have witnessed.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Edited 12/20/2008 8:20 am ET by Riversong