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Insulating attic with slanted ceilings, advice needed...

Insulating attic with slanted ceilings, advice needed...


  #1  
Old 10-02-09, 10:59 AM
H
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Insulating attic with slanted ceilings, advice needed...

I own a three family triple decker (as we call them here in New England). The house was built around 1910 and has no insulation. I would like to start adding some by insulating the attic before this winter. The 3rd floor has slanted ceilings in most rooms, and the ceiling joist spacing is 24”. I’m trying to figure out the easiest and most effective way to insulate. The slanted ceilings create a problem as I need to maintain airflow under the roof, so I can’t just fill the cavity. My two options are to use 24” wide R19 fiberglass bats and slide them down from the attic, or try to install rafter vent spacers and use blown in cellulous insulation. The two methods are cost neutral, since I will need to use rafter vent spacers with the blown in. I figure the R19 fiberglass will not be thick enough to block airflow under the roof and therefore I don’t need the spacers. Either way it will be difficult to insulate these areas. I don’t have any experience with blown in, so I can’t decide what will make more sense. One advantage with blown in is that I can also insulate about 20 percent of the wall cavities which are accessible from the attic. However, a disadvantage is that I will need to have the blowing machine in the third floor apartment, which will make one hell of a mess (as I understand). I don’t think the blower hose will reach the 3rd floor attic from ground level blowing straight up.

I could really use some advice or insight on what to use and how to use it before I go purchase materials. Especially since if I go with the cellulous, I only have one day to complete the job. Thanks for reading (my book).
 
  #2  
Old 10-02-09, 12:29 PM
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Hi hjs, if your wall cavities are accessible from the attic, it sounds like balloon construction. If so it will be good to insulate and air seal those walls. Also check interior walls, as you need to eliminate any air leakage into the attic.

Do you have soffit vents and gable, roof, or ridge vents to utilize the air gap you are intending to leave. The gap would be of little use with no vents.

Here is a link with some reading: Welcome To Home Energy Magazine Online

There are different opinions as to the r-value needed for slanted roof areas, is it r=20 like walls or r=40 for ceilings. My thoughts are, if air is moving over it, it would be more like a ceiling. That being the case, there isn't enough space inside the cavity, so adding a layer to the inside of the plaster becomes a consideration. I mention this, because sometimes there are areas that can't be reached and can't be omitted, so it becomes an option.

stay warm
Bud
 
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Old 10-05-09, 05:10 AM
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Thanks for the reply bud. The roof has soffit vents and a ridge vent, so maintaining the airflow is necessary. I’m starting to lean toward going the blown in rout. I can make my own rafter vent spacers using cardboard that I already have. If the machine that they rent at the big blue box is capable of blowing to the third floor attic from ground level, then my choice is clear. If not, then it just might not be worth making a huge mess of the apartment and lugging 25 bags of cellulous up three flights, rather then only 7 or 8 rolls of fiberglass.
 
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Old 10-23-09, 05:24 AM
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As an update for anyone else who needs to insulate slanted ceilings through the attic… I ended up buying the thinnest, cheapest 4x8 panel that I could find, which was tempered masonite (hate that crap). I cut this into 8’ long strips, wide enough to fit between the roof rafters. I then cut 7/16 plywood into 8’ long by 4 ¾” strips. I attached these plywood strips on edge to the center of the cut masonite sheets. This held the sheets 4 ¾” above the ceiling, creating a space to blow in the insulation. I then cut cardboard to cover the end of the panel, to prevent insulation from falling into the soffit area. So to clear things up, I made an 8’ long sheet of masonite cut wide enough to fit between the roof rafters, with a 4 ¾” x 8’ plywood spine running down the center and a cardboard cap folded over the end. I then slid this down between the rafters from the attic. I needed the full 8’ length, with only about 1’ sticking out past the slanted to flat ceiling transition (which will be perfect for when insulation is blown onto the flat part of the ceiling). I have old 2x6 rafters, so this setup should maintain about a 1 ¼ airspace below the roof deck (above the masonite sheet). I will start blowing in the insulation this weekend. Hopefully things continue to go smoothly. My new tenants better appreciate all of my hard work (yeah, right).

I hope this helps someone with the same dilemma that I had.
 
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Old 10-23-09, 06:49 AM
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might check on the availability of fiberglass for the blown in option,in 'my' opnion, it is not as messy as the celulose.
 
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Old 10-23-09, 12:07 PM
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Without a -1" spacer between the roof deck/masonite, what prevents the force of the blow-in insulation from pushing the whole unit up tight to the roof? Or did you nail the ends and middles? For any others thinking just fiberglass batts without air baffles, it won't work. You need to keep the attic venting air away (seperate) from the insulation to create the dead air space the glass provides for any use.
Be safe, Gary
 
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Old 11-05-09, 05:32 AM
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Originally Posted by GBR in WA
Without a -1" spacer between the roof deck/masonite, what prevents the force of the blow-in insulation from pushing the whole unit up tight to the roof? Or did you nail the ends and middles? For any others thinking just fiberglass batts without air baffles, it won't work. You need to keep the attic venting air away (seperate) from the insulation to create the dead air space the glass provides for any use.
Be safe, Gary
I chose to use cellulose because it has a higher r-value per inch and prevents infiltration better (so I read).
The house is about 100 years old and has many, many nails protruding about 3/4" - 1" through the roof decking. I was relying on this "bed of nails" to prevent the masonite from being pushed up against the roof deck.
I agree 100 percent that fiberglass batts with no venting is not good. I went through all the work of making baffles for a reason. When I was a kid, my father gutted and remodeled a bedroom with slanted ceilings at our house. Whenever it snows, the whole roof melts except for this one square. Now, 20 years later, the shingles in that section are almost completely gone, while the rest of the roof is not bad.
 
 

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