Insulation


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Old 09-30-15, 06:38 PM
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Insulation

Hello,

I have 2 x 6 rafters in my shallow attic. I want to get R30 + using roxul, but not sure how to go about it.

1. should i use firring strips to make the joist taller?
2. sister a 2 x 8? I think that would be difficult with how the structure is
3. Use R19 then run another piece perpendicular? (to me this is a better idea
4. use rigid insulation with R19? So i guess you would put the rigid first and then the roxul over?
5. Or can i just use R30 and not worry about it sticking up taller than the joist? Not sure if that would be a code issue?
 
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Old 09-30-15, 06:41 PM
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Standard practice is to use two layers, one parallel to the joists, the second perpendicular to the joists. If it were me, I would blow in cellulose.
 
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Old 09-30-15, 06:51 PM
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Thanks. Because it is cheaper and easier?
 
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Old 09-30-15, 06:59 PM
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In a shallow attic the blown in insulation Keith suggested would be a lot easier.

With traditional framing the joist and rafter are side by side which complicates keeping the insulation fit as neat as it should be, right where it needs to be neat. To go with Roxul batts I would install 6" batts flush with the tops of the joists all the way out to the baffle that is against the roof deck.

Now comes the detailing. To fit the next layer over the top you will need to piece in between the rafters flush with the baffles and then match the slope with the first cross piece. Sounds more difficult than it actually is, as Roxul is easy to cut.

Bud
 
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Old 09-30-15, 08:10 PM
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Could you explain what you mean by this:

"Now comes the detailing. To fit the next layer over the top you will need to piece in between the rafters flush with the baffles and then match the slope with the first cross piece. Sounds more difficult than it actually is, as Roxul is easy to cut."

My house is a 1929 house and I do not have soffit venting. Probably will do that when I need to reroof in 5 years.

I am assuming that you think that if I use Roxul I should use R21 (lowes and hd dont have r19 by 24 OC)
 
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Old 10-01-15, 04:51 AM
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If you have soffits, then you will want baffles for that future soffit air flow. The baffles provide an air channel below the bottom of the roof and prevent your insulation from closing off that space or falling into the soffits. Do you have soffits?

The detailing I'm referring to creates a uniform layer of insulation with on air gaps. any gaps between batts allows air circulation and greatly reduces the effectiveness. Thus, as your insulation is installed down near those soffits it needs to be cut to conform to the baffles and framing.

Whatever insulation you use should fill the joist cavities so that a cross layer doesn't leave a huge air space.

Bud
 
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Old 10-01-15, 12:51 PM
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Cellulose is generally about half the price and yes, easier to install. But beyond that, it fills in all the little spaces that can be next to impossible to do with batt insulation.
 
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Old 10-01-15, 03:47 PM
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I have gable venting and no soffits. However, I believe if I made holes where the soffits should be I would get the airflow if I change to ridge roof when i reshingle the roof in a few years.
 
 

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