Must the ceiling joists be as tall as the batts?

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Old 06-26-07, 06:55 AM
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Must the ceiling joists be as tall as the batts?

Hey y'all,

I have 2*4 ceiling joists in my old 1950's 800 sqft house. I want to put in r30 insulation. Most of the attic has 2" of blown stuff so I'm just adding about 7 or 8" more per instructions on package.

HOWEVER - I had to replace the ceiling in one of the rooms and I haven't put any insulation back on top of it. I've sprayed 24 bales so far on the other parts of the attic and then I ran out. I can't stand the process of spraying this stuff and I have no idea whether it's better or not. I'm considering using batts over the room with new ceiling because it's clean and the installation *might* be easier than blowing that crap.

The question: Since R30 batts are likely ot be about 9" tall, would I have to have joists the same height oin which to staple the edges of the insulation? OR can I just roll it out and be done? Is the batt insulation significantly less effective if I don't have 9" joists?

(Side note: I'm not even going to consider adding 2*6's on top of the 2*4's to make it ~9" higher; too much work)

Thanks,

Chris
 
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Old 06-26-07, 07:19 AM
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Must the ceiling joists be as tall as the batts?

Blown cellulose would be best since it performs, fills and covers better. Unface batts would be a second choice.

It is preferred to have the insulation over the ceiling joists because you eliminate the heat loss through the wood.

If you do use batts, run them perpendicular to the direction of the joists.

Make sure you do not obstruct the flow of the air through the soffit vents.

Dick
 
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Old 06-26-07, 08:49 AM
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Thanks for the reply.

It's interesting that cellulose performs better - it's half the cost of the batts in my area, so I assumed it was the inferior product. It sure is the messier product! I'm using the cocoon brand; not sure if it's cellulose, but it's the only blown stuff they sell at the big box retailer in my area.

I see what you mean about insulation loss through the wood.

Why do they make the batts 16" wide if they would be placed perpendicular to the joists? If I did make them perpendicular to the joists, will the air gap between the drywall and the top of the joists be a problem?
 
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Old 06-26-07, 09:53 AM
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Must the ceiling joists be as tall as the batts?

When I was referring to going perpendicular to the joists, I was assuming the area between the joist was filled to the top.

With blown in, you do not have a problem since it fills in and can give you 100% coverage over what you have.
 
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Old 06-27-07, 10:30 AM
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I gotcha.

My latest question still remains: Because I don't have any fill to the top of the joists that hold the new ceiling, will the batt insulation 'work' effectively with the 3'5" air gap that would be below it?
 
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Old 06-27-07, 10:34 AM
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Must the ceiling joists be as tall as the batts?

Definitely fill between the joists with the insulation of your choice.

Then blow over the joists with cellulose.

Dick
 
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Old 06-28-07, 04:56 AM
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I think the previous poster is missing the fact that you just don't want to blow any more.

My suggestion is to fill the cavities between your joists with R-13 batts then run a layer of R-30 batts perpindicular to your joists on top.
 
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Old 06-29-07, 12:27 PM
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I decided to press on with the blown stuff because running batts perpendicular to the joists would be even more aggravating in several ways:

1) more time spent crawling out towards the narrow parts of the roof instead of just holding a hose
2) Can only run batts in the areas *without* the 1*6's that connect the horizontal joists with the diagonal roof supports, thus, not as good coverage (and it's just irritating to know you can't cover it completely
3) More expensive (IF the blown stuff covers to specs; I had to use a lot more than I planned. Batts, while more expensive, would cover to spec because you can measure the length, etc)

HOWEVER, Batts would be less mess, if that was the only type of insulation up there. (I already had 2" of old blown insulation, so I just wanted to add to it.)

I'm glad it's done! I just have to keep a supply of dust masks for whenever I venture into the attic.
 
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