Rigid Foam over Stone Wool?


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Old 05-11-19, 03:03 PM
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Rigid Foam over Stone Wool?

Hello,

I am re-doing my attic and have removed the old fiberglass blown in insulation. I am replacing it with Rockwool and would also like to add rigid foam on top of the joists. Will this create any problems?

Thanks
 
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Old 05-11-19, 03:41 PM
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The biggest problem would be that you would not be able to see the joists, not know where to step, and might put your foot through it. Drawing lines across the 4' dimension of the foam with a drywall square to represent where the joists are then using 3 or 4 cap nails in each joist to secure the foam would at least give you a visual reminder of where the joists are. The thicker the foam the better. 2" would be best.
 
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Old 05-11-19, 03:56 PM
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Thank you, but this was not my question. Nice thought although.
 
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Old 05-11-19, 04:05 PM
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uhhhhhh, then what exactly are you asking?

Will this create any problems?
If your foam is too thin it will create a condensation problem. Thats why I said it should be at least 2" thick. Also joints should be taped so that there is no air leakage.
 
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Old 05-11-19, 04:13 PM
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Better re-ask your question with a lot more detail because I think X did a great job of answering what you asked in your first post.
 
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Old 05-11-19, 04:16 PM
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Seeing it that way, yes, he did indeed give a good answer.
How I am doing it:

Air seal first with spray foam all gaps, then place Rockwool batts between joist. Now, my thinking was on top of that, rigid foam. I am not sure if the foams should be taped after air sealing?
 
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Old 05-11-19, 04:36 PM
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joints should be taped so that there is no air leakage.
And it needs to be at least 2" thick. This is so that the inside surface of the foam has a better chance of staying above the dewpoint during winter months. You tape the seams because you don't want a way for cold air to meet warm moist air.
 
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Old 05-11-19, 05:00 PM
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Ahh, but as I read, doesn't it create some kind of sandwich environment?
I air seal between joists and drywall and then the rigid too?

I am now totally confused, because I also posted this in another forum and they say rigid on top is not good.
 
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Old 05-11-19, 05:23 PM
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You're getting the same people answering here as on the other forum. We all agree it's not what we would do but X explained how to do it if you insist.
 
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Old 05-11-19, 05:49 PM
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It's not a "sandwich" because none of the other components are vapor barriers. Vapor can dry to the interior if needed. The key is using thick rigid foam, because ideally the vapor barrier should be on the warm side. But since heat rises, the interior surface of the 2" thick foam should still warm enough. If you use thin foam it is more likely to have problems. If it is not air sealed on top, more likely to have problems. Due to the liklihood of these problems, most might discourage it. Done properly it should be fine.
 
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Old 05-12-19, 01:14 AM
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The real question here is why are you thinking you need the rigid insulation in the first place, if your installing new rockwool just install the recommended amount and your done!
 
 

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