# Rockwool in 2x4 cathedral Ceiling?



## XSleeper (Sep 23, 2007)

> vapor barrier and then Hardie board.


I hope you mean housewrap, which is vapor permeable. The complete opposite of a vapor barrier.

Are there soffit vents to let air in those rafter bays from below? Regardless this is a case of not being able to have your cake and eat it too... unless you want to add 2x2s onto your rafters... there is no way to do both.

IMO if you can only pick one, your insulation probably wins. But consider adding 2x2s so you can also ventilate. Will be plenty hot up there already and you're just making it worse by blocking the airflow.


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## chandler48 (Jun 5, 2017)

I agree, too, with adding 2x2's to the face of the rafters, then adding styrofoam baffles then your Rockwool. Make sure you have soffit venting and ridge venting in place as well.


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## Bud9051 (Nov 11, 2015)

Zone 5 probably wants a lot more than the r-15 you suggested and with an air gap you will have less than that. All bays need soffit to ridge venting.

Ditto on above advice.
Bud


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## SPS-1 (Oct 21, 2008)

Ditto on what the others say. Your roof needs ventilation.

Space out those 2x4's with 2x2's or 2x4's for more depth. I like Rockwool too, but if you need the most R per inch, go with XPS. R15 doesn't cut it.


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## davidjames (Jan 26, 2017)

House wrap was used on exterior before Hardie, sorry for confusion. 

There are soffit vents installed on both sides of the house, as well as 3 attic fans in the crawl spaces sucking air to the outside. 

Just to be clear, you guys are suggesting furring our the 2x4 rafters with 2x2 material, and still using the r15 batts? Allowing for that 1-2” of air gap behind the batt? Would using a product like MemBrain before drywall be beneficial? I have never seen 2x2 material, assuming people are ripping 2x4 material down?


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## APA (Jul 13, 2018)

davidjames said:


> House wrap was used on exterior before Hardie, sorry for confusion.
> 
> There are soffit vents installed on both sides of the house, as well as 3 attic fans in the crawl spaces sucking air to the outside.
> 
> Just to be clear, you guys are suggesting furring our the 2x4 rafters with 2x2 material, and still using the r15 batts? Allowing for that 1-2” of air gap behind the batt? Would using a product like MemBrain before drywall be beneficial? I have never seen 2x2 material, assuming people are ripping 2x4 material down?



If you have never seen a 2x2, this job might be too much for you to tackle...


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## APA (Jul 13, 2018)

Closed cell foam. Those guys would be done before you could even get the boards cut to extend your rafter depth...


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## XSleeper (Sep 23, 2007)

If it doesnt get frigidly cold there, or doesnt snow often, you can probably skip the Membrain without any problems. But if the roof is covered with snow during the winter, it might be a good idea. Or if you just want to use it to help hold the insulation up until you drywall, it would work fine.


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