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Discussion Starter · #1 ·
I'm getting ready to insulate my basement HT and will be using Roxul. I've not done this before and have several questions:


1. One of my walls is about 8" from the foundation and all of my ceiling joists are at least 12" deep. Is a single layer of 4" Roxul enough or should I fill the entire space. If so, would standard fiberglass insulation be OK in addition to the Roxul?


2. Is it alright if the Roxul is several inches from the surface of the walls. I ask this for several reasons. My ceiling joists are the i-type and I think I'll let the Roxul rest on the lip of the "I" which will be 1.5" from the surface of the ceiling. I'll have the same situation with my soffits as they were framed with 2x4 and so the insulation will actually be 3.5" from the surface.


Thanks
 

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I used Roxul in my ceiling


1) I let it rest on the lips


2) I found it pretty easy to cut with a long kitchen knife and a straight edge


3) From my research I've concluded that there is two different 4 inch roxul products ...the Safe and the AFB batts. The only difference is the density. The Safe is more dense. The acoustic properties are different. Review the spec sheets at their web site. I used the "safe" because that's what the supplier stocked.


4) If you are concerned about sound containment here is something to think about.... on the ceiling if you simply attach the drywall directly to the joists what you've done is created a large passive radiator. The sound in the room will vibrate the drywall, the drywall transmits the vibration to the joists and the joists to the upstairs flooring. The more mass you add and the more de-coupling you do to the ceiling surface will give you a pay back. Think about extra layers of drywall, hat channel on isolation mounts or RC channel. These will be more important than some extra fiberglass in addition to the roxul.
 

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Discussion Starter · #3 ·
Quote:
Originally posted by BIGmouthinDC
I used Roxul in my ceiling


1) I let it rest on the lips
So, if I understand this correctly you also have a gap between your Roxul and the sheetrock.
 

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Yes, Plus the additional air gap of the 1/2 inch drop for the RC channel. I put two layers of DW on the double legged RC2. 1/2 and 5/8. Only half of my ceiling is done but I can tell that the half done is very quiet. Some where I read that air gaps actually help in sound control.


I have recessed lights and I built "boxes" of roxul that went up and over each light. on the first layer (resting on the lips of the Joists) of roxul I put a 6 inch strip sitting on top, then put a cover over the light that sat on the two strips. The lights are rated air-tight and in contact. There is actually an air gap in my bridges. Again I used the very dense and semi rigid "Safe" 4 inch batts. The AFB may be more flexible and not support this design.
 

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Discussion Starter · #5 ·
I will also do 1/2 + 5/8 all around. Great idea for the can lights. I think I'll try and do the same as my lights are also IC rated. Great tips - thanks.
 
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