Replies

1. 
   davidmeiland | Jun 12, 2010 05:41pm | #1

   I don't know the R-value

   of the black board you now have, but I'll guess about R-4 and so you have maybe an R-16 roof. 

   You could consider replacing with something like 6" (or more) of polyisocyanurate foam, in staggered layers. At 6" and R-7 per inch you'd have roughly R-42. The 1x3 battens and the metal would go back on. I would tend to put the roofing felt over the foam and under the battens. You would have to adjust the height of your fascia boards to deal with the added foam thickness.

2. 
   Clewless1 | Jun 13, 2010 08:53am | #2

   Well if you want to improve the insulation value and save money, you probably won't do it. The black insulation could very well be polyiso board ... I've seen it in black ... called Blackore I think. Regardless, as the other poster indicated, you have at least R-4 per inch and maybe as high as around R-7 per inch if it is polyiso. that is R-16-28. Adding to this to reduce heat loss/gain will make a pretty small dent in your energy bills. You may not even notice it really. Let's say you add another R-14 to it (e.g. 2" of polyiso). Generally, the first 4 inches that was added to your roof cut your heat loss over no insulation by say 85% (or more). Your next 2" may only cut it another 10% (through the roof), which is pretty tiny in the grand scheme of things.

   I'm not trying to discourage you from doing it, just making sure you understand what you are doing. It's not as inexpensive as blowing another 6" in your attic. You will be putting forth a fair amount of effort and/or money.

   As for sound. Are you sure the sound is coming through the ceiling or simply into the house? Added rigid insulation may not do a whole lot for the sound aspect. 2" decking on the roof is a substantial amount of mass (relatively speaking). Mass works best on sound which is why building codes require e.g. dual layers of drywall to separate e.g. apartments ... the mass does a lot to control/reduce sound transmission. Sound batt insulation is designed to absorb sound. I doubt that rigid insulation would do too much. Maybe rigid fiberglass, but I'm not sure.

   From an energy perspective, make sure you have your priorities right. You might consider other areas/aspects of the house that you might get a better bang for your buck (e.g. air leakage and ventilation).

3. 
   Matt | Jun 13, 2010 09:22pm | #3

   first...

   I think you need to identify what the black rigid insulation is so we will know what the starting point is. 

   Secondly, like the other poster said mass is often a better sound deadner than insulation.  The mass (more sheetrock?) might well be isolated from the existing ceiling with resiliant channel.

1. 
   davidmeiland | Jun 13, 2010 10:31pm | #4

   The thing that will

   reduce the noise of rain on the roof is to install the metal over a solid deck rather than the 1x3.

1. 
   Clewless1 | Jun 14, 2010 12:39am | #5

   So ... conceptually, what might the OP do? And why do you say that a solid deck would be superior to 1x3? So if I come back and install say 5/8 plywood instead of 1x3, it's better? My 1x3 has some air space which might be better for sound, but the plywood has more mass. Your response lacks reason and detail (both important for the OP to begin to make decisions on what to do AND why).

1. 
   davidmeiland | Jun 14, 2010 12:58am | #6

   It's not my job

   to explain every last detail. Metal roofing spanning over purlins will ring more in the rain than metal roofing dampened by a solid deck. That's elementary. A person spec'ing anything important should be using more than just random strangers on the internet for his design phase.

   I find your suggestion that the OP has polyiso on his house to be unlikely. Maybe I've never seen it in black, I dunno, maybe it exists, but I have seen cheap black fiber roof insulation panels and they are lower in R value and more dense than polyiso, they are common under flat roofs on older commercial buildings. 6" of polyiso with CDX over it followed by the same metal will be significantly quieter than the existing. He could also install SIPS panels over his existing car decking.

   No need to take my word for any of this, you could have an acoustical engineer give you the STC rating of various sandwiches and pick the one you like.

1. 
   Clewless1 | Jun 14, 2010 08:34am | #7

   You're right, it's not your job to explain every last detail. But it's helpful for the OP to understand in general why you are saying what you say so he understands why he might act on your suggestion. No one asked you or expects you to "explain every last detail", just enough to paint the picture of why you suggest what you are saying. The OP doesn't know you from Adam and for anyone to take a suggestion w/out the benefit of learning a bit as to why he should do it would be a fool (in which case you'd be wasting your breath (or type in this case). No offense intended in my suggestion you add a bit more details into the whys and wherefores of your response.

   For a while, polyiso was made in black ... as I said under a trade name Blacore or some such name. I'd be a bit surprised that he has 4 inches of e.g. mineral fiber board (that I think you are referring to). This is still commonly used as a roofing base in e.g. commercial roof applications. It is tough to tell what he has. The more I think about it, the more I lean toward the polyiso as, again, I'd be surprised that it would be 4 inches of the other ... but about the time I think my logic is 'reasonably sound', reality kicks my butt and makes me humble again.

   My bet he's not going to consult an acoustical engineer on this matter ... but again, you never know.