batt insulation over a drop ceiling

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  #1  
Old 02-11-15, 04:23 AM
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batt insulation over a drop ceiling

Hello. My church in South MS is a metal building and has have a layer of insulation next to the metal on all sides and roof. We have a drop ceiling (2x2 tiles) throughout the building along with can lighting and 2x4 troffers. Wanting to reduce our power bill, we were considering adding 2x4 batts of R30 insulation directly above the ceiling tile. Given our 9 ft ceiling height and 20 ft high pitched roof, we have considerable air volume above the ceiling tiles that we are paying to heat/cool monthly (20,000 cubic feet of air above the suspended ceiling vs. 30,600 cubic feet below it).

So, is batt insulation laid directly onto the suspended grid the best option provided we keep it 3" away from all lighting fixtures? Also, shouldn't unfaced batts be sufficient given that we are not insulating against an exterior wall?

Any help or information is greatly appreciated.
 
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Old 02-11-15, 06:15 AM
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Hi squid and welcome to the forum.
Sounds like a real problem. Batt insulation over a drop ceiling is not going to be air tight, thus the air up there will carry a lot of moisture which will now have more potential to condense being colder.

We need to know what is up there for current insulation and how well air sealed it is? Also, is there any ventilation built into this metal roof?

Often a commercial style metal roof will have fiberglass batt insulation in plastic batts draped over the roof framing structure making it difficult to add more.

Bud
 
  #3  
Old 02-11-15, 01:00 PM
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Thanks for the reply. The insulation that is there now was installed with the metal building and is as you described: plastic batts draped over the red iron and directly beneath the metal. That is all the insulation we have at the moment. The attic is not vented at all though we have talked about adding ventilation fans in the past.
 
  #4  
Old 02-11-15, 01:20 PM
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Difficult, as this falls under commercial codes and your final approval must come from your local AHJ (authority having jurisdiction). But if I were to approach a solution, I would look to abandon the insulation under the metal roof. Leave it in place for sound reduction, but create my air, moisture, and thermal barriers right at the ceiling level. They belong together, in contact, and continuous.

Now, how to achieve that with the current drop ceiling is difficult for me to guess from here. I assume this is an unsupported wide span, with the drop ceiling suspended from far above with a forest of wires to get into the way of anything continuous.

Now, there are rigid insulation materials that are rather light that could perhaps be attached below the drop ceiling, air sealed, and painted to look reasonable. Then the batt insulation would go above as you have suggested, but venting would also be necessary.

Bud
 
  #5  
Old 02-11-15, 04:42 PM
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Bud, although I would agree it would be nice to have the air/thermal barrier together, you have to remember this building is in MS and if air conditioned we may have to consider the vapor retarder placement in its relation to other insulation.

If a rigid foam is attached below the suspended ceil, and I presume it would have to be "Thermax" for fire resistance, although there may be others, this may constitute a cold side vapor retarder. Again, I'm only speculating. MS could be pretty warm and humid during a Sunday church service when the building would be most likely air conditioned. That could present a topside condensation issue.

Since most churches have limited use and do constitute large air volumes, I'm not sure being too concerned with internal moisture should be a major concern. Speculating that they have a ducted heat/air conditioning system, that can be used to help control humidity in the occupied space.

Dealing with the penetrations is of course, critical. Perhaps replacing fixtures or building enclosures for them could aid in that endeavor. Maybe a review of the suspended ceil support system would provide assurance that a drywall suspended ceiling assembly could be attached to it or even just light gauge furring channels to support the new lite weight drywall. Maybe some R-21 rock-wool batts over the ceiling blocks or one of the high density commercial fiberglass board products similar to duct board could be installed carefully into the suspended ceiling grid .

These forums are good for concepts but in a case like this I feel a local pro might be the best way to assess the potential issues and solutions.

These are all just ideas to get the process started, maybe you could add some review of what my suggestions are so the OP could get a feel for the issues at hand.
 
  #6  
Old 02-11-15, 08:34 PM
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Hi calvert, I definitely agree with a local pro needs to be doing this. There are probably lots of mechanicals up there as well.

As for the vapor barrier on the cold side, since I don't know how it could be installed above with all of those wires, this is similar to buildings up here in the north country getting a 2" layer of foil faced rigid insulation installed under the new siding, a VB on the cold side. When they break the "VB to the warm side" rule there is a ratio of rigid insulation to permeable insulation that will ensure the cold surface doesn't fall below the dew point. Too much insulation above and the rigid layer below can become too cold. But all of this is held in place by those wires so I don't believe they should/can get too carried away with weight. Maybe 6" of fiberglass (r-19) if the ratio down there will allow.

But r-14 from the Thermax, plus r-19 above would make a reasonable thermal layer. Of course all of the ducts and equipment up there would have to be air sealed and insulated.

With the op asking about just adding some insulation above the drop ceiling I'm assuming they are looking for as low of a cost improvement as possible and being a one or two day a week use building makes payback very difficult.

Bud
 
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