Flat Roof Insulation

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  #1  
Old 02-28-10, 01:36 PM
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Flat Roof Insulation

Have 1974 construction single story home with flat metal roof and steel truss construction. There is a relatively thin layer of insulation, perhaps three inches, under steel roof. Attic space is low, perhaps four feet, and crisscrossed by insulated air ducts for heating and cooling. Almost impossible to move through attic to get any work done. Energy loss is of course terrible. Heat in winter goes right through ceiling sheetrock into attic area. Very roughly speaking, living space is 66F, attic space is maybe 20F less, and outside is whatever outside is.

Concensus is that I CANNOT put a thick roll of insulation on the ceiling sheetrock, no vapor barrier, because that will cause mold in the attic. I can put a layer of polyurethane on the roof, which will increase its R-factor, but I'll still be heating the attic area.

I don't understand the mold trigger. There already are three separate temperatures, living space, attic space and outside. Putting simple insulation without vapor barrier on the ceiling sheetrock will simply increase the temperature delta between living space and attic space, maybe to 30F from the present 20F. How does that cause mold?

Any other options? Thanks.
 
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  #2  
Old 02-28-10, 02:03 PM
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Hi Frank, mold is produced by moisture, ie above 50%RH, and can form in a wide range of temperatures. Your current situation is probably not good as it is. Warm air can hold more moisture than cool/cold air. So, if any of your heated air is leaking into the attic, it may be depositing moisture. If that attic were well ventilated, then much of the warm moist air would be going directly outside. But where your rood is insulated, doubtful that space is ventilated.

Adding insulation on top of your sheetrock would lower the temperature in the attic and IF warm moist air is entering, increase the chances of condensation.

So, some options and remember I'm here and you are there, so tough to give good advice.

Add a new roof over what is there with ,say, 4" of rigid insulation, ouch.

If possible, spray insulation under the roof, from the attic space. May not even be possible.

Move your thermal barrier to the sheetrocked ceiling, with lots of new insulation and air sealing of the sheetrock transition, plus ventilate the attic. This would be more like a traditional house, but consideration would have to be given to heat ducts and any water pipes.

Since all heat ducts and ceilings leak some air, trying to simply add insulation to the current ceiling is questionable. Your choice will also depend on your location, warn, cold or in between. Is there any chance of adding to what is currently up there?

Bud
 
  #3  
Old 02-28-10, 04:49 PM
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The only place in the house where the ceiling is open to the attic area, and therefore where warm moist air can rise into the attic, is the utility room where the furnace and ducting is located. The only other holes between living space and attic space are the ventilation outlets and while they are not airtight there shouldn't be an airflow from living space to attic space, right?

I would like to assume that most of the transfer from living space to attic space is thermal radiation through the ceiling sheetrock and not the movement of air.

And if that wonderful assumption is correct, then simple insulation on top of the ceiling sheetrock would reduce the thermal radiation transfer, right?

As for adding to current insulation, the present stuff is sandwiched between the steel trusses and the flat metal roof. Any new insulation immediately under the roof would have to be glued, I guess, to the existing stuff. That doesn't sound very hopeful and certainly not stable over the long term. [But like Mark Twain said, in the long term, we're all dead.]
 
  #4  
Old 02-28-10, 05:31 PM
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If your walls are also sheetrock, then the wall to floor, all windows and doors, and all electrical boxes anywhere in the house, provide access for air into the wall cavities. Once inside a wall cavity, air can move through many adventurous paths and end up in the attic. In your case, if the attic is not vented, air will have a hard time entering, because none is exiting. Only a convective flow of cold air flowing down through perhaps those same adventurous paths would allow warm air to rise. Ceiling lights, recessed cans, holes for electrical and plumbing all need to be sealed. Drop ceilings, openings for plumbing under sinks, showers, and tubs as well.

You still didn't mention what region you live in and that is part of the analysis.

Bud
 
  #5  
Old 02-28-10, 06:00 PM
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Sorry. Am in northwestern NJ, so get northeastern PA winter weather, which can be quite cold. Far enough from the Atlantic to not have atmospheric moisture.

Last November got Energy Kinetics System-2000 installation, single burner replacing two 1974 vintage GE oil hot air furnaces and one oil hot water heater. Fancy humidifiers on both air handlers, but given the subject of mold in the attic, maybe that wasn't so smart. So far, so good.
 
  #6  
Old 02-28-10, 06:58 PM
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North Jersey, Delaware water gap, I wonder what the family camp lots look like now???

Not typical new construction for NJ, sounds like a former commercial building. Now, I just can't get comfortable with what you want to do. Here's why. Dew point is where condensation occurs. Your inside air in your house might be 35% RH at 70 degrees. But move that air into the attic and reduce it's temperature to 35 degrees and you will be at or near that dew point. Once you cross the line, you have moisture in the attic and even long before that, you are well above the RH feeding threshold for mold, 50%.

If you could seal as best possible. Then bury that attic in blown in insulation, enough so you could open some vents to exhaust any moisture that does get up there, then the mold/moisture issue would be gone.

But, there is always a "but". Some designs rely on the snow melting off of the roof. Once you vent the remaining attic, the roof will be cold and snow melt eliminated. If that is what you want and it is the most energy efficient, that is what I would recommend.

Be sure to check the perimeter walls in that attic to see how they extend from below into the attic. Sometimes they are open cavities, like balloon construction, and dump a lot of heat into the attic.

Bud
 
  #7  
Old 03-01-10, 05:32 AM
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You can see the Water Gap from a rise not far from my home. When I make a long cycle loop [in other than the worst winter months] I'm right on the river itself for several miles.

Home wasn't literally commercial, but was built by a commercial builder for himself and the whole thing effectively is a commercial building subdivided into an ordinary residence.

Regarding blown in insulation, then "open vents". Where might those vents be? From the attic space to what? Outside?

Roof is almost perfectly flat [which upset my home insurer immensely -- Prudential/HighPoint after 20 years somewhere else wouldn't underwrite me here]. There's a very, very slight pitch from the centerline to the perimeter gutters. That's it. No snow runoff except by melting.

When you say perimeter walls dump heat into the attic, you mean leaks into the wall cavity and then up, if open at the top, right? All the hatches into the attic are some distance from the perimeter walls. I'll see if I can crawl [on the metal joists, no flooring] to a perimeter and look.

Thanks for your time.
 
  #8  
Old 03-01-10, 06:15 AM
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Just to think out loud, I've seen flat roof buildings converted to traditional roof style.

Yes vented to outside, thus lots of new insulation on top of the sheetrock to take over the entire job. But from the sound of what is up there, you need a lot more anyway.

Vents would be high and low as best you can and on more than one side. A HRV could be used for some heat recovery, but primarily for a powered exchange. They use low power 24/7 fans that run for 10 plus years.

Good luck up in the rafters .

Bud
 
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