Replies

1. 
   Malcolm_Taylor | Sep 17, 2024 04:33pm | #1

   Mike,

   Don't worry, it's entirely expected - and while it would have been a lot better to have a synthetic roof underlayment, it's not the end of the world.

   Poets taught us "dew falls", but it doesn't. Dew is moisture in the air condensing when it hits something cold. That can be the top or the bottom of the panels. Two things limit the amount on the bottom: The heat of the attic space (it's always warmer there than outside), and air-sealing the ceiling so the amount of moisture getting up there is reduced. Once you finish insulating and air-sealing the attic, the amount of condensation will be greatly reduced.

1. 
   xtal_01 | Sep 17, 2024 04:53pm | #2

   Awesome ... that is what I was hoping. I left a 2 ft space ... ceiling to roof panel ... at the tightest point so I can get say 18" of insulation and still have a 6" air gap.

   Is there any benefit of moving the insulation in the "wings" toward the roof ... like in the blue on the picture rather than the red (right on the ceiling)?

   It is more work but I am thinking it might cut down the air volume in that unused attic area (above the side areas) and thus have less warm air?

   Thanks!

1. 
   Malcolm_Taylor | Sep 17, 2024 05:51pm | #3

   Mike,

   Warm air is good. The hotter those attics get the better. It will dry everything out and keep the roof panels hotter, which reduces condensation.

   Another advantage of leaving the insulation at the bottom of the trusses is the ease of air-sealing. Getting a good air-barrier where the insulation follows the roof-line is going to be difficult with all the truss webs interrupting it.

1. 
   xtal_01 | Sep 17, 2024 06:08pm | #4

   Fantastic! Yes, it is much much easier to seal the bottom of the trusses.

   I have been super careful so far ... even calked in the plate to the pad.

   The pad is insulated and there is insulation down the outside of the pad and 24" outward.

   Definitely not a perfect building but if was relatively cheap to do and saves energy (money out of my pocket to heat) in the long run, I have done it.

   Thanks so much!

2. Expert Member
   Michael Maines | Sep 17, 2024 06:26pm | #5

   I am less optimistic about your situation than Malcolm is. That doesn't mean he's wrong, as every project is different, but I believe what you are seeing is condensation caused by night sky radiation. On clear nights, the roofing radiates heat directly to outer space, similar to how the sun radiates its warmth directly to our skin and other surfaces.

   Night sky radiation cools the roofing below the ambient air temperature, and if it drops below the dewpoint temperature of the air, moisture will condense from the air on both the top and bottom surfaces of the roofing. How much moisture is produced depends on many factors; it can be little, or it can be a lot.

   Most of the condensate that forms on the bottom side will find its way down the roofing and drip harmlessly, some will be absorbed and released by the purlins, and some will fall onto whatever is below the roofing.

   If you were to keep the roofing warm than you would eliminate the condensation, but if you have insulation in the roof then you almost certainly won't have enough heat loss to keep the roofing warm enough all of the time that condensation could occur.

   The two ways I address this are to include a watertight membrane below the purlins, or to install the roofing on sheet goods, which provides enough insulation to prevent condensation on the bottom of the roofing.

   In any case, I wouldn't panic but I would hold off on installing insulation until you have a better sense of when and where condensation is occurring. I bet it's worst on mornings after clear nights when the outdoor relative humidity is fairly high (aka when the air is close to the dewpoint temperature).

   If it seems like a significant amount, installing rigid foam between the purlins, and taping the seams so outdoor air can't reach the underside of the roofing, would almost certainly solve the problem.

1. 
   Malcolm_Taylor | Sep 17, 2024 06:52pm | #6

   Mike,

   I think it's mainly night sky radiance causing the roof panels to condense too. It's bad enough here in the PNW that the underside of our roofs sheathing often shows discoloration even in well built attics.

   I'm optimistic for two reasons:

   - Unlike the moisture accumulation you get due to air-leakage from the interior, night-sky radiance is sporadic, giving the roof time to dry out.
   - My experience has been that on metal roofs with purlins, even without an underlay, the moisture accumulation doesn't do much harm, as (much like walls) it's the sheathing that usually sees the damage.

   I would be more worried about a cathedral ceiling with those conditions that attics - which is another reason not to have the insulation follow the roof-line on this shop.

3. 
   xtal_01 | Sep 17, 2024 07:59pm | #7

   I appreciate all the input!

   I was worried about this when I started looking at metal roofing.

   I put up a few posts about three years ago now. I kept getting reassurances that as long as I had good insulation and a good vapor barrier, there should be no problems.

   I am guessing everyone was thinking about the moisture from the building but not considering the moisture from the air gap I left.

   I am even thinking maybe a fan to vent the area on the days it detects moisture? I was told the idea is to keep a "cold roof" ... keep it the same temperature on both the top and bottom? Maybe a small fan would blow cold air into the attic area? Or as you said, do I want a warm area?

   Seems nothing is simple when building.

   Mike