first off I am a woodworker with some building experience, I am building a new shop myself to save money. well I put 2×4 purlins 24″ o.c. directly over the trusses and a classic rib metal over the purlins. after I closed in the walls, in Sept., I began to notice a mist on the metal that disappeared by mid morning, one day when the cool weather hit it began to drip – so far just the one day. my question is – do I need to insulate the bottom side of the metal- if so what type of insulation should I use? I intend to have a ceiling with cellulose blown over it and the soffit and ridge are vented.
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Replies
I think you are looking at condensation. Vapor barrier the work space and it'll go away?
Maybe.
The condensation is forming on the cold surface of the metal. If all you do is put a plastic sheet on the bottom of the ceiling joists, with no insulation, the plastic Vaporbar becomes the cold dew point where it will condense..
Excellence is its own reward!
"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius
anything you don't know? you seem to be some sort of building genus LoL
Genius sorry having problems typing tonight
LMAO
If they clone me, genus might be right!
Keep reading here. I still learn something every day. There's plenty of guys here who know more than I do - just don't post as much.
And then there's guys who post a lot but know less than they think they do.
I used to be a genius but then I got old.
Thanks for the laugh!.
Excellence is its own reward!
"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius
Easy on those guys that think they know more then they do, Im one of them, but this board is the best thing I have ever found, Ive learned more reading here then in ten issues of the mag and I have a long way to go yet
" i used to be genius than i got old".............
piffin with that comment you put a deer slug in my ethos...
the shame ,the shame......
I think what your saying is that I need to insulate, what do you think about 3/4" extruded foam ripped to width and fit between the purlins against the bottom of the metal?
Not sure, but I think you need to keep the same ventilation practice you get when you make a composition roof. That is, you need a soffit & ridge vent system to ventilate the condensation when it gets warm. It is a cruddy part of roofing but there is always moisture even on the impervious surface you have. Rain on top and condensation from below. If you were to spray on the urethane foam you'd break the cycle but if you have even a crack it'll get wet and mold up.
what scares me about the spraying foam is the possibility of it getting between the purlins and the metal and expanding, resulting in a deformed roof.
the soffit and ridge are vented.
I would more likely fasten it to the bottoms of the purlins and tape the seams. That way you don't have breaks for the moisture to migrate up into and hide from your eyes behind it. Better yet, use one inch thick.
Bob is right to suggest seeking the source of the moisture. I am presuming it is green wood drying as it is in mine. If it is unventer heat then you could have another headache..
Excellence is its own reward!
"The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius
I think going to the under side of the metal and applying a bead of silicone would be better because going under the purlins would create a damb at every purlin.
the ventless heater is only temperary, I do have a dehumidifier in the basement with the heater.
And where is that moisture coming from? Some form of "ventless" heater? A plugged flue? Some other water source?
Treat the problem, not the symptom.
________________________________________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
I have neglecked to mention, the shop is built over a basement with a concrete floor and ceiling. and the heat sorce to date has been a kerosine salamander type heater. the basement however is not without air infilltration.
heat sorce to date has been a kerosine salamander type heater
There's your source of mositure; water vapor is a significant byproduct of combustion and there are several gallons per 100,000 btu, I forget the exact figure.
And be SURE to use a carbon monoxide detector with a digital readout, high levels of CO can kill, low levels can cause significant adverse health problems.
Just becaus you aren't dead doesn't mean you don't have a CO problem!________________________________________________
"I may have said the same thing before... But my explanation, I am sure, will always be different." Oscar Wilde
That salamander is kicking out a ton of moisture.
I might run 2x2 nailers up the sides of the trusses along the undersides of the purlins before putting in the foam. That way any moisture that may find its way into the space between the metal and the foam has a clear run down the bay and out w/out getting pooled-up against the purlins. It provides a clear vent channel from soffit to ridge as well.
Be sure to seal the edges of the foam to the rafters with spray-foam. It's actually easier to cut the foam a little loose and foam the gap.
Steve
I think the other guys are right in diagnosing condensation.
But I wouldn't suggest putting something on the purlins. I'd look more at adding a ceiling, and insulating it. (If the trusses were designed for ceiling loads)
That way you get more light down closer to your work, and it will take less insulation. You can bolw insulation on top of the ceiling a lot cheaper and easier than fitting stuff to the purlins under the roof.
What part of the country are you in? Will you need heat in the shop part of the year?
The hardness of the butter is proportional to the softness of the bread.
I fully intend to install a ceiling, 5/8" plywood and blow in an R-30 over it.
since the addition of the ceiling will prevent access to the roof , due to the shallow pitch 4/12, I feel that I need to insulate the bottom of the metal now to prevent moisture dripping on the cellulose later. I first noticed the condensation before the heater was being used.
this is not from me to me, I am new to this site and not sure how to post to all exept on the first post.
first, I want to thank all for taking time to respond. I did a small test with a piece of 3/4" extruded polystyrene by ripping to fit between purlins,and wedging it tite to the under side of the metal. for about a week there was no condesation at all, seems like I cured the problem! not! , on sat. the moiture was back in the form of frost and it was on the top as well as the under side of the metal, everywhere exept under my test peice. it was dry as a bone, this has inspiered me to do the entire job. perhaps this is overkill, but it will make me sleep better. and the extra R-10 my help keep a cooler attic in the summer. for what its worth the foam will be vented by the ribs in the metal on its top and vented by the soffit and ridge on its under side.
thanks again for the responces and please continue to comment, I will begin insulating today, yes on Sunday the last day of deer season, no rest for the weary.
"I feel that I need to insulate the bottom of the metal now to prevent moisture dripping on the cellulose later."
Actually, the reason I suggested it is because I think the ceiling will solve your condensation problem.
The condensation is occurring because the warm, moist air inside the building is hitting the underside of the cold roof. If the ceiling is insulated (And has a vapor barrier), the air in the attic will be colder, and won't have a lot of moisture in it. That should solve the problem.In my house on the ceilings I have paintings of the rooms above... So I never have to go upstairs.
I would agree with you, except the moisture began before the heat was running. I think it was late Aug. - Sept. this is why I'm going ahead with the project, that and the fact that if I only put the ceiling and insulate and the problem still existed... I would have to remove the roof to get access, I dont like that idea. Worst case, I am wasting $500 worth of insulation and my time, but I dont think so, since I began I've had mornings with frost and moisture only in the areas that where not insulated and no trace of moisture on the insulated parts.
next time you can bet the farm there will be a plywood deck and tar paper under the metal !!!