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Condensation issues
I am looking at re-roofing my wood-shingled barn in metal roofing. I have been told that the roofs can sweat so much in our cold climate that it can rain inside the barn. How can this be avoided? If 2x4 nailers are used to support the roofing, must the remaining shingles be removed?
Depends on the use of the barn? Livestock produce heat and urine vapors to the interior atmosphere. Thus both causing vapors that will rise and condensate on the bottom side of the panel if exposed. Urine vapor is a definite "no-no". The amonia content is highly corrosive to the panel. Storage of grains and grasses will also induce a moisture content with barely no corrosive effect. The best bet is to use a vapor barior below the roof to protect the panel, and, provide proper ventilation to the structure to eliminate "raining".
As a Canadian I can attest that condensation will happen no matter what the roofing material. Metal in some climates may cause more dew point crossings that create condenstion and on the other hand it may reduce them. If you did have a barn where the metal was the only barrier between the inside and out coupled with a moist air source inside and poor ventilation, you could get it to drip inside. This can be easily corrected by having a barrier.
There are many roof systems designed to install on strapping and approved for roofing over existing roofs including wood provided they are in reasonable condition. You should install a moisture barrier as an underlayment over theold roof and ensure that the strapping is fastened securely down through into the supporting stucture. For more info contact the individual manufacturer of your chosen product.
Ok, I think I understand.

Now the plan as it stands is to clear off a row of shingles, and nail a 2x4 to the rafter, then screw the pannel to the 2x4. Would the vapor barrier go between the panel and the 2x4. It there anything special about the vapor barrier itself, or will 6mil poly. work. What works best? Is there any sort of condesation/ventilation issue between the barrier and the metal itself?

BTW- the barn might have a horse or two, but not much for how big it is (old dairy barn)

Thanks in advance,

Last November I had a fellow worker install a metal roof. He does odd jobs on the side for people so I thought he knew what he was doing. Well, during the winter there was a sweating problem. He didn't put any insulation or a wood barrier between the wood where he nailed it. What can I do now? It also leaks water where it joins the house. That was why I wanted a new roof. It's worse now than before. What product can take care of this? Help! Thank you.
Condensation occurs, in essence, when the dewpoint is hit. The highest likelihood, then, is when temperatures are cool outside, making the metal cool and there is warm moist air on the inside.

For maximum efficiency, you need to have insulation on top of your ceilings so that heat cannot escape the home. Then, you need to have the attic space above that (beneath the metal roof) to be well ventilated. This will avoid build-up of excess moisture which can then condense.

I have seen this problem occur with all types of construction and all types of roofing. My guess is that maybe you've also added new windows, siding, and other things to the home which have made it tighter. As a result, moisture cannot escape the way it used to escape from the home and that moisture is instead ending up in the attic.

Also, look for other ways to minimize moisture -- make sure that all plumbing and fan vents for the home are vented to the outside and not just into the attic. Same for the dryer vent. Try to minimize production of moisture inside the home if you can. Ventless gas fireplaces are to be run only a few hours each day, not cobtinuously, because they generate moisture as well.

Also, it sounds like you need a new flashing between the house and the new roof. This flashing should go up behind the siding or other material on the house if at all possible and then come out on top of the roofing by 4 - 6". It then needs to be sealed where it joins the sidewall and the roof. Consult the roofing manufacturer for additional details on this.

Thank you.
Sorry for the delay but I just found this one hanging here and hope its not too late.

The old terms of the industry were called vapour barrier and underlayment. The vapour barrier went on the warm in winter side of the ceiling and the underlayment under the roof material. Many people mix these terms and purposes up.
Today the terminaology is an air barrier to the warm in winter side to stop the moist air from entering into the celing assembly and condesating as it cools and a moisture barrier to the underside on the roofing material that breathes while trapping condensation on the surface under the roof material.
In a barn with no insulation amd animals there will be a good amount of moist warm air cooling and condensating on the underside of the roof material. I would suggest leaving the wood shingles, strapping the roof vertically over the rafters with a 1 " board, laying a good quality moisture barrier loode over the vertical strapping followed by horizontal strapping and the roof metal. I would suggest using the vented closure strips at the eaves and ridge if possible and you will have a good roof assemble. Hope this helps.
I have a self storage facility with a standing seam roof. There is nothing on the bottom side of the metal roof and I'm getting condensation. Is there something I can go in and retrofit on the undesid of the metal roof to act as a vapor barrier. I guess it would have to have some adhesive or be glued to the undrside metal.
I would suggest polyicynene spray insulation


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