21sapptk

We recently bought an existing metal building and relocated our woodworking shop. We insulated it overhead with R-19 fiberglass up between the purlins. And heated it with infra-red overhead gas heaters..

The insulation has a vapor barrier on the warm side (UNDERSIDE). And we have six large (16" diameter) vents at the ridge vented to the outside. The building is about 9,000 sq. ft.

We are still getting condensation on the underside of the metal.

We are needing a new roof so we've decided to install a new metal roof over the old one using "hat sections" to created about a 2" air space between the old & new roofs. This air space will be vented and we'll continue to use the ridge vents to vent from the inside of the building out.
Will this eliminate our roof condensation problem?

joasis

Is the building engineered to take the added weight of a second roof?

Condensation is occurring because 1, fiberglass in your application is not going to work, and 2, if you need the roof replaced, why not actually strip the building and install a roof with vinyl backed insulation correctly.

If I were looking for a quick fix, I would consider a spray foam insulation application, and remind you that the roof purlins can and do conduct temperature efficiently. When a building is insulated during erection, a thermal break is created between the purlins and roof panels by the rolled insulation.

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21sapptk

Thanks Joasis for your reply.
Do you think the air space between the two roofs would create enough thermal break or would we be wiser to tear the old roof off , add rolled insullation and put new roof on?

precisionbuild

Tear off, use correct insulation installed correctly, and replace roof if possible. Plus, a packaged engineer building is usually engineered to the minimum standards for the use it was sold for. Adding extra weight, or modifying is a bad idea without an engineer.

joasis

What exactly is wrong with the old roof? Unless it is rusting through, or has damage, a competent crew should be able to pull the old fasteners and lift the sheets, rolling insulation and resealing the sheets as they are replaced.

If you try to add a roof to a steel roof, you are only asking for trouble.

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21sapptk

Thanks to both Precisionbuilt and Joasis for your response.
We're going to follow your advice Joasis and lift the roof, apply the rolled insullation and reinstall the old roof. Later we'll have it coated with a special paint designed for this purpose to cover some surface rust spots.
Much cheaper than a new roof and safer than a double roof.
Someimes you just need someone to point out the obvious.
Thanks Again

Dr Heat

If I were looking for a quick fix, I would consider a spray foam insulation application, and remind you that the roof purlins can and do conduct temperature efficiently. When a building is insulated during erection, a thermal break is created between the purlins and roof panels by the rolled insulation.[/quote]

We have run into this on several occasions, here is what I have learned.

1. The rolled insulation does not create a sufficient thermal break. You still have a purlin that drops below the dew point.

2. Dehumidifying is not the answer because the cost is too high

3. Ventilation and insulation are the key

So I have found the only way to stop or minimize condensation is to create a vented space between the insulation and the roof.

Some fixes we have used.

1 drop the ceiling you need to have a sufficient vapor break. you will also need a thermal break sufficient to lower the dew point above the ceiling. This requirement will vary from area to area.

2 you can also insulate the roof itself wrapping the purlins. This may be cheaper but will only work in warmer climates than my own I live and work in northern Il.

3. we also once laid two inches of roof insulation on top and covered it with rubber roofing material. This worked very well for the first two years then we had some trouble in the corners, mold began to form. This was a trucking depot.

One word of caution. We have consulted the builders ( Butler Morton and Lester ) they all told us that rolled insulation solves the problem. The culprit here is the tips of the screws that penetrate the purlins. You can look up and see the drips forming.