Condensation issue

[Richwoods]

Trying to decide on how to fix a metal roof condensation issue. I have 3inch foam board screwed to the purlins. All seams are spray foamed. Roof ridge vent complete span of roof, 30x40. It got down in the 20s a few weeks back and I left the heat on 65 overnight. Next morning the roof was dripping water in two spots.

I'm thinking about taking the roof off and laying down a vapor barrier. Good/bad idea?

Thanks!

 

[racecougar]

The ridge vent is above the insulation or open to the inhabited space? Is there soffit venting? Where was the condensation dripping from? What method of heating are you using?

 

[Richwoods]

The ridge vent is above the insulation or open to the inhabited space? Is there soffit venting? Where was the condensation dripping from? What method of heating are you using?

The ridge vent is above the insulation. There's a 1.5inch gap between insulation and metal roof.

One foot overhangs, but the soffits are metal.

Condensation was dripping from the ceiling in two spots. Coming through a seam I'm assuming.

Split unit for heat/ac

 

[PCustoms]

What type of heater?

 

[Richwoods]

What type of heater?

split unit heat and air

 

[racecougar]

I'd lean toward the lack of venting being the main culprit. The ridge vent isn't particularly functional without intake air at the soffits, which is allowing warmer-than-ambient air to condensate on the inside of your roof sheeting.

 

[tmp]

Some photos would be helpful, but if I’m understanding correctly your roof consists of foam insulation directly to purlins, foam is strapped with wood leaving a 1.5” air space, followed by metal roofing panels. Where does the ridge vent come into play? Just to ventilate the roof deck, not the living space?

So your assumption is that moisture condensed on the underside of the metal, dripped on the foam, and rather than running off down the roof it came in through a seam in the insulation panels. This doesn’t seem all that likely to me and I think you’d be wasting time and money pulling the roof off until you are absolutely certain of the cause.

What was the temperature of the underside of the foam panels?

 

[Richwoods]

Some photos would be helpful, but if I’m understanding correctly your roof consists of foam insulation directly to purlins, foam is strapped with wood leaving a 1.5” air space, followed by metal roofing panels. Where does the ridge vent come into play? Just to ventilate the roof deck, not the living space?

So your assumption is that moisture condensed on the underside of the metal, dripped on the foam, and rather than running off down the roof it came in through a seam in the insulation panels. This doesn’t seem all that likely to me and I think you’d be wasting time and money pulling the roof off until you are absolutely certain of the cause.

What was the temperature of the underside of the foam panels?

Ridge vent is for ventilation behind the insulation, like an attic. With some soffit ventilation, maybe it would actually work.

I wouldn't call it an assumption. Without rain, condensation is the only logically answer to where the water is coming from.

It was 20 something outside and 65 in the barn. I left the heat on overnight. The next morning, water dripping.

Warm air, cold metal = condensation. I was hoping it would be sealed up airtight to not let the warm air out, but that didn't work.

 

[giddygoat]

There needs to be good air flow from the softits to the ridge vent best would be the entire length of the wall and ridge vent as to avoid any dead spots. My thoughts are there should be a vapor barrier, when I did my shop the ceiling is vapor barrier then metal ceiling, blow in fiberglass with venting softit and ridge. Walls are sealed with vapor barrier down from the ceiling over lapped by leaving a couple of feet ceiling plastic down on the walls, wall plastic was then put inside to help seal the wall to ceiling joint . 1 by 1 strips over the vapor barrier on cement wall with foam inserts, tin over all of it. Never had any issues with water/moisture even 30 below with 70 degree inside temp.

 

[tmp]

Ridge vent is for ventilation behind the insulation, like an attic. With some soffit ventilation, maybe it would actually work.

I wouldn't call it an assumption. Without rain, condensation is the only logically answer to where the water is coming from.

It was 20 something outside and 65 in the barn. I left the heat on overnight. The next morning, water dripping.

Warm air, cold metal = condensation. I was hoping it would be sealed up airtight to not let the warm air out, but that didn't work.

For sure, I don’t disagree that the source of the drip is condensation. I’m just confused on where the condensing surface is and how it’s making its way in. With a decent pitch I would assume any water dripping from the roof panels onto the insulation would run off the top of the insulation rather than working its way down a crack.

I’m assuming your assembly goes:
-purlins
-insulation
-1.5” strapping
-roof panels

What am I missing with the soffit venting? Your soffits are not connected to the airspace between the insulation and the roof surface.

 

[finn]

The ridge vent has to have a continuous air pathway from the soffit vent in order to be effective. The idea is to allow enough airflow beneath the roof tin that the underside is the same temperature as the top side and thus outside air.

I’m assuming you have wide temperature swings daily, The warmer more moisture laden outside air gets into the building during the day, then the temperature drops at night, and so does the temperature of the roof. Moisture trapped inside condenses on the cold underside when metal temperature dips below the dew point of the air trapped in the cavity above the insulation.

Perhaps running the mini split in dehumidifier mode might help. Anything to drop the relative humidity in the building.

 

[billconner]

I believe the lack of balance between soffit and ridge is key to this. High - ridge - should be less than half. of high and low combined. Your result is negative pressure sucking interior moist air above the foam, where condensing on roof or top of foam or both.

 

[tmp]

I understand how roof venting works in a typical roof. The OP does not have a typical roof - they have a roof with “exterior” insulation (despite having no sheathing layer). Because they put their insulation layer on top of the purlins, then strapped it, then put on metal there is no place the soffits connect to this airspace. Here’s a detail that shows the way this could be done on a house.



The place that needs airflow here is the edge of the roof to facilitate airflow to the ridge. OP has some major differences with what they describe in this thread, no sheathing, no WRB, no air control later at all.

You add soffit venting, and all you get is outdoor air making it into your building. It still never makes it to your ridge vent which is above your insulation layer.

 

[firebirdparts]

One thing is for sure. If you get some ventilation, it'll be dry as toast in there with 20 degree weather and the building heated up to 65. The ventilation is going to cost you some heat, of course.

You didn't mention anything about your life to indicate how it got humidified in there. Normally, when you heat in the winter, it just gets super dry inside and you skin cracks. it's not easy to humidify a barn.

Heating air doesn't make it wet, it makes it dry.

 

[racecougar]

The soffits should be vented and routed above the "hot roof" insulation to the ridge. That is simple enough to correct. Kick the roof insulation down at the walls instead of carrying it all the way into the soffit.

 

[Richwoods]

My original plan was to spray foam. I ran across these foam boards for a great price and decided to save a lot of money. 3inch foam screwed to the perlins.

I ordered soffit vents for the entire both sides.

 

[Richwoods]

Thanks for all the comments and discussion.

Here is what I am gathering so far. PLEASE agree/disagree with me. Just trying to fix this.

Solutions to eliminate condenation:
1. Add soffit ventilation to hopefully create some airflow from soffits to ridge vent.
2. Remove roof, add vapor barrier, replace roof. OR add sheathing, vapor barrier, replace roof.


Need anymore pics?

 

[racecougar]

I'd advise against #2, as you'll trap your purlins between two vapor barriers (the foam boards and the vapor barrier you'd install).

If you're able to vent ambient air in through your soffits, between the foam board and the roof sheeting, and out the ridge vent, you'll prevent the roof sheeting from condensating.

 

[billconner]

You don't want vapor barrier on cold side of foam.

You should try to locate the condensation, even if it means removing some sheets of foam. I was in particular wondering if you could locate where interior air is escaping.

You might also try to locate source of moisture - just seems like a lot more than I'd expect in a shop. Maybe tape a piece of clear plastic to floor and look for condensation under it in morning.

What climate is building in?

 

[Richwoods]

You don't want vapor barrier on cold side of foam.

You should try to locate the condensation, even if it means removing some sheets of foam. I was in particular wondering if you could locate where interior air is escaping.

You might also try to locate source of moisture - just seems like a lot more than I'd expect in a shop. Maybe tape a piece of clear plastic to floor and look for condensation under it in morning.

What climate is building in?

Middle TN. Humidity in the shop stays low. I think it was just a perfect storm that night (could be totally wrong of course). High humidity outside, cold metal, warm air from shop hitting cold metal.

I'll check the floor tonight, but I don't think it's the floor. About a year-old concrete with a vapor barrier under it. Epoxy flooring on top.

 

[finn]

Thanks for all the comments and discussion.

Here is what I am gathering so far. PLEASE agree/disagree with me. Just trying to fix this.

Solutions to eliminate condenation:
1. Add soffit ventilation to hopefully create some airflow from soffits to ridge vent.
2. Remove roof, add vapor barrier, replace roof. OR add sheathing, vapor barrier, replace roof.


Need anymore pics?

How does adding a vapor barrier to the roof help? It’s condensation, not water migrating through the roof.

Condensation occurs when moist air inside the room envelope contacts a surface that’s below the dew point for that air.

A vapor barrier stops or reduces water vapor migration.

Two totally different phenomena.

 

[Richwoods]

How does adding a vapor barrier to the roof help? It’s condensation, not water migrating through the roof.

Condensation occurs when moist air inside the room envelope contacts a surface that’s below the dew point for that air.

A vapor barrier stops or reduces water vapor migration.

Two totally different phenomena.

I'm not sure, that's why I'm here. But I thought by preventing the warm air from contacting the metal roof?

 

[finn]

I'm not sure, that's why I'm here. But I thought by preventing the warm air from contacting the metal roof?

If it’s condensing, it will just condense on the backside of the vapor barrier.

 

[tmp]

Ok I’m seeing what I missed now. In the first posts I was assuming wrongly that your foam was on top of the purlins.

In this configuration I’m still thinking soffit vents alone won’t fix it. You need to have a clear path for air to move from the soffit to the ridge. In this case your purlins interrupt this path. For that to work, you would need create a vent channel. One way to do that is to pull the foam and add perpendicular strapping to the bottom of the purlins, then reinstall your foam.

You do need to stop as much air as possible from escaping up through the foam. If you can’t effectively air seal it you will need to add an air control layer to the bottom side. This could be drywall or it could be as simple a sheet of poly very carefully installed and taped at all the seams and penetrations.

 

[billconner]

was just a perfect storm that night (could be totally wrong of course). High humidity outside, cold metal, warm air from shop hitting cold metal.

If it was very wet outside, are you sure it wasn't a roof leak? Humid exterior air that came in through roof vents and condensed? Have to ask. Does seem odd it never happened before. I think might wait for a second occurrence before doing much.

 

[Fav Onefour]

Pull the foam. Put in the soffit venting. Install vapor barrier and ceiling on the bottom of your rafters. Insulate ceiling.
It's a bummer deal because you are already doing a nice job with the foam board installation. I'd use the foam board as the first layer above the ceiling and seal as you have done above.

 

[Richwoods]

What is another option besides pulling foam off? Shop is finished.

 

[Allan00]

Is there anything in common with the 2 areas that seems to be condensing? Location, size of spot, material, etc?

 

[Fav Onefour]

What is another option besides pulling foam off? Shop is finished.

Dang it. I didn't catch that the whole thing is done. It's a bummer deal. The shop looks good.

I don't see a simple button option.
With the foam board directly attached to the purlins, there isn't any real air flow. It's essentially limited to what flows through the metal roof ribs. That amount of air negates hot roof function but still allows temp differential.

The condensation you see is only part of what is happening. The purlins are catching and holding some of the condensation. In the long term, it will damage structure.

Peeling off steel and reusing is generally not a simple option. Screw holes grow in size no matter how fussy you are reusing them. The steel is part of your building structure. Add a little wobble and it just grows worse over time. If you can, try not to go that route unless adding a proper ventilation cross. And that's where it gets tricky. I can't envision a simple way without adding another layer of structure. Sure, you could do 2X8s on their sides on top of existing rafters and, or purlins. Then do purlins on top to rerun steel. Is it worth the effort? It's a lot of fiddling with a roof and I'm not fond of the structural characteristics.

 

[My Old Tools]

You could add a vapor barrier to the ceiling inside, either plastic or a barrier paint. You have to stop warm air from getting to the cold surface. 65 degree air at 35% humidity is 100% humidity when cooled to 20 degrees.

 

[Richwoods]

Thanks fellas. I haven't seen any water or wet foam since that one occasion. I've also been keeping my temperature gap from the inside to the outside within 20 degrees.

My plan is to add vented soffits and maybe a whirlybird to help get that air flowing. Thoughts on a whirlybird?

I'm also going to tape all the seams. I used the great stuff and/or caulk on all the seams, but obviously I have some warm air getting out.

After that, I'm not sure how to see if I have some wet metal on the inside of the roof.

 

[Dagny]

dehumidifier

 

[Dagny]

If your furnace will allow it **** combustion air from inside.

 

[C-S-H]

I love your shop! If you want to keep the look inside, I would have insulated above the roof deck. The naked metal roof will sweat whether exposed to inside or outside air. It needs a substrate.

An example layup from the top down:
Outside air
Metal roof
30# felt
1x pressure treated diagonal strapping vented to soffit and ridge
Polyiso foil face out, seams taped
Peel-and-stick air-water-vapor barrier
Plywood or OSB
Purlins
Roof trusses
Inside air

I can't think of a simple solution to your problem. The bright minds on here have some good ideas.

 

[Richwoods]

dehumidifier

The humidity stays low due to the split unit. How low do I need it? Currently 41%

 

[Richwoods]

The soffits should be vented and routed above the "hot roof" insulation to the ridge. That is simple enough to correct. Kick the roof insulation down at the walls instead of carrying it all the way into the soffit.

The roof insulation does NOT carry into the soffit. Air can flow from behind the foam boards from the ground, all the way to the ridge vent.

 

[racecougar]

Good. What do you mean that air can flow from the ground though?

 

[Richwoods]

 

[Richwoods]

Good. What do you mean that air can flow from the ground though?

Dark blue is the foam board insulation

 

[Firstram]

Dark blue is the foam board insulation

If the foam is nailed to the bottom of the 2x roof purlin, the flow of air is blocked by the purlin! The only clear pathway from the eave to the ridge is the small opening where the metal is crimped!

Unfortunately, spacing the foam down is the only way to get the air flowing.