We have an old 6″ kitchen fan vent pipe that went out through an older, abandoned coal stove 10″ vent in the old roof, originally — the top of the vent had a high “top hat” and huge opening for airflow out, and the inside pipe went right to the top of the outside — never had any blowback from it.

The roofers replaced the 10″ roof cap with another 10″ one, new style, low flat top and small openings.

They left the old 6″ kitchen fan vent pipe lying on the floor of the attic. I’d caught that weeks ago.
I’d gotten them back twice to fix that and thought they finally had — it looked Ok from across the attic.
And I’d hurt a knee and been unable to get over to double check it.

Finally got my injured knee in shape, in desperation, and clambered over there just now.

All they had done was drop an 8″ pipe down in from outside — just covering up the problem.

We still had the kitchen vent fan blowing all that moist air through the 6″ opening.
Much of it was escaping right at the base, in the space where the 8″ pipe didn’t mate to anything.
More was escaping between the top of the 8″ new pipe and the 10″ new cap.

(These are flat top caps with little rectangular spaces around them, lots of back pressure I guess.

So — for weeks — all that moisture has been going into the attic.

I guess all this time it has been condensing in the DensDeck material under the IB Roof and some of it had soaked into the old roof boards (luckily the old roof deck boards are still tar covered in many places, in fact a lot of the old tar paper is still on them).
So — I hope — this explains the water that has been trickling back down along the screws.

Well, duh.

Trust, but verify, as Mr. Reagan said.

For now I’ve stuffed plastic bags up around the outside of the 8″ pipe inside the 10″ roof vent cap — not blocking the proper airflow out the top, just blocking the backflow down around the sides.

And duct-taped the gaps between the 8″ pipe and the 6″ pipe below that.

Air is now flowing out from the kitchen vent fan to the outdoors.

I’m running heat and fans to dry out the attic — down to 66 percent humidity now.

Still got water trickling down the screws on the north slope of the roof, that part of the DensDeck must have gotten the wettest being both the coldest and nearest the kitchen fan source.

I’m still concerned about ordinary condensation under the IB Roof.
But I’m a little less worried now that I have an explanation for the immediate large problem.

I thought we were getting the IB ‘mushroom’ aluminum vents but apparently the IB rep. decided we were not going to need them – we had stood on the roof and talked about having five of the large ones.

There’s a big difference between venting hot air (which really isn’t likely to be a problem in our area) and dealing with moisture condensing at night under a cold roof.

Well one last hope is maybe the roofers failed to connect the exhaust fans, I’ll do the crawl to the back of the attic tomorrow and make sure that’s not where the moisture’s coming from.

But the copper plumbing vent pipes go right up through the attic and out, they’re plenty cold, and there’s no moisture condensing on them at all – it’s all coming down from the screws that poke through the wood.

There were screws coming through before from the old asphalt/tar and gravel roof — but those never dripped water.

It’s got to be something happening under the IB roof somehow.

It’s mechanically attached, big screws through plates and down through the 1″ boards that make the roof deck here, with a quarter inch of DensDeck under the IB.

It’s plain IB, 80 mil, white, edge to edge (we have what they call ‘Dutch gutters’ out here so the membrane wraps up over a 6″ board at the outside, making gutters).

I asked repeatedly about venting. The IB factory rep had negotiated something with the city about how much but never told me what was supposed to be there. But out here the inspectors don’t even go up on the roof when they sign off on the work. The IB factory rep got tired of talking to me and just referred everything to the roofer. The roofer came back and put on two 5″ vents through the IB roof (not penetrating through the DensDeck or the wooden roof deck) at the very end because I reminded him that the city insisted some venting was needed. I don’t know if he did what IB told him was right or not, yet — can’t get the IB rep to tell me! Really frustrating.

I”ll be asking til I get answers.

And I lost most of a month after screwing up a knee, I’m an old guy, so I just got up the attic ladder tonight. I have a lot of asphalt dust and debris to clean out up there, and insulation to get back in place.

And — we’ve got water dripping off the screws where they poke down into the attic. It’s about 45 degrees, cold and clear outside.

There has always been water condensing aplenty on the outside of the IB material — as soon as the sun goes down — but apparently it has also been condensing all along underneath it, as the screws are all pretty rusty where they stick down through the wood planks that make up the deck.

Tonight some of them have liquid water dripping off them.

Pretty … well, I shouldn’t use the language that comes to mind. I emailed the IB rep here, and the roofer, and the IB tech representative and will see what they say.

The ex-NASA guy who pointed out to me originally that this might happen advises completely sealing the attic — so no moist air can get into it at all. He said that adding regular attic ventilation under a cool roof is counterproductive because the high emissivity surface is a heat pump radiating heat away to the nighttime sky just like during the daytime (except with a colder sink to send it to) — and that letting nighttime air circulate under a cool roof just keeps bringing in warm moist air that will condense on the underside.

Others say add more vents (well there are no vents at all right now except the two 5″ IB ones above the roof deck.

Another guy said get a foam roof company in to spray the underside of the roof deck with 3″ of foam, except that water can still get between that and the IB and rot out the wood.

This has the feeling of being a mistake, so far.

First of all, you did not specify whether you got an all IB roof or an IB + Shingles combination. Second, I would need to know if you have a mechanically attached or fully adhered membrane assembly. Third, since you have a low slope roof with open attic space, you should probably have a ridge / soffit ventilation added to get rid of moisture in the attic. IB roof is not only a waterproofing membrane but also is a vapor barrier. Here is and article I wrote on venting of the low slope roofs to prevent condensation and Ice Dams – http://www.coolflatroof.com/flat-roofing-blog/ice-dams-prevention-ventilation-of-low-slope-roofs/

We usually add membrane vents to a mechanically attached IB roof to get rid of the condensation under the membrane. In fully adhered assembly, there is no air space between the membrane and the substrate (DensDeck in your case) so condensation occurs in the attic space. You can also use large (8 inch wide) attic vents to improve ventilation, if you opt not to use ridge / soffit ventilation.

I would never recommend using both IB and shingles on a low sloped roof. It creates more problems than saves money.

If you want, send me a few pictures of your roof + attic space, and I will have a better idea of your potential issues, and see if I can recommend anything. my email is Leo @ coolflatroof.com (remove spaces).

I just got an IB roof put on, reroofing an old house (low slope not flat) after tearing tar and gravel off. I got bids from five roofers for ‘cool roof’ — some shingles, some IB, some combining the two using the IB around the edges where asphalt rots really fast. This goes over an old 1×8 board roof deck with big gaps between boards.

All of the bids proposed using a quarter-inch “DensDeck” layer or equivalent fiberglass between the “white roof” and the boards — apparently standard for fire resistance here.

None proposed using a layer of foam insulation. I asked each one about adding a layer of insulation thinking why not take the opportunity. All said it was useless and throwing money away, since there is ample insulation on the floor of the attic. I believed them, they were consistent about the answer.

But I didn’t know how much water these high emissivity materials condense — it’s astonishing, the roof gets as wet as roofs of vehicles do.

After we got the IB roof on, a neighbor engineer asked “how much insulation did they put under that white high-emissivity roof? You know it’s a heat pipe and will be condensing water out of the air on both sides …..”

Argh. Do the roofers know the area, or not know anything about this high-emissivity material?

We rarely get freezing, but we always have about 60 percent humidity year round, near the coast.

Under the IB roof, the roof deck on a hot sunny day is about 85 degrees — 25 degrees cooler than under an asphalt roof next to it. That’s with air temp around 85-90 degrees.

Dunno what it will do on a cold clear winter night. I’m realizing that if there’s frost on the outside, odds are there will be frost underneath as well.