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barnowlbakery 04-13-2013 08:26 PM

Condensation with metal roof
Hey guys,
First time poster but boy have i pored over these forums during the last few months as I built an Alan Scott style WFO with a 3'x4' hearth for a small bakery my wife and I are building on Lopez Isl. Soon I'll put up a post with lots of pics and some things i learned along the way, but first a question!

I boxed the whole thing in with metal studs, sheathed with concrete board. Insulation is ~4" of perlite covered with ~18" of mineral wool insulation, essentially filling up the box. I roofed the whole thing with corrugated roofing screwed directly to the steel studs. There's one large vent just under the eaves, but no ridge vent.

The oven cured for ~5 weeks, with lots of small fires, and eventually a few big ones before the insulation went in.

Now the problem!

I've got lots of moisture in the oven box and from the looks of it it's condensation on the metal roofing. Enough to soak down through the insulation on the side of the oven, through the insulating slab and out into the chamber below the oven. At first I just thought it might be a leaky screw in the roofing but I think there's too much water for that.

Any other oven builder ever have experienced with condensation? It makes sense that condensation would happen as the warm oven air hit the cold metal roofing, but where's the moisture coming from? in through the vent? out of the concrete?

Any thoughts would be appreciated!

I have a few pictures up on the web for on our kickstarter project that we used to originally fund the oven.

check them out here:

Wood fired bakery on Midnight's Farm by sage dilts Oven is done, bread on the bake! — Kickstarter

cobblerdave 04-16-2013 02:35 PM

Re: Condensation with metal roof
Nice looking oven
The moisture comes from the pearlite layer I recon , just remember the amount of water required when you mixed it. You have I closed it before its had time to escape and the tin provides a condenser for the moist air. Is it possible to open the enclosure up till this problem stops? It might take a while. But it has to be done sounds like to moisture will stay in the enclosure for a long while if you don't.
Regards dave

Tscarborough 04-16-2013 03:19 PM

Re: Condensation with metal roof
I used wood studs, but I put a layer of plastic under the tin to catch the moisture, although I have never noticed any at all. FYI, by the time I built the house it had been cured and cooking for weeks.

SableSprings 04-16-2013 04:38 PM

Re: Condensation with metal roof
2 Attachment(s)
I built my oven during the summer of 2009. Since we're in Oregon I knew I would need a temporary cover through that winter and spring. I simply built a wood frame and nailed metal roofing to it over the oven. I left an open space of 6"-8" between the roof and the top of the dome thinking that would allow good air circulation and eliminate condensation....WRONG!

My oven was fully cured as was it's perlite/cement insulation layer--so the moisture was not coming from my build. I finally figured out that just the moist winter air passing through the open space would condense on the slightly colder metal of the roof. Lots of dripping onto my oven every morning as the air temps rose and the dew point reared its ugly head.

I ended up taking the metal roof off, laying down plywood on the frame, putting a layer of roofing felt on top, and then replacing the metal roof (seemed like a lot of work on my temporary oven cover!). That roofing layer system resolved the problem. When I built the Dragonfly Den the next year, I used plywood/roofing felt/metal roofing system and have not had any moisture problems from the roof. I've attached pictures of the final temporary structure and the current building where the oven now lives.

FYI: I have a deck on the front of the house that is covered with corrugated plastic/fiberglass panels (designed for roofing) and I get drips under the plastic roofing from condensation especially in the spring and fall. You just can't have a single layer of a thin, roofing material around here and not get this condensation "drippage" because of slight temperature differences.

Sorry to babble on, but I suspect that the exposed metal will continue to act as a condensation plate and drip fairly often on your oven/insulation. You are probably going to need to get something such as the insulated metal roofing, or redo the roof somehow to eliminate that situation.

silvfox 04-16-2013 05:30 PM

Re: Condensation with metal roof
If I interpet your photos correctly your oven is housed beneath a water proof roof in an unheated structure. Condensation will always be likely on the under side of your steel cover whenever the temperature swings up or down across the freezing point. You indicate that you have soffit vents but not top escape for moisture. Creating a vent would help. Removing the steel panels and installing 1/4" thick Astrofoil or Plyfoil under the lid would give you a water proof thermal break. Foil tape the seams. Its available 48" wide. If the edges extend beyond the outside wall any condensation that does occur will be on the top side and will weep outside your enclosure.

cobblerdave 04-16-2013 06:50 PM

Re: Condensation with metal roof
Gudday all
Thanks for stepping in with the detailed explanations, I certainly learned something there. I know now I was definitly out of my "climate zone"
( sub tropics)
Regards dave

Faith In Virginia 04-16-2013 08:32 PM

Re: Condensation with metal roof
Hay, barnowlbakery

I answered this same question from you over at The Fresh Loaf a few days ago. I'm not much on repeating myself. TFL just changed it's format and I can't find a thing.

In short ...Vent your roofing properly and your moisture will go away!!! That includes a full ridge vent and soffit vents.

Edit. Found it on TFL This is my post from over there.

"Sounds like a venting issue. One large vent in the eave could be one of your problems. The eave vents should run the full length of the eaves. Ridge vent is a must. For metal roofing they make a ridge vent that fits under the cap (looks like a scotch bright pad). If the outside temperature is cool and the area over the oven is warm you will get condensation. Think of a cold glass of tea on a hot summer day , and the condensation builds on the warm side. Proper venting will keep both sides of the metal roofing at a similar temperature and condensation won’t build.

Take what you described and high humidity, you could generate a lot of water."

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