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Condensation forming?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Cudos, Nov 29, 2009.

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  1. Cudos

    Cudos Member

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    Hello, I need some advice as I'm somewhat concerned. My cabin is almost complete, we have R20 insulation in the ceiling and R12 in the walls. The exterior walls are 4x4 tongue & grove. I have a tin roof. As you can see by the enclosed pics there is condensation forming on the flue box. I also have condensation building up on the inside windows as well. The temperature today inside the cabin about 15-17c , heated only by a couple of space heaters at this point.
    The outside temp was around 0c, maybe +1 but was around -10c the night before.
    Why would condensation be forming on the flue box? is this a negative house pressure problem? I really have no idea, any ideas. The stove has not been used recently.


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  2. jtp10181

    jtp10181 Minister of Fire

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    Seen it happen with this kind of install on gas stoves. That ceiling support is basically exposed to outside air, so its going to get cold. Just like a window.

    Bottom line is, you have too much humidity, get a dehumidifier or start running the stove some more to dry the place out.

    Do you have one of those whole house humidifiers? Turn it down or off...
  3. LLigetfa

    LLigetfa Minister of Fire

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    Therein lies your problem. Relative humidity is well... relative, so you need to get the temp up or the humidity down. Contrary to what many people think, new kiln dried wood used to build homes is not really all that dry. There is excess humidity being released from the new wood. I've seen that with my last two new homes.

    If there are any voids in the insulation, there will be cold spots with subsequent condensation or frost forming. The chimney roof support cannot have sufficient R-value because of clearance requirements.
  4. AlaskanRedneck

    AlaskanRedneck New Member

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    This is completely normal. What happens is that moisture is drawn to cold surfaces. Just like in the summer when you have an ice cold glass of tea outside on a hot summer day, the glass is wet on the outside.

    Especially in a new to newer home, 0-3 years old, moisture is coming off the wood and products made in your house, everything from the flooring, cabinents, walls, ect, as well as your crawlspace emit moisture as they acclimate, your crawlspace should be sealed BTW with a vapor barrier.

    So as the moisture in the house increases, humidity, it condenses to the cold spots. If you had a wood stove, you could keep it going and it would do a great job of keeping the condensation down. The reason it condenses to the box in the ceiling is becuase that box is colder than the other spots in the house. Mine has actually done the same thing when i don't run it for a while and the outside temp is way down.

    You will also get condensation on windows, especially when the temp is below freezing. Easy way to stop these problems is to seal your crawlspace floor with a vapor barrier, and that means not just laying it down there, it needs to be sealed up to the foundation wall at the bottom. Also, you need to run your wood stove as your primary source of heat. If that is not possible, make sure you ventaliate your home with either a bathroom vent fan, or you could install a Venmar mechanical ventilation unit (what I have). Lastly, if the window condensation is a problem, keep your blinds slightly raised and run those small clip on fans blowing on the window as needed, this will keep warm air moving over the surface of the glass and condensation will not be able to form.


    Hope that helps you. If you have more questions, I would be glad to help.
  5. Cudos

    Cudos Member

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    Thank-You all. I do have vapor barrier being put down in the next week or so in the crawl space. It makes sense that this is a ventilation problem. We don't have bathroom fan in yet either. Looking in to a Venmar mechanical ventilation unit may be an answer as well. At least I know what is causing this now and what is needed to correct it. Better ventilation and ensuring I have a proper vapor barrier installed.

    I was thinking also of applying heat resistant sealant around the flue support box where the wood meets the box and installing the 2" metal heat shields that came with the pipe kit. I guess that can't hurt but likely won't help much.

    Thanks again!
  6. jtp10181

    jtp10181 Minister of Fire

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    You should seal air leaks everywhere you can around the support box. The metal strips are trim strips, not heat shields. They are made to hide the joint between the support box and the ceiling. Yours does look pretty good without them though.
  7. littlesmokey

    littlesmokey Minister of Fire

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    JTP, is this one of those cases where the thimble should be insulated, at least on the upper side? Seems the house is a pretty tight envelope and the flashing is acting as a condensor.

    Thoughts....????
  8. AlaskanRedneck

    AlaskanRedneck New Member

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    You can use a high temp caulk known as Proseal, it's clear and would work in that gap you are referring to, then put on the 2" metal trim ring. Another thing is that the Venmar will help circulate the heat from your stove to the rest of the house if you have ductwork (force air furnance) installed, it can also do this if you don't have forced air furnace heat too. Depends on how you install the Venmar. I do not know if you have another heating source or not.

    Several different scenarios you can use.
  9. John the Painter

    John the Painter Member

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    Like most of the other posts have said all the new wood is still emmitting moisture because kiln dried isn't actually all that dry.Something to try is when the temp. outside is low (ie -10 or so) open up a couple windows and let some fresh air in.The frosty air is great for drying and it acts something like an air exchanger.Beautiful home.Love all the wood finishing.
  10. Cudos

    Cudos Member

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    More great advice Thanks everyone, also no kiln dried wood used in this build. All the wood was cut, stacked and then planed at a later date so the wood is likely wetter than kiln dried.

    Thank-You
  11. jtp10181

    jtp10181 Minister of Fire

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    What thimble? Everything looks installed fine to me. And yes, the support box is acting as a condenser because its so cold.
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