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Inspected two duplexes last weekend in the same neighborhood. Same floor plan and everything except that one building faced west and the other faced north. The facing north had its eave vents east to west and the builing facing west had its eave vents north and south.

In other words the two buildings which were built at the same time were perpendicular to each other.

The one facing north with its eave vents east and west had blackened roof decking in the attic and the top of the insulation was blackening and it appeared that condensation was a problem. This was no spot occurrence the blackened decking was everywhere and appeared to be mildew.

The other building was clean as a whistle.

Could it be just the orientation of the building to prevailing winds that is making the difference?

Both crawlspaces were dry however the building with the problem with the attic also had substantial moisture damage to bathroom and kitchen floors. The other building also had moisture damage to the same floors just not to that degree.

Do you think that the orientation of the building is more likely the issue or do you think its just higher internal moisture due to the moisture damaged floors?

Chris, Oregon

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In the past I might have said increase ventilation for example by installing gable venting. That I think is wrong advice. The only thing I can think of is to increase venting with a continuous roof vent and installing venting in each rafter bay.

Is that sound advice? Or is that not likely to really work either? Usually I will see the mildew lighter or non-existent in the rafter bays with eave venting or only the north side etc. but in this case it appeared to evenly coat everywhere as if no venting is occuring and I could see daylight at the vents of the eaves.

Chris, Oregon

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Hi Chris,

"The only thing I can think of is to increase venting with a continuous roof vent and installing venting in each rafter bay."id="blue">

Oh, so it's one of those with eave vents every other rafter bay? If they increase that by boring additional vent holes, make sure they install chutes so that the air can pass up the underside of the roof.

You didn't say how old the buildings were or whether they're equipped with air exchangers and timers to remove the damp interior air that's been diffusing and leaking up into the attic. If they are relatively new, you need to make sure that the timer is actually working, connected to something and has been set to ventilate the home. You'll also need to make sure that any damper on any intake duct is functioning or that, if it doesn't have an intake duct, that the other intake source -usually slot vents at windows and doors here - are actually being used. Had a 4 month old home the week before last where the underside of the roof was all black and green with mold. The air changer wasn't working and there were lots of passages into the attic. The house sits in a draw with high trees all around it. Lots of shade. Lots of fungal growth!

They can treat the underside of the roof and the framing with BoraCare® and it will knock back the fungi and make it impossible for it to grow on the wood, but with that much moisture you've probably got a gazillion air passages up into the attic in both units that should have been sealed.

OT - OF!!!

M.

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A few years back I bought a party smoke making machine with a remote control. The idea was to demonstrate to the judge and jury how a blower door test could be manipulated and a visual representation of a pascal.

That case went well and we won. Being something of a tight-wad, I needed to justify the cost of that expensive ($49.99) equipment. Soooo I started running it everywhere; in the office, in every attic I own, outside on the deck, in the barn, in the bathroom, EVERYWHERE! I got to me an expert on blowing smoke.

A builder friend of mine had a new house inspected by one of our inspectors and we wrote poor ventilation; soffit and ridge, house facing west with north south ridge. Most of our weather comes from the west so you would think that venting set-up would work. The builder and me set the smoker dead center in attic and were amazed at where the smoke came out - the rear soffit vents and the east side of the ridge vent, mostly from the rear soffits! I suspect what was happening was the ridge acted as an airfoil with pressure on the front and vacumn on the rear, so no attic air was being dis-placed, just blowing thru the ridge vent. Front soffit vents were admitting air as intended and pressure was pushing that air out the rear soffits. A view from the skuttle showed exactly that. Builder added 4-5 roof pot style vents on the rear and problem solved!

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Questions, Questions & more questions. Both heating systems were identical, correct? If furnace, was there a humidifier installed @ problem house? Insulation in attic equal, or does problem house have more warm/moist house air seeping into attic (possible vapor barrier issues w/one house & ceiling)? Does one have air handler & the other not? The air handler is typically used to create a reasonable interior air exchange rate in new/tight construction. I would expect to find the air to air exchanger in the home w/out the problem, because moisture will be expelled with the air. Also, consider the fact that one system may be working while the other is not - all things being equal.

I don't believe that simple orientation would cause widespread conditions, but I may be wrong. Is the problem roof showing any signs of moss growth on the leeward face? Either way the recommendation of a baffle type continuous roof vent is probably the most adequate remediation technique. Is the sheathing salvageable? (i.e. Will the roof have to be replaced anyway)

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I believe the smoke machine would indicate current conditions & may not be indicative of all/or normal operating conditions. In any event, adding all the extra ventillation would have done the trick no matter what the original cause. Hate to think I have to start smoking the attic as part of typical inspection, I'm not that dedicated.

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Orientation is very important, if there is a problem. In other words if I have a slimey attic space, the first thought is how the building is orientated. Then I would progress thru the other factors. It is my experience that ridge vent can work, but don't bet the farm on them. If a person thinks thru attic ventilation carefully, you possibly will form a different opinion abt ventilation.

And NO I will not blow smoke into every attic. Although it is a preferred place to get rid of a little stomach gas from lunch!

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  • 1 month later...

I ran into another duplex laid out in an L and again one attic was clear and the other was black.

Is a mechanical air exchanger type system, which I am not at all familiar with, better or more likely to be more effective then air sealing the ceiling and providing continuous eave and ridge venting?

Is this a problem that insulation contractors are familiar with solving?

Chris, Oregon

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Originally posted by Chris Bernhardt

In the past I might have said increase ventilation for example by installing gable venting. That I think is wrong advice. The only thing I can think of is to increase venting with a continuous roof vent and installing venting in each rafter bay.

Is that sound advice? Or is that not likely to really work either? Usually I will see the mildew lighter or non-existent in the rafter bays with eave venting or only the north side etc. but in this case it appeared to evenly coat everywhere as if no venting is occuring and I could see daylight at the vents of the eaves.

Chris, Oregon

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