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Roof Rafters & Condensation


kitferre
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I built a home with a general contractor in the mountains near Lake Tahoe (elev. = 6500 ft.) last year. The issue I recently had condensation dripping from the peak of the interior ceiling when the temperature (60 + degrees) & the solar gain increase recently. The roof assembly is new to me, for I have previously done scissor trusses with an attic. This one is built with 2 x 14 lvl @ 16" o.c. (i.e. big snow load) The pitch of the roof is 5/12, & runs 28' both sides to sit on a center beam. The soffit has a continuous vent with holes in the blocking, & a ridge with a continuous vent. The construction assembly is asphalt shingles, building paper, 5/8" plywood, 2" of air space, R-38 batt w/kraft paper w/ baffles at each end, 1/2" rock, & 1x8 cedar boards. The builder & myself went up & look at the ridge vent. The roof framing has an engineering detail of nailing the roof sheathing into a center peak block which was notched for air flow. We have decided that we need create bigger notches/holes to increase the air-flow to the ridge vent. A light bulb went off in my head, & my question to this journal is about humidifiers. We install a Honeywell TruSteam humidifiers to the furnaces, & only recently (past 2 months) got it integrated with a byrant thermostat. Our climate is typically dry, so we have run it to 40 + % of R.H. The question I have could this be the source of the condensation?

Thanks for any practical responses in advanced.

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Assuming that it is condensation, I can't figure any mechanism whereby condensation would form at this time of the year barring any breaches in the building envelope in the vicinity of the ridge. Why not try turning the humidifyer off for a while, monitor the relative humidity values and see if the dripping stops? Perhaps the new connections to the thermostat or the thermostat settings are incorrect. There should be a relative humidity indicator installed within the conditioned space to help with diagnostics.

Some of the other guys here are much more experienced than I. Check back later.

Marc

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Why not try turning the humidifyer off for a while, monitor the relative humidity values and see if the dripping stops?

Marc

What Marc said, shut off your humidifier for a while. Most folks really over humidify their homes. The next thing that comes with that type of moisture is mold growth.

Yep, I have seen more problems caused by humidifiers than good. If you really want an efficient way to add humidity to the air get a 20-30 gallon fresh water aquarium. They work great at adding extra moisture to the air of a home! Best of all, they don't grow mold!

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It could easily be the humidifier. Who knows what 40% on the device means; I've found those humidistats to be all over the map.

Get yourself a Fluke 971. Use it to figure out what the real relative humidities and dew points are in the house and roof structure. I have rountinely measured RH in the 60% ranges in some attics and vaulted rafter spaces.

Near as I can tell, all the insulation and ventilation calculations in the IRC are not necessarily accurate depending on a very wide range of particulars in the structure. Sometimes you want less, sometimes more.

New building methods and materials, matched up in ways that aren't necessarily thoroughly thought out by the building industry, can have a startling effect on how structures perform and how vapor and moisture move into and through them.

Read Water in Buildings by William B. Rose, then go here and read about vapor movement in structure.

You have to have some background for understanding moisture in modern structures; it's all pretty much in those two resources.

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You're in a dry climate already. Vapor diffusion is going to push that warmer/moisture air right through the ceiling plane wherever it can. It's pushing through at the ridge because the batting stops on either side of the ridge board. There is an air space where the air passes out through the ridge vent and they didn't do a stellar job sealing the roof plane there. It's cooler there. That warm/moist air pushed through the ceiling plane, hits that cold ridge beam, is cooled to dew point, condenses and then drips from the ridge.

They should have used full-length chutes, foamed the underside with a couple of inches of closed-cell foam, so no air could pass around the chutes at the sides or ends and then installed the batting. Also, instead of the foil faced insulation, they could have used unfaced batting with a layer of foil-faced Blackcore foam (R7 per inch) directly behind the drywall with the seams taped with aluminum tape. You would have ended up with a ceiling near an R50 and noplace for moisture to cool and condense. An overhead paddle fan to keep things circulating up there and thing would be nice and dry.

ONE TEAM - ONE FIGHT!!!

Mike

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