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Community and Q&A

Unvented attic assembly and rafters ghosting through roofing material

SCBNick | Posted in Energy Efficiency and Durability on


I recently completed a 2,500 square foot two story home in climate zone 3C, about ten miles inland on the coast of California.  The climate is very moderate, with few days ever exceeding 85 degrees, or falling below 45 degrees.  It does not rain often here, approximately 12″ a year.

The house was sheathed in zip panels, and all seams were taped to manufacturer’s specifications.  It’s a crawl space foundation, with fiberglass batts between the floor joists, and a 12mil vapor barrier on the floor of the crawl space.  The roof was also paneled with zip product, seams taped, and backed up with GAF tiger paw synthetic roofing felt.

Insulation in the walls is dense packed cellulose (2×6 wall construction), and the roof is unvented, with R-30 open cell icynene spray foam sprayed under the 4:12 pitch roof.

Air conditioning and a forced air furnace are present; however, they rarely run due to the moderate climate and seemingly functional insulation.  During the hottest days of the year, the air conditioner runs for approximately 2 hours.

An interesting phenomenon began directly after occupancy.  The upstairs bathroom ceiling would condensate quickly, a Panasonic 150CFM fan was used with an external humidistat.  No amount of exhausting seemed to curb the condensation on the ceiling.  The fan is located directly outside of the shower area, the bathroom is approximately 10×15.

The fan was checked during HERS testing and passed.  I figured that it must be undersized, so I upgraded it to a Panasonic 380CFM fan.  There is sufficient makeup air via several windows, and the door is not closed.  I have not noticed any rusting, or other signs of moisture damage in the bathroom.  There is no visible fog of moisture during a shower; however, the ceiling seems to condensate rapidly.  Furthermore, the walls begin to drip after an extended period of time.  I have never installed a fan larger than 150cfm.

I installed two hygrometers, one at the bathroom ceiling and one in the attic against the spray foam.  The RH in the attic seems to remain constant at about 55%, which seems high to me.  The bathroom ceiling hovers at the same RH until a shower is taken, where it can rise up to 80%.  The interior temperature of the house is around 70 degrees, and the attic is somewhere around 72-74 degrees.  The RH in the attic does not seem to climb more than a percent or two during a shower.

The ceiling condensation is one issue.  The newest issue I have noticed, is that the rafters are visible from the outside when looking at the roof.  They are ghosting through the triple laminated shingles, there is no physical bump, just visual evidence that some type of moisture or thermal bridging is occurring during the summer.  I’m not sure if this is of concern or not, or if it indicates roof rot or not.

It would seem to me that in California, an RH of 55% is a little high.  I have debated installing a supply duct into the attic area; however, since the HVAC is rarely running, I don’t see how this could help.  I could install a fan into the attic space to exchange air, or I could install a dehumidifier which also seems extreme for this climate zone.

I welcome any and all advice:

How do I solve the ceiling condensation issue?  I have the largest fan with makeup air available, and it doesn’t seem to be helping.  Is the RH prior to a shower too high to begin with (55%)?

Are the ghosting rafters a sign of roof rot and moisture movement due to the spray foam?  I thought I would see a higher RH in the attic (55% all day) if that was the case?  See attached photo.

Thank you,


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what's going on at your house. But I know that when open-cell spray foam is installed on the underside of roof sheathing, the attic below the insulation is often very humid. For more information on this phenomenon, see these two articles:

    "High Humidity in Unvented Conditioned Attics"

    "Open-Cell Spray Foam and Damp Roof Sheathing"

    I advise you to start by installing a dehumidifier in your attic. You may only need to run this dehumidifier for a few months, in order to remove construction moisture.

  2. SCBNick | | #2

    Thank you for the article links, I have read both of them -- they, along with an Energy Vanguard post, are the only information I can find on this topic

    I have a small dehumidifier on order, I will try that and see. I am not certain if 55% RH is too high to begin with.

    The rafters ghosting through the roofing material is also worrying me, that much moisture migration seems catastrophic. In the winter I would understand ghosting through frost in a colder climate, I am not certain why it is happening during the summer in a moderate climate.

  3. Expert Member
    Dana Dorsett | | #3

    Terminology: "condensate " is a noun. The active form is "condensation".

    Without the temperature to which it relative the raw number "55% RH" isn't very meaningful. A body of 65F air at 55% RH will have an RH of 39% if heated to 75F or 28% if heated to 85F.

  4. SCBNick | | #4

    Hi Dana,

    I should have been more clear:

    The temperature indoors ranges from 70-72 degrees, I’ve been logging it for a week now. The attic temperature is 72-74 degrees.

    The RH averages 55% in the attic, with a 1% increase during a shower. The temperature remains 72-74 degrees depending on time of day.

    The RH averages 63% at the bathroom ceiling, with spikes up to 90% during a shower. The temperature at the bathroom ceiling averages 71 degrees with a 2 degree increase during a shower.

    Does that help provide any information? I am at a loss. The RH recovery post shower to 63% takes approximately two hours

  5. Expert Member
    Dana Dorsett | | #5

    The dew point temp is a measure of the absolute humidity rather than relative humidity.

    Attic air at 55%RH @ 73F has a dew point of 56F. Move the same body of air into a bathroom at 71F the RH would be 59%.

    So if the bathroom ceiling is 63%RH @ 71F (dew point = ~58F) it has substantially more moisture than the attic air. It's not surprising that the humidity spikes during showers and takes time to recover, but the fact that it never drops below 63% even with substantial active ventilation may indicate an ongoing humidity source, which could be stored moisture in the construction materials if it's a new, tight home. But that source is NOT the attic, which is consistently drier than the bathroom.

    Ghosting of the rafters in the morning is common and not of much concern. Overnight the radiational cooling of the roof drops the surface temperature to below the outdoor temperature. The thermal bridging of the rafters keeps those stripes of roof at a warmer temperature than the R30 part of the roof. When the roof temp reaches the dew point of the outdoor air (typically in the 40s or low 50s in CA) dew forms on the colder part over the insulation, but not (or not as heavily) over the rafters. Those effects would not persist over the course of the day, but can be very pronounced in the morning after a clear night.

    An interior RH of 55% @ 73F is not a health problem for either you or the house. An interior RH of 63% @71F is a bit above the optimal health range for humans but not into the hazardous range, and not a problem for the building. The sweet spot for human health is 30-50%. Above 50% dust mites are able to reproduce, and those allergic to dust mites can have symptoms. Above 60% fungal infections of skin & nails become more prevalent, but not a high risk until it's chronically well north of 70%.

  6. SCBNick | | #6

    Thank you so much for the detailed reply. It is 8 months old, and perhaps the moisture is due to the construction materials. I will have the fan turn on more often automatically for a few weeks and see what happens.

    Regarding the rafters — that picture I sent you was taken at 5pm, not in the morning. Would the same theory prove true? At 11am, with no direct sunlight, the lines are not visible.

  7. Expert Member
    Dana Dorsett | | #7

    During the day direct sun heats the whole roof deck fairly evenly to ABOVE the ambient air temperature. Several hours after the roof goes into shade that changes- in an air conditioned house the rafters might be below the outdoor ambient temperature, but not by very much.

    It might be worth testing the roof deck's moisture content in a few places to see if it's different at the rafter striping than mid-way between rafters, and whether it's elevated enough to be of concern. A site visit might turn up other clues as to what's going on with the visually apparent striping.

  8. Expert Member
    ARMANDO COBO | | #8

    You said: "It’s a crawl space foundation, with fiberglass batts between the floor joists, and a 12mil vapor barrier on the floor of the crawl space". Is this a ventilated crawl space? with batts under the floor?

  9. Expert Member
    Dana Dorsett | | #9

    Vented crawlspaces with batts between the joists are fairly common in CA, where outdoor dew point average are low enough that it's really never a problem even in air conditioned houses (unlike the Gulf Coast states, or the eastern seaboard.)

    R408.1 of the California Residential Code calls out crawlspace venting as a requirement:

    "The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement) shall have ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall be not less than 1 square foot (0.0929 m2) for each 150 square feet (14 m2) of under-floor space area, unless the ground surface is covered by a Class 1 vapor retarder material. Where a Class 1 vapor retarder material is used, the minimum net area of ventilation openings shall be not less than 1 square foot (0.0929 m2) for each 1,500 square feet (140 m2) of under-floor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building."

    1. Expert Member
      ARMANDO COBO | | #10

      I'm concerned with the batts insulation installed in a ventilated crawl space, not the ventilated crawl space by itself. I'm designing a Builder's own house in the same area (CA CZ3C north of SF) which is marine, and they get humid air in the afternoons (think of the humid fog SF gets avery afternoon and evening). We are designing his house as a humid climate job. I would use a layer of 1' min taped rigid foam attached to the bottom of the floor joists.
      Just FYI, I've seen plenty of nasty, rotted and humid vented crawl spaces in New Mexico and Arizona, which is much dryer than Mid-Cal.

      1. Expert Member
        Dana Dorsett | | #11

        A continuous layer of taped 1" (or thicker) rigid foam is definitely a superior, less-risky build for vented crawlspaces in almost any climate.

        I'm sure there are exceptions to the rule, but with a ground vapor barrier the track record is still pretty good for vented crawlspaces in zone 3C, despite the foggy-dew days. On the foggy days the temps in the crawlspaces still tend to be warmer than the outdoors, which helps. The monsoon season in AZ/NM often sees super high humidity concurrent with high temperatures, higher than the temps in crawlspaces which increases the risk, even if the annual rainfall amounts are much lower.

        The first time I ever visited Tucson in July it was 107F outside and raining when the plane landed. As luck would have it my change of clothes got sent to Mexico City for the weekend by the airlines' baggage mis-handling system. :-) That kind of weather isn't very common in zone 3C, but happens every year at some point in much of AZ.

        I'm not suggesting that the CA code prescriptives are rational on this, but it's not as risky as in other locations where crawlspace venting is often still required by code and completely IRRATIONAL, causing more problems than it solves.

  10. SCBNick | | #12

    Ventilated crawl spaces have two different requirements in California -- if there is no vapor barrier, the ventilation requirement is much higher. With the vapor barrier, the ventilation requirement is much less. The footprint of the subfloor is approximately 1200 square feet, this particular house required 6 vents in the crawl space, whereas if it had no vapor barrier, there would be much more. I agree that rigid, taped foam would be much nicer; however, we stub all of our plumbing out into the crawl space for obvious reasons. In fact, it's so rare to use a vapor barrier in our area that I had to look up quite a few details to even pull it off, it's a new thing in this area. San Francisco is more coastal than where I am, I do see the advantage there. Radon is not as much of a concern here either.

    I inspected the roof this morning. It appears that the shadow lines of the rafters are actually stains on the roofing material. I'm not sure if it's fading differently due to the thermal bridging causing different heat levels, but it does not appear to be moisture migration.

    I re-inspected the bathroom fan, the roof cap damper is functional, and there doesn't appear to be any impedance. I reset the humidistat to automatically cycle every 45 minutes for 20 minutes on at a time, and installed a temporary dehumidifier.

    I have attached photos of the roof, to me it looks like staining rather than any kind of moisture migration.

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