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  1. #1
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    Insulation acting like a dessicant?

    I think I know the answer to this question. But I would liek to get some input.

    Mow that hte weather has cooled off quite a bit and humidty levels are more moderate, I'm noticing that my indoor humidity levels upstairs have not changed much. My dewpoint mosture levels indoors are often higher than the outdoors. thsi was not hte case when it was hotter outside.

    When I mved in, I had the underside of my roof deck spray foamed wiht 5-7" of open cell foam. I did not remove the existing cellulose at the time (I now realize I shold have removed ALL of it first). and as such they did the best job they could but I think it didn't get completely sealed at the rim joist where the hip roof meets the attic floor on the perimeter.

    My theory is that all the loose fill cellulose insulation (That I probably need to remove form my attic) is absorbing moisture in the afternoon when reverse stack effect is most prevelent, then slowly releasing it in the evening.

    I wonder is removing it would make a noticeable impact. I'm also concerned that the opposite will occur in winter. War moist air form inside hte house will exfiltrate out the same leaks and as those surfaces get cold, moisture will form.

    Of course before I insualted the roof, the same issues would have been occuring.

    If nothing else, the stuff is really nasty and would make doing any work much, much easier up there and would make it a more useable space for storage if desired.


    Any thoughts?

  2. #2
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    A desiccant aDsorbs moisture. The cellulose insulation aBsorbs moisture. May seem trivial but there is a difference.

    If you can get the air gap between the exterior wall top plates and the roof deck completely air tight, at that point you can consider removing the cellulose. Getting the attic sealed against exterior atmospheric conditions pretty much nips stack effect and reverse stack effect in the bud. It also makes the attic conditioned space, meaning it should reside at or near what conditions you maintain in the house.

    Moisture migration from the house into the attic in winter is only a concern if any surface in the attic develops a temperature lower than the average dew point temperature your house maintains in winter. If your attic roof deck was foamed properly (rafter/truss members also covered and not exposed - to leave them bare exposes them to thermal bridging and possible condensation problems) AND is sealed I would not expect moisture levels in your attic to be significantly different than what's in the house. Regarding moisture levels I would expect the same to hold true in summer.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
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    The goal was a sealed attic that would be semi-conditioned space. I think not removing the cellulose first, made sealing the top plate near impossible. It's a low pitch hip roof and they used a small rake to try and pull the cellulose away form the wall, but spraying the foam was very challenging. It took them at least 3X longer than they expected.

    I think this fall I may rent a machine to vacuum out the cellulose, then have the sprayer come back in to get the perimeter sealed-up.

  4. #4
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    Quote Originally Posted by motoguy128 View Post
    I think this fall I may rent a machine to vacuum out the cellulose, then have the sprayer come back in to get the perimeter sealed-up.
    Sounds like a plan.

    Ain't it a drag that hindsight is 20/20?

    Look at it this way; you're getting great field training in building science. It's a discipline that by itself is only starting to be widely understood.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
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    I've always thought that if I get tired of my current profession, I'd go into doing energy audits and home inspections. I'd love to hve an infrared camera and see what my actual insulation values are and what behind my walls.

  6. #6
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    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by motoguy128 View Post
    I think I know the answer to this question. But I would liek to get some input.

    Mow that hte weather has cooled off quite a bit and humidty levels are more moderate, I'm noticing that my indoor humidity levels upstairs have not changed much. My dewpoint mosture levels indoors are often higher than the outdoors. thsi was not hte case when it was hotter outside.

    When I mved in, I had the underside of my roof deck spray foamed wiht 5-7" of open cell foam. I did not remove the existing cellulose at the time (I now realize I shold have removed ALL of it first). and as such they did the best job they could but I think it didn't get completely sealed at the rim joist where the hip roof meets the attic floor on the perimeter.

    My theory is that all the loose fill cellulose insulation (That I probably need to remove form my attic) is absorbing moisture in the afternoon when reverse stack effect is most prevelent, then slowly releasing it in the evening.

    I wonder is removing it would make a noticeable impact. I'm also concerned that the opposite will occur in winter. War moist air form inside hte house will exfiltrate out the same leaks and as those surfaces get cold, moisture will form.

    Of course before I insualted the roof, the same issues would have been occuring.

    If nothing else, the stuff is really nasty and would make doing any work much, much easier up there and would make it a more useable space for storage if desired.


    Any thoughts?
    If you talking dew point comparisons outside verses inside, I would expect inside to be higher than the outside with low/no cooling loads.
    Expect that the inside dew point to be the outside dew point plus the moisture added by the occupants/activities minus the amount of moisture removed by the a/c or dehumidifier.
    Here is an example.
    1. Assume that the outside dew point is 65^F.
    2. Assume you have a infiltration or ventilation rate of 100 cfm.
    3. Assume you maintain 75^F, 50%RH, which is a 55^F dew point.
    4. Assume a family of 4 that adds 2 lbs. of moisture per hour.

    The moisture load is 2 lbs./hr. from the fresh air and 2 lbs./hr. from the occupants. As long as the a/c or dehumidifier removes the 4 lbs. per hour and the home is 75^F, 50%RH is maintained.

    If the it was rainy and cool like 70^F, 84%RH, outside with a 65^F dew point and the home was 75^F with the same moisture loads including 100 cfm infiltration but without any moisture removed, the indoor humidity will rise slowly to +80% as the indoor materials absorb moisture. The indoor dew point will slowly rise to 73^F. Slowing the infiltration will increase the indoor dew point. Increasing the infiltration/ventilation rate reduces the indoor dew point. Opening all windows and door will reduce the indoor dew point to slightly above outdoor dew point.
    All materials in the home absorb moisture or dry to air based on the %RH of the surrounding of the air and water content of the material.
    Every evening and wet cool days are similar regarding dew point.
    Reduce the fresh air flow to the minimum needed to purge indoor pollutants and renew oxygen at a minimum when occupied.
    If you use the above example, ventilation with outside dew points <50^F will keep the home dry. Higher dew points will need some inside moisture removal from the a/c or dehumidifier. Typically, during with 80^F outside dew points and occupants, 6-8 lbs. hour of moisture is removed by the a/c to maintain <50%RH.
    As you grasp this concept, a cold coil a/c and whole house dehumidifier will make sense to maintain <50%RH throughout the seasons in green grass climates.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  7. #7
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    Quote Originally Posted by motoguy128 View Post
    I'd love to hve an infrared camera and see what my actual insulation values are and what behind my walls.
    You'd likely be surprised. You'd also quickly learn about thermal bridging and thermal bypasses. Both exist in higher quantities in most houses than is commonly realized.

    I don't have an IR camera, but I have developed a sense over time where a building's thermal boundaries are weak. That comes from understanding how a building performs in all weather, which takes a good bit of study and casting out of preconceived notions. Can be humbling at times.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Teddy,

    The reason I point towards the attic insulation, is that the downstairs has only shown a small increase in humidity levels. An increase that I'd associate with shorter run tiems of the equipment.

    The humidity levels remain high even when there is no activity in the home, and when reverse stack effect would be minimal (late evening & morning). I'd actually expect the natural convection of cool air falling and warmer downstairs air raising to mix hte upstairs and downstair air somewhat. But I'm not seeing that. The downstairs dewpoint is always 50-55F. The upstairs swings from 51-58F. It swings within the course of just 1-2 hours as the equipment cycles on and off. Downstairs slowly goes down in the afternoon and rises in the evening when the unit is shut down.

    It hasn't been rainy or terribly cool. Dewpoints overnight have dropped to 54F the last 2 nights. Actually we'd have <1/4" of rain in 7 weeks. Its' the most humid in the late afternoon when nobody has been in the house.

    I'm just trying to get a grasp on the volatility of the humidity cycles upstairs and find a root case. Reverse stack effect alone doesn't seem to explain it. Normal loads from showers and humans doesn't explain it either. We have a fairly powerful bath fan (>100CFM with 2 pick-up points).


    Come to think of it. I do need to track down where my drain goes for my attic furance/ac. I'm guessing they simply tied it to one of the cast iron drain vent pipes in the attic (probably without a trap). No trap was probably intentional so warm air form the furnace keep the condensate from freezing (before I insualted the roof deck). I'm not aying that's good practice.... it's actually why condensing furnaces are normally not installed in attics.

  9. #9
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    Aug 2003
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    Go buy some cheap temperature/humidity sensors from Radio Shack that broadcast a signal wirelessly. Set one up in your attic. Another one upstairs. Another one downstairs. One more outdoors.

    Monitor them over the course of a week. Convert temp/RH to dew point for all stations monitored. Compare the results and you'll get a better picture of the moisture flow in through your house.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,062
    Quote Originally Posted by motoguy128 View Post
    Teddy,

    The reason I point towards the attic insulation, is that the downstairs has only shown a small increase in humidity levels. An increase that I'd associate with shorter run tiems of the equipment.

    The humidity levels remain high even when there is no activity in the home, and when reverse stack effect would be minimal (late evening & morning). I'd actually expect the natural convection of cool air falling and warmer downstairs air raising to mix hte upstairs and downstair air somewhat. But I'm not seeing that. The downstairs dewpoint is always 50-55F. The upstairs swings from 51-58F. It swings within the course of just 1-2 hours as the equipment cycles on and off. Downstairs slowly goes down in the afternoon and rises in the evening when the unit is shut down.

    It hasn't been rainy or terribly cool. Dewpoints overnight have dropped to 54F the last 2 nights. Actually we'd have <1/4" of rain in 7 weeks. Its' the most humid in the late afternoon when nobody has been in the house.

    I'm just trying to get a grasp on the volatility of the humidity cycles upstairs and find a root case. Reverse stack effect alone doesn't seem to explain it. Normal loads from showers and humans doesn't explain it either. We have a fairly powerful bath fan (>100CFM with 2 pick-up points).


    Come to think of it. I do need to track down where my drain goes for my attic furance/ac. I'm guessing they simply tied it to one of the cast iron drain vent pipes in the attic (probably without a trap). No trap was probably intentional so warm air form the furnace keep the condensate from freezing (before I insualted the roof deck). I'm not aying that's good practice.... it's actually why condensing furnaces are normally not installed in attics.
    All of your dew points are similar. Exhausting air by bath fan and clothes drier suck in outside air. Reverse stack is a small force compared to minor wind. All inside materials will absorb and dry to the inside air depending on %RH.
    You have a fairly stable inside condition. You see the varible %RH depending on the temperatures on the various levels of your home.
    .25 lbs. of moisture per adult from respiration/presperation per hour plus activities.
    Basements may contribute depending on water table and quality of masonary interface with earth. Walmart/Radio Shack remote monitoring of %RH/dew point is a good ideal. Get an extra couple of remotes for outside and basement. Fresh air change (.2 ach) when occupied would also be recommended. 50-55%RH is good.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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