Page 1 of 2 12 LastLast
Results 1 to 13 of 15
  1. #1
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125

    typical hi RH-winter, here

    at 6am, dry bulb= 23F, wet= 21F = ~95%RH
    so,
    sealing attics & crawlspaces prevents sucking in moist air;
    probably best to forget vapor barrier --

    this hi RH will typically be until 10am --

    so,
    don't make firm statements that one should draw in outside air;
    unless
    it is conditioned --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    It's 95% RH... the key term is RH. That amount of moisture at the indoor temp of 70F, is only around 25%RH if I read my Psychrometric chart right.

    A <25F dewpoint is still pretty dry.

    Maybe I'm understanding your statement wrong. As a pro, I'd think you'd know this already. The saturation point of air it terms of how much moisture it will hold decreases as temeprature decreases. So saturated air at 20F, hold 1/4 the amount of moisture as air at 70F.

    Ideal conditions of 40-45% RH at 70F is a dewpoint of around 45-50F. So to better understand this, if the home is an idea 45% RH at 70F, then any indoor surface that is <50F will have moisture on it. This is why windows have moisture at low outdoor tempratures.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,706
    I could be wrong.

    But, at 23F DB and 21F WB the RH is 75.1%, 13.0588 grains per pound of air.

    At 70F DB and the same 13.0588 grains of moisture, it would be 12%RH.

    According to my Carrier chart.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,283
    Quote Originally Posted by beenthere View Post
    I could be wrong.

    But, at 23F DB and 21F WB the RH is 75.1%, 13.0588 grains per pound of air.

    At 70F DB and the same 13.0588 grains of moisture, it would be 12%RH.

    According to my Carrier chart.
    You are correct on both counts.

    cems-bee wrote:
    so,
    sealing attics & crawlspaces prevents sucking in moist air;
    probably best to forget vapor barrier --
    this hi RH will typically be until 10am --
    so,
    don't make firm statements that one should draw in outside air;
    unless
    it is conditioned --
    Beenthere (along with motoguy) is correct again, in that drawing in air under the conditions you gave is still dry air, and will lower indoor RH levels if not humidified (or if indoor moisture generation is low).

    Any vapor barrier, to my understanding, should be on the drainage plane of a wall (behind the siding or cladding, in front of the sheathing), not on the interior. Housewraps are air and liquid moisture barriers, but allow vapor diffusion (allowing a wall cavity to dry to the exterior as well as to the interior).
    • 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
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post
    You are correct on both counts.

    Any vapor barrier, to my understanding, should be on the drainage plane of a wall (behind the siding or cladding, in front of the sheathing), not on the interior. Housewraps are air and liquid moisture barriers, but allow vapor diffusion (allowing a wall cavity to dry to the exterior as well as to the interior).
    How about fiberglass insulation w/paper vapor barrier, applied to the sill (not sure if this is the correct term, but is first wood above grade on top of cinder block).

    The barrier is on the inside face of the insulation.

    Thanks.

    Amp

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,283
    Quote Originally Posted by ampulman View Post
    How about fiberglass insulation w/paper vapor barrier, applied to the sill (not sure if this is the correct term, but is first wood above grade on top of cinder block).

    The barrier is on the inside face of the insulation.

    Thanks.

    Amp
    Do you mean something like this, where a framed stem wall sits on top of the basement wall or crawl space wall? :



    In the drawing, notice the lack of a vapor barrier to the interior, and why. The interior plane shows drywall painted with latex, to allow drying of the wall cavity via vapor diffusion to the interior.

    If you're speaking about a different configuration (such as an above grade block house) please specify where this is found in the 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.

  7. #7
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post
    Do you mean something like this, where a framed stem wall sits on top of the basement wall or crawl space wall? :

    Yes, this is a basement w/10 ft cinder block walls. The cinder block extends about 1.5 feet above grade. There is an attached crawl space which has the same treatment.

    In the drawing, notice the lack of a vapor barrier to the interior, and why. The interior plane shows drywall painted with latex, to allow drying of the wall cavity via vapor diffusion to the interior.
    The basement is unfinished; hence no interior walls.

    Would I assume correctly, that my 'configuration' doesn't pose any problem?

    Thanks.

    Amp

  8. #8
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,283
    Quote Originally Posted by ampulman View Post
    Yes, this is a basement w/10 ft cinder block walls. The cinder block extends about 1.5 feet above grade. There is an attached crawl space which has the same treatment.



    The basement is unfinished; hence no interior walls.

    Would I assume correctly, that my 'configuration' doesn't pose any problem?

    Thanks.

    Amp
    The fiberglass batts...are they attached to the basement wall, or the crawl space wall? Is the basement wall exposed block, or does it have a framed wall in front of the block to contain insulation? The crawl space...is it ventilated? Is the floor over the crawl space insulated?
    • 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.

  9. #9
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by beenthere View Post
    I could be wrong.

    But, at 23F DB and 21F WB the RH is 75.1%, 13.0588 grains per pound of air.

    At 70F DB and the same 13.0588 grains of moisture, it would be 12%RH.

    According to my Carrier chart.
    That's probably right. I just "eyeballed" a chart I found on google that wasn't specific to HVAC. The Carrier chart is probably a little more precise (easier to read). I had to convert from celcius as well.

  10. #10
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by shophound View Post
    The fiberglass batts...are they attached to the basement wall, or the crawl space wall? Is the basement wall exposed block, or does it have a framed wall in front of the block to contain insulation? The crawl space...is it ventilated? Is the floor over the crawl space insulated?
    The basement wall (cinder block) is below grade, with the exception of about 1.5 ft. On the top block (above grade), there is an approximately 2 x 5" board laid flat, on which 2 x 8 joists are placed. At the top of joists is the first level sub-floor. This (8") is the space which contains the (kraft paper inward) faced fiberglass. This is all open to the basement.

    The crawl space is basically the same, except that the joist level is split (lowered) from the main (first floor) about 8". The crawl space is under a section of house which has no second floor, and contains heating ducts, pvc furnace vent pipes, and plumbing.

    There is fiberglass (about 1") board on the walls; some thick material on the floor. The space is open to the basement and is not vented to the outside. It is on the cool side.

    Thanks.

    Amp

  11. #11
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    read about vapor barriers at BUILDINGSCIENCE.com

    so, I did not carefully note my dry bulb, I focused on the wet bulb & RH%.

    nevertheless, every thing I have heard or read from the health 'experts' is that RH% should be kept between 35 & 55%.
    No body stated that it was dependent upon temperature.
    so, maybe my bdrm @ 57F does not have much moisture per cubic ft, but it is still on the high side [60+ percent], especially during the early morning --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,706
    If you know WB and RH, you can find/determine DB.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  13. #13
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    why look-up or calculate when I can just read my instrument?
    I suspect that I did my first look-up in 1961 --

    again, what is your 'take' on RH% needed ID for good health, even at 57F?
    at 67F?
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event