+ Reply to Thread
Results 1 to 5 of 5

Thread: ice in attic

  1. #1
    Join Date
    Dec 2008
    Posts
    2

    Default ice in attic

    i finished ventilation sistem in attic. i install in each rafter provent vents.also i put the second insulation layer cross the excisting.now is about 13". iwas in the attic after the first snow ,all the nails were iced and some part of the roof. is this normal?
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Picture 005.jpg 
Views:	270 
Size:	10.6 KB 
ID:	1898   Click image for larger version. 

Name:	Picture 002.jpg 
Views:	506 
Size:	16.2 KB 
ID:	1899   Click image for larger version. 

Name:	Picture 003.jpg 
Views:	268 
Size:	11.5 KB 
ID:	1900  

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: ice in attic

    is this normal?
    Pretty much .... yes.

    What you see is whatever moisture ( humidity) that exists in the attic space being attracted to the coldest point ...... mainly the roofing nails that are piercing the sheathing.
    This moisture is basically the relative humidity ( RH ) that exists in the attic space.

    When the temperature drops to freezing ... or below ..... this moisture freezes forming the ice crystals you are seeing.

    So where does this humidity come from ?

    A common source is from the living area in the home. The mechanisms that contribute to vapor passing from the living space to the attic ..... air leakage and vapor drive ( vapor pressure differential ).

    Air leakage is simple .... penetrations that allow moisture laden air from the living space to freely enter the cold attic.

    Unfortunately more consideration is given to adding insulation .... which is beneficial to prevent heat transfer .... but less is considered to air sealing. The penetrations from things like electrical , plumbing and gaps in framing are rarely properly addressed.
    For example ..... adding fiberglass insulation has the benefit of preventing heat transfer but has little resistance as an air barrier.

    In other words .... unless the penetrations are properly sealed the fiber glass will allow the air from the living space to freely pass through this material into the attic.

    Vapor drive from pressure differential occurs because of the higher pressure within the living space will migrate by diffusion through materials like drywall to the lower pressure in the colder attic and forming condensation..... the law of nature.

    This is the role of vapor retarders or vapor barriers ..... to retard the vapor diffusion.

    However .........

    Humidity in the attic is also naturally existing.
    Having the attic vented to the outdoors allows humidity form outside to enter this space. As the seasons change this humidity may not have a chance to balance if temperature swings rapidly.

    So , if the fall season may have been humid and the temperature drops this doesn't allow the moisture level to decrease.
    Having the vents open to the outside allows cold air which doesn't absorb moisture as well as warm air ..... not allowing that humidity in the attic to be removed as quickly. The remaining moisture will freeze as the temperatures drop.

    The short of the long .... if you are basically seeing frost on the nails .... it shouldn't be too much of a concern.
    If you see frost covering the underside of the sheathing you will have to investigate where the source of the vapor is coming from.

    To be honest .... I haven't seen an attic yet without frost on the nails during the winter.
    Considering up here right now it's 15 below zero f ( - 43 with the wind ) I'm sure all the nails up in the attic are covered with frost .... I ain't going up there to check.



    Hopefully this makes sense and helps.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  3. #3
    Join Date
    May 2008
    Location
    Pacific Northwet
    Posts
    1,661

    Default Re: ice in attic

    Warning: don't put a vapor barrier on TOP of the insulation. Moisture will condense on the insulation side of the barrier and soak the insulation defeating its value and potentially damaging the ceiling below.

  4. #4
    Join Date
    Dec 2008
    Posts
    5

    Smile Re: ice in attic

    Quote Originally Posted by canuk View Post
    Pretty much .... yes.

    What you see is whatever moisture ( humidity) that exists in the attic space being attracted to the coldest point ...... mainly the roofing nails that are piercing the sheathing.
    This moisture is basically the relative humidity ( RH ) that exists in the attic space.

    When the temperature drops to freezing ... or below ..... this moisture freezes forming the ice crystals you are seeing.

    So where does this humidity come from ?

    A common source is from the living area in the home. The mechanisms that contribute to vapor passing from the living space to the attic ..... air leakage and vapor drive ( vapor pressure differential ).

    Air leakage is simple .... penetrations that allow moisture laden air from the living space to freely enter the cold attic.

    Unfortunately more consideration is given to adding insulation .... which is beneficial to prevent heat transfer .... but less is considered to air sealing. The penetrations from things like electrical , plumbing and gaps in framing are rarely properly addressed.
    For example ..... adding fiberglass insulation has the benefit of preventing heat transfer but has little resistance as an air barrier.

    In other words .... unless the penetrations are properly sealed the fiber glass will allow the air from the living space to freely pass through this material into the attic.

    Vapor drive from pressure differential occurs because of the higher pressure within the living space will migrate by diffusion through materials like drywall to the lower pressure in the colder attic and forming condensation..... the law of nature.

    This is the role of vapor retarders or vapor barriers ..... to retard the vapor diffusion.

    However .........

    Humidity in the attic is also naturally existing.
    Having the attic vented to the outdoors allows humidity form outside to enter this space. As the seasons change this humidity may not have a chance to balance if temperature swings rapidly.

    So , if the fall season may have been humid and the temperature drops this doesn't allow the moisture level to decrease.
    Having the vents open to the outside allows cold air which doesn't absorb moisture as well as warm air ..... not allowing that humidity in the attic to be removed as quickly. The remaining moisture will freeze as the temperatures drop.

    The short of the long .... if you are basically seeing frost on the nails .... it shouldn't be too much of a concern.
    If you see frost covering the underside of the sheathing you will have to investigate where the source of the vapor is coming from.

    To be honest .... I haven't seen an attic yet without frost on the nails during the winter.
    Considering up here right now it's 15 below zero f ( - 43 with the wind ) I'm sure all the nails up in the attic are covered with frost .... I ain't going up there to check.



    Hopefully this makes sense and helps.
    I would like to add insulation to my attic. Live in Indianapolis, IN. Hot humid summers, pretty cold winters. Some background: Last year, I had ridge vents cut in to my rooflines, with the soffit vents, there is plenty of airflow through this space, BUT, I am wondering if I invited my home to be colder in the winter. So, my thought was to ADD BATT insulation over the top of the existing blown in insulation. Here are my concerns:

    1.) Changing the attic environment, causing a negative effect (condensation, mold, etc..,).
    2.) Properly addressing the whole "air pressure issue". What should I do to address the areas where ceiling fans, light fixtures etc..., penetrate the ceiling?
    3.)The "panels" that fit between the roof joists, what are those for? Where do those get installed? Are they necessary?
    4.) Were the ridge vents a bad idea? I always was told that adding those will help keep the attic from over heating and reducing the life of the shingles.
    5.) Any other insulation tips? Will this really help? My home is drafty, and uncomfortable, will this help?

    Thanks for any input

  5. #5
    Join Date
    Aug 2007
    Location
    Maryland
    Posts
    1,694

    Default Re: ice in attic

    Quote Originally Posted by ScottInIndy View Post
    3.)The "panels" that fit between the roof joists, what are those for? Where do those get installed? Are they necessary?
    4.) Were the ridge vents a bad idea? I always was told that adding those will help keep the attic from over heating and reducing the life of the shingles.

    Thanks for any input
    The panels between the rafters are insulation baffles that keep the insulation from blocking off the airflow from the vented soffit. This is essential for the ridge vent to operate properly.
    Ridge vents are a good idea and should make the shingles last longer and keep the attic cooler in the summer. And also help to dissipate the moisture you are seeing
    More insulation would probably help up to a point.
    If your house is drafty you might investigate for air leaks around window and doors.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •