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  1. #1
    Join Date
    Dec 2010
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    Default Roof Ventilation Issues?

    Could you tell me if this situation is normal?

    We have a 1919 brick bungalow in Chicago and have lived here for a year and a half. It's our first home. It has the original cement tile roof which is in fair shape, though WAY past service life. A roof inspector said the first three courses have basically no felt left under them, but higher than that it looks ok. There are two roof vents, one on the north and one on the south faces of the roof.

    In the winter, when it snows with a blowing wind, snow comes in through the roof vents. I know because I work in the attic, which is mostly finished and there were vents cut into the ceiling of the finished space right below where the roof vents are. Hence, snow in my office. Also, on at least one occasion, there was a fine layer of snow all over the attic insulation as far as we could see.

    Now in the summer, I'm dying because the attic reeks of mold when it gets above 85. I suspect the insulation all has to go. But my main question is: should snow be coming in through the attic vents at all? Is replacing the attic insulation at this point throwing away money until we replace the roof? Any insight you might have will be a lifeline.

    Thank you!

    Naomi

  2. #2
    Join Date
    Jun 2011
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    208

    Default Re: Roof Ventilation Issues?

    Naomi:

    No, a bit of snow getting into your attic through a roof vent isn't anything to be concerned about. Normally, when snow lands on a roof or on the ground, it quickly recrystalizes and becomes part of the snow cover already on the ground or on the roof, so that the wind doesn't easily blow it away. But, if it's a really cold night with a strong wind, then I can see snow being blown up into your roof vents and getting inside your attic.

    The time when you saw "snow" all over the attic insulation was probably a situation where hoar frost occured. If you have warm moist air displacing cold dry air, what happens is that the warm moist air will form frost on everything it comes into contact with that's still cold. So, the warm moist air could have filled your attic and formed frost all over the still cold roof and insulation. The same thing happens outdoors to form "hoar" frost on trees: (I see it frequently where I live.)




    Welcome to Attic Ventilation 101:

    Regardless of how well they build a house, warm moist air from the insulated living space will always find it's way up into the cold attic in winter time. Once in the attic, the moisture from that air will condense (or sublimate to form frost) on the coldest surfaces in the attic, which is normally the underside of the roof.

    The problem is that when this frost melts in the spring, it can get the insulation between the ceiling joists wet, and that can cause the wooden ceiling joists to rot in places.

    The whole idea behind roof ventilation is to allow cold dry outdoor air into the attic. That cold dry outdoor air will warm up because of heat loss from the house and as it does so it's relative humidity will go way down and it'll absorb any moisture present in the attic. That wetter warmer air will then be blown out of the attic by a wind (or whatever) and be replaced with cold dry outdoor air again. In this way, attic ventilation serves to keep the attic dry, and dry wood in your attic is happy wood cuz it knows that as long as it's dry, it'll never rot.

    So, the easiest and most effective way of seeing if you have sufficient attic ventilation is to simply go up into your attic on the coldest day of the year and look for condensation or frost forming up there. It'll form on any uninsulated areas on the underside of your roof.

    If that frost or condensation disappears within a day or two on it's own, there's no real call for alarm. However, if it persists for weeks on end, it'd be better to increase the amount of ventilation in the attic to eliminate that frost sooner and thereby prevent it from causing any problems.

    For a much more elaborate discussion of roof ventilation, read this thread (which starts out talking about drywall nail pops, and then moves on to attic ventilation):

    https://advice.thisoldhouse.com/showthread.php?t=118086

  3. #3
    Join Date
    Aug 2007
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    The Great White North
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    Default Re: Roof Ventilation Issues?

    Quote Originally Posted by nmillan View Post
    Could you tell me if this situation is normal?

    We have a 1919 brick bungalow in Chicago and have lived here for a year and a half. It's our first home. It has the original cement tile roof which is in fair shape, though WAY past service life. A roof inspector said the first three courses have basically no felt left under them, but higher than that it looks ok. There are two roof vents, one on the north and one on the south faces of the roof.

    In the winter, when it snows with a blowing wind, snow comes in through the roof vents. I know because I work in the attic, which is mostly finished and there were vents cut into the ceiling of the finished space right below where the roof vents are. Hence, snow in my office. Also, on at least one occasion, there was a fine layer of snow all over the attic insulation as far as we could see.

    Now in the summer, I'm dying because the attic reeks of mold when it gets above 85. I suspect the insulation all has to go. But my main question is: should snow be coming in through the attic vents at all? Is replacing the attic insulation at this point throwing away money until we replace the roof? Any insight you might have will be a lifeline.

    Thank you!

    Naomi
    Naomi -- There really shouldn't be any snow coming in through the roof vents --- providing they are proper vents.
    Sounds to me your issue is because of the 2 vents on opposing sides of the roof. Because of pressure differential one vent is acting as an intake drawing in the snow --- this is more the case if soffit venting is lacking or insufficent to be the intake source.
    In your case the roof vents should be on the same side and oppsite to the side of the prevailing winds.
    Ridge venting combined with soffit vents--- eliminating the roof top vents altogether ---- would the ideal setup

    As for roof venting removing moisture out the attic in the cold winter is really not correct. The temperature if at or below freezing will not remove much moisture. The main purpose of venting an attic during cold winter months is to keep the underside of the roof cold -- not allowing heat escape from the living space to melt snow on the roof causing ice damns.

    The moisture issues you are describing could be from a number of causes -- roof leaking -- too much warm moist air from living sppace entering the attic space --- for examples.

    If you are planning to redo the roof shortly then have the venting corrected then.
    You should also seal any avenues of air leakage from the living space into the attic space.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  4. #4
    Join Date
    Aug 2007
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    Default Re: Roof Ventilation Issues?

    Quote Originally Posted by Nestor View Post
    Naomi:

    No, a bit of snow getting into your attic through a roof vent isn't anything to be concerned about. Normally, when snow lands on a roof or on the ground, it quickly recrystalizes and becomes part of the snow cover already on the ground or on the roof, so that the wind doesn't easily blow it away. But, if it's a really cold night with a strong wind, then I can see snow being blown up into your roof vents and getting inside your attic.

    The time when you saw "snow" all over the attic insulation was probably a situation where hoar frost occured. If you have warm moist air displacing cold dry air, what happens is that the warm moist air will form frost on everything it comes into contact with that's still cold. So, the warm moist air could have filled your attic and formed frost all over the still cold roof and insulation. The same thing happens outdoors to form "hoar" frost on trees: (I see it frequently where I live.)




    Welcome to Attic Ventilation 101:

    Regardless of how well they build a house, warm moist air from the insulated living space will always find it's way up into the cold attic in winter time. Once in the attic, the moisture from that air will condense (or sublimate to form frost) on the coldest surfaces in the attic, which is normally the underside of the roof.

    The problem is that when this frost melts in the spring, it can get the insulation between the ceiling joists wet, and that can cause the wooden ceiling joists to rot in places.

    The whole idea behind roof ventilation is to allow cold dry outdoor air into the attic. That cold dry outdoor air will warm up because of heat loss from the house and as it does so it's relative humidity will go way down and it'll absorb any moisture present in the attic. That wetter warmer air will then be blown out of the attic by a wind (or whatever) and be replaced with cold dry outdoor air again. In this way, attic ventilation serves to keep the attic dry, and dry wood in your attic is happy wood cuz it knows that as long as it's dry, it'll never rot.

    So, the easiest and most effective way of seeing if you have sufficient attic ventilation is to simply go up into your attic on the coldest day of the year and look for condensation or frost forming up there. It'll form on any uninsulated areas on the underside of your roof.

    If that frost or condensation disappears within a day or two on it's own, there's no real call for alarm. However, if it persists for weeks on end, it'd be better to increase the amount of ventilation in the attic to eliminate that frost sooner and thereby prevent it from causing any problems.

    For a much more elaborate discussion of roof ventilation, read this thread (which starts out talking about drywall nail pops, and then moves on to attic ventilation):

    https://advice.thisoldhouse.com/showthread.php?t=118086
    I'll have to disagree to a point with some of those comments.
    A lot depends on the severity and the swing of the winter temperatures.

    For example in Winterpeg the temperatures are too cold for too long for attic venting to be effective for drying during the winter months. There's no way minus 20 , 30 , 40 C air entering the attic space will be warmed enough by any air leakage from the living space to be able to carry enough excess moisture out.

    As a matter of fact the CMHC did a study on attic venting and one finding supports that attic venting in certain regions is over rated. Other than keeping the roof cold ( for preventing ice damns ) it's even suggested to close off attic venting during the winter in those regions of extreme cold temps for long durations -- opened only for spring and summer use only.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
    Join Date
    May 2011
    Posts
    251

    Default Re: Roof Ventilation Issues?

    IF you have access to the unfinished attic space, another option could be to spray foam the entire roof deck and seal the attic, eliminating venting.

    I did this on my 1920 home that had no venting when I purchased it. Rather than cut holes all over the face boards and roof, I insulated the attic. As a result, it never gets over 90F even when its' sunny and 95F outside. In the early afternoon it sometimes cooler in the attic than it is upstairs. I still need to remove the existing 12" of cellulose. Otherwise it would stay under 85F.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  6. #6
    Join Date
    Dec 2010
    Posts
    4

    Default Re: Roof Ventilation Issues?

    Thank you all for your considerate responses. I really appreciate that you'd take the time! It gives me somewhere to start with figuring things out. Thank you!

  7. #7
    Join Date
    Sep 2011
    Posts
    53

    Default Re: Roof Ventilation Issues?

    regardless of the weather or season, you need ventilation to aid the circulation of the air inside and outside the house. It's great to have adjustable hole size for the roof like something sliding shutters I guess.

  8. #8
    Join Date
    Sep 2011
    Location
    Waukesha, WI
    Posts
    2

    Default Re: Roof Ventilation Issues?

    Roof ventilation is essential to the proper longevity and integrity of the entire roofing system. If the ventilation is not correct then problems will occur and you will have bad experiences.

    The general rule of thumb is that during winter months the temperature in your attic should always be within 10-20 degrees of the outside temperature, even if its 20 below zero. Although many people may dispute this, the fact remains that maintaining equalized temperatures between the inside and outside reduces the chances of ice damming and the build-up of condensation on the underside of the roof deck.

    NOTE: Proper ventilation consists of:
    1. Intake
    2. Exhaust


    Both intake and exhaust should be proportionate to one another, and the volume is determined based upon the square footage of your attic. The Federal Housing Administration recommends a minimum of at least 1 square foot of attic ventilation (both intake and exhaust) for every 300 square feet of attic space. I suspect that if your vent is allowing snow to enter that there are too many roof vents -- likely due to someone adding more to let the heat escape in the summer...

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