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  1. #1
    Join Date
    Nov 2008
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    Default Open cell between rafters - to do or not to do?

    Boston, MA suburb. 1894 house, balloon framing, second floor rooms all have knee walls on side walls. The attic has fiberglass batting between the floor joists with plywood flooring on top. Energy audit prescribes open cell insulation sprayed between the rafters (along with closed cell around the sill in the basement) as an additional draft break. The exterior walls are insulated on the first floor only. MY QUESTION/PROBLEM: The attic open cell insulation would completely cover the roof sheathing and fill the space between the rafters as well as cover them and block the ridge vent and gable vents. All of my previous research has said that air is SUPPOSED to be able to flow through the attic but this would seal the space in an attempt to stem the chimney effect. Has the philosophy changed? What method is correct? Am I going to save myself money or am I going to create a mold problem?
    Last edited by rey; 01-19-2010 at 11:20 AM.

  2. #2
    Join Date
    Jun 2007
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    Coventry, RI
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    340

    Default Re: Open cell between rafters - to do or not to do?

    I had my entire house done with open cell foam during a major rehab. sounds like I have a similar style house with knee walls on the second floor. I was informed by the installer that by spraying the underside of the roof with the insulation venting is not necessary. They actually told me that if a house has a section of roof that would be difficult if not impossible to vent (I guess by its shape) that inspectors are now in most cases requiring that foam be sprayed in those areas. The building inspector passed the insulation without any problems so it seems to be the way to go. I had mine done 4 years ago and have had no problems with the roof or moisture on the ceiling or anything like that. I live in RI so we pretty much have the same weather as you in Boston. Hope this helps you out.

    Mike

  3. #3
    Join Date
    Jan 2009
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    Chicago
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    Default Re: Open cell between rafters - to do or not to do?

    I am in a similar situation where I am considering installing open cell insulation in an old sun porch in Chicago. Will I have to add a vapor barrier and if so, will I then need to vent the roof?

    Also, when I read articles ****** it states to never use open cell insulation on exterior walls. To me, this means the roof and the outside walls of my room, which is where I need tight insulation to begin with.

    So confusing!

  4. #4
    Join Date
    Nov 2008
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    Default Re: Open cell between rafters - to do or not to do?

    Thanks all for your responses. Further research has revealed that the "hot roof" practice has become a generally accepted means of insulating the attic, along with at least the rim joist, in an attempt to prevent the chimney effect whereby cold air is drawn into the basement and vents its way up through the house, taking all the warm air with it. Given all that I have learned though I'm still not comfortable covering the underside of my roof sheathing, open cell or not, and possibly holding any moisture there between the foam and wood. I guess I'm an old dog but I just can't get beyond what I've been told my whole life, vent the attic and let the moisture escape. I'll need some more years of others experience before I can do it myself.

  5. #5
    Join Date
    Aug 2007
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    Default Re: Open cell between rafters - to do or not to do?

    rey ---- a couple of thoughts.

    Insulating the underside of the roof ( hot roof ) shouldn't be an issue with moisture.

    You have to think of the reasoning for venting the attic in the first place. A long time ago there was little consideration in preventing warm moist air entering this space. Instead it was discovered a long time ago that warm moist air was entering this space and condensing. The long time remedy was to allow lots of venting to try and remove the moisture rather than preventing it into this space in the first place.

    A long time ago I learned that venting is over rated.

    The principal behind insulating the attic is turning this into a semi conditioned space.
    I will point out that it's insulating all the exterior surfaces of the attic and not just the underside of the roof. The reason is to minimize heat gain during summer months reducing the cooling load for A/C.
    Durning the winter this would reduce the heat loss and reduce the heating load for the furnace.
    The other benefit is keeping the temperature above the dew point of the moisture and preventing condensation during the winter.

    This is not dissimilar to insualting a wall.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
    Join Date
    Nov 2008
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    8

    Default Re: Open cell between rafters - to do or not to do?

    Thanks canuk, I agree, it all sounds right and logical but our attic is a major storage space for us and the proposal is not to insulate the entire space, just the rafters and underside of the sheathing down to what would be the top of the slanted ceiling above the knee-wall. Due to the fiberglass between the joists, which does not prevent the moisture from entering, it will not become a semi-conditioned space and air from the house will still be able to go up there. I can understand with new construction or gut reno. that sealing the space would be possible but it seems that it wouldn't be ideal in this old house. And I guess I just think a house needs to breath in some way, energy conservation or not... Thanks again though, your explanation definitely makes the concept much clearer.

  7. #7
    Join Date
    Aug 2007
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    The Great White North
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    Default Re: Open cell between rafters - to do or not to do?

    In your case if there are ceilings already in place then I understand what you are saying.
    If you already have an air chase above the ceilings then you probably don't have much insualtion for this space. Also you would want a continious chase from the area on the sides of the knee walls to outside intake vents.
    In which case you could attach rigid foam to the underside of the rafters which will provide insualtion and thermo break but also leaves the rafter bays open for an air chase.
    The thing is the breathing part. If you prevent the warm moist air and vapour from reaching the cold uninsulated areas then there is no reason for the house to breathe per se.

    I don't agree that only insulating the underside of the roof is the way to go --- it's only half the space.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #8
    Join Date
    Feb 2010
    Location
    Eastern MA
    Posts
    75

    Default Re: Open cell between rafters - to do or not to do?

    Rey,
    I am a spray foam contractor in MA. I have done quite a few unvented attics in the gas rebate program. It is allowed by the building code and they work. I did a house in Newton. See attached photo. Click image for larger version. 

Name:	Barry roof1.jpg 
Views:	36 
Size:	29.0 KB 
ID:	3190 The heat was trapped in his attic, while his neighbors' houses' roofs no longer had snow because it melted due to the heat lost through the roof. We compared gas bills and found he used 30% less gas.

    Attics do not need to be vented. The MA code does call for a vapor barrier on open cell foam. This may not be really necessary, except in houses with a lot of moisture. Closed cell foam does not need a vapor barrier.

    A bigger concern I would have in your situation is the use of the attic for storage. For an applicator to spray the foam, he has to be able to spray to the surface. Although I cover things, it seems inevitable that foam gets on things. Secondly, foam presents a risk to a person who might be in the attic if a fire starts. It would intially flare up and be dangerous for anyone there. After the intial flare, the char would start to protect he building. In these cases I spray the foam with an intumescent paint to give the necessary fire protection.

    If you would liketo contact me, I am at Robert@Dr-Warm.com

  9. #9
    Join Date
    Nov 2008
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    8

    Default Re: Open cell between rafters - to do or not to do?

    RJordan - Thanks for the message. In fact we did this audit hoping to take advantage of the gas rebate but it's my understanding that that incentive no longer exists so, therefore, our budget prevents us from proceeding with the insulation no matter what. Also, not using the attic for storage isn't an option. With only 1700 sq. ft. of living space, the attic and, to a lesser degree, the basement are our prime storage spaces. I understand that we would realize a monetary savings but, if the foam would prevent using the space for storage, we wouldn't be able to stand the loss of space. But all that aside, my concern still is that we're not talking about a gut renovation. Due to the knee walls, we'd only be partially insulating the underside of the sheathing since the slanted sections of the 2nd floor ceilings are directly under the roof. There is between 5-6 feet of ceiling/roof from the top of the knee wall to the drip edge of the roof that isn't "insulatable" (not a word, I know). It seems we'd only be insulating under the peak of the roof and that the heat loss would continue unabated everywhere below that. All of the responses have been very helpful though. Thanks again.

  10. #10
    Join Date
    Feb 2010
    Location
    Eastern MA
    Posts
    75

    Default Re: Open cell between rafters - to do or not to do?

    Rey,
    The rebate program still exists. It is undergoing some changes and the people running the program don't seem to know what's going on yet. They give conflicting advice. In fact, the gas companies, by regulation, have to have the rebate program.

    You can still use the space for storage. After the work is finished, the foam needs to be covered for your safety in the event of a fire. This can be done with a fire retardant paint. Somewhat costly. The other issue is being able to access the roof surface to be able to spray it, which probably means clearing things out while the work is done.

    One of my first projects was in Watertown. He had renovated half of his attic to be a bedroom and insulated it with fiberglass and had the heat brought up to the room. He hired me to insulate the rafters in the unfinished side. He later told me the unfinished, unheated attic was warmer than his bedroom. All of the heat previously being lost from the floor below into the attic was trapped by the insulation and clearly doing a better job than the fiberglass in his bedroom.
    Last edited by RJordan; 02-13-2010 at 06:37 PM.

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