Attic vented or not? | Ask Mason Knowles - SPF Consultant | Spray Foam Technical Questions and Answers | SprayFoam Magazine Q&A Forums

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Posted: Nov 11, 2007 11:50 PM	Attic vented or not? I have rancher, using foam in exterior walls, my foam contractor suggested(due to my wild vaults and high ceilings) to spray my entire envelope. Do the exterior walls and all rafter bays up to ridge with cc foam. This would give me an unvented attic, I am now foaming my ducts in the attic, and covering with ignition barrier as per your knowledge,(only metal ducts in attic) but my inspector will not allow me to have an unvented attic, one without conditioned air. But that would be a waste, there is no space usable up there due to vault angles and design elements. I'm not sure why I see that in pics, but cannot do it myself, also, if I can have an unvented attic and foamed rafters(with 1 1/2 fiberglass ignition barrier over all foam) what do i do at ceiling levels that are not vaulted but 8', and small vertical side walls adjacent to vaults? I will loose heat up into space won,t I, should I do like exterior walls and use batts with no vapor barrier? Will moisture go from house to unvented space through batts and affect wood structure in attic? My inspector is telling me basically not to have an unvented attic, I think he's wrong, why not? My whole envelope is sealed, isn't that better for efficiency of heating and cooling? I would appreciate any suggestions on this I,m not sure how to proceed, I know one option is not to foam any celling areas and just do outer walls, but it seems there is more heat loss up than out, right? Sorry its a lot, but I'm getting frustrated dealing with my inspector and trying to educate him.
mason Posted: Nov 12, 2007 07:44 AM	Derek, This is one of the more popular topics on my forum. Yes, the building codes have allowed unvented attics since 2004. If your area is using ICC codes from that date forward, you can have unvented attics using an air impermeable insulation. You can download the ICC codes online at iccsafe.org. You have the greatest efficiency from spraying foam to the underside of the roof deck (ie ceiling of the attic) and definitely spray the side walls to make the foam continuous where possible. Spraying foam to the underside of the roof deck will stop air infiltration and minimize the amount of moisture traveling to the attic ceiling along with the potential for condensation. You do not have to condition the attic in this type of application. Just spraying the foam to the underside of the roof deck will prevent your interior temperature from escaping to the outside and the attic space will naturally be within 6-8 degrees F of the interior space. You do not want to spray the floor of the attic. It would not help energy efficiency and would separate the interior temperature from the attic space. You would not want to use fiberglass on the underside of the roof deck without venting the attic space. That would cause moisture problems. Hope that helps.
Posted: Nov 12, 2007 10:30 AM	If i did have unvented attic, and had foam in rafter bays, I would have to cover it wtih an ignition barrier correct, I was going to use 1 1/2 of fiberglass(no paper) since i have alot of it. Also like you stated foaming my ducts and then wrapping them. Also, your saying not to do anything to drywall ceilings that are 8' and not in vault areas, and side walls. (Not to exterior they would be foamed) I know that heat would escape into that unvented attic space, isn't that defeating the purpose of trying to keep in down in my living space, or is that a by product of not venting attic, but still getting some form of heat into the space. My Hvac is having a hard time with that, because I loose heat to attic, and for me its a great air barrier between my conditioned space and the exterior. I understand that an air gap in the walls between foam and drywall is good, is this the same principle, or I am wasting heat to an unused area? And should I try to stop it? I really aprreciate you input, thanx!
Posted: Nov 12, 2007 06:48 PM	Your previous statement said not to use fiberglass under roof decking , is that fiberglass only, or if I use foam can I still use fiberglass, under foam as an ignition barrier and still not vent attic? I think I got it, just want to be sure raters bays foam and fiberglass, and ceilings, nothing at all. anywhere ther is foam in attic I have to cover it with fiberglass, but never insulate with any product the ceilings at 8' or interior exposed to attic only side walls. All exterior surfaces with have foam and fiberglassin attic space if exposed, if not exposed then vault areas with drywall cover foam for ignition? Correct?
mason Posted: Nov 13, 2007 08:14 AM	It is a very easy concept. You want the temperature inside the interior of the house to "leak" into the attic. By spraying the foam to the underside of the roof deck, you have separated the interior environment (temperature and humidity) from the exterior environment. It takes much less energy to cool or heat the attic space this way, than trying to have your furnace or AC maintain a consistent temperature inside the house with the attic temperatures much colder or hotter (depending on the time of the year) Tests performed by Oakridge National Labs show that when you insulated the floor of an attic with 14 inches of blown in fiberglass that with outside temperatures of 0 degrees F, the attic temperature will be around 6-8 degrees F. At outside temperatures of 110 degrees F, the attic temperature would be well over 100 degrees F. However, after spraying sprayfoam to the underside of the attic the temperatures maintain within 6-8 degrees of interior temperatures. (note: in an unvented attic or cathedral ceiling application, insulating your attic ducts would not be a great energy saver, but if you insulated the floor of the attic and let the attic stay hot or cold, then insulating the ducts would save a great deal of energy, but not as much as spraying to the underside of the roof deck without venting. For more information go to sprayfoam.org and in the news and media section scroll down to an article titled SPF Outperforms Fiberglass in ORNL Attic Tests.
mason Posted: Nov 13, 2007 08:33 AM	Yes, when you spray foam in the attic, you need to cover it with an ignition barrier and unfaced fiberglass will suffice. I was only mentioning that if fiberglass was the only insulation used against the underside of the roof deck, that it would have to be ventilated to minimize condensation
Posted: Nov 13, 2007 04:56 PM	THANK YOU VERY MUCH!!!! :) I am going to have an unvented attic, and will probably not cover ducts as it is not worth the money as you stated. I will seal them of all holes and voids , but thats it. I guess alittle temperature hot or cold coming off ducts will help my attic space. Thank your again for your input.
Darren Gomez Posted: Dec 19, 2007 11:36 PM	Mason, WHere can I get a copy of the study that you mentioned by Oakridge?
mason Posted: Dec 20, 2007 07:50 AM	Contact SPFA at 800 523 6154 or visit their website at sprayfoam.org. An article on the attic testing is downloadable from the website.

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Comments

Posted: Nov 11, 2007 11:50 PM

Attic vented or not?

I have rancher, using foam in exterior walls, my foam contractor suggested(due to my wild vaults and high ceilings) to spray my entire envelope. Do the exterior walls and all rafter bays up to ridge with cc foam. This would give me an unvented attic, I am now foaming my ducts in the attic, and covering with ignition barrier as per your knowledge,(only metal ducts in attic) but my inspector will not allow me to have an unvented attic, one without conditioned air. But that would be a waste, there is no space usable up there due to vault angles and design elements. I'm not sure why I see that in pics, but cannot do it myself, also, if I can have an unvented attic and foamed rafters(with 1 1/2 fiberglass ignition barrier over all foam) what do i do at ceiling levels that are not vaulted but 8', and small vertical side walls adjacent to vaults? I will loose heat up into space won,t I, should I do like exterior walls and use batts with no vapor barrier? Will moisture go from house to unvented space through batts and affect wood structure in attic? My inspector is telling me basically not to have an unvented attic, I think he's wrong, why not? My whole envelope is sealed, isn't that better for efficiency of heating and cooling? I would appreciate any suggestions on this I,m not sure how to proceed, I know one option is not to foam any celling areas and just do outer walls, but it seems there is more heat loss up than out, right?
Sorry its a lot, but I'm getting frustrated dealing with my inspector and trying to educate him.

mason

Posted: Nov 12, 2007 07:44 AM

Derek,

This is one of the more popular topics on my forum. Yes, the building codes have allowed unvented attics since 2004. If your area is using ICC codes from that date forward, you can have unvented attics using an air impermeable insulation. You can download the ICC codes online at iccsafe.org.

You have the greatest efficiency from spraying foam to the underside of the roof deck (ie ceiling of the attic) and definitely spray the side walls to make the foam continuous where possible. Spraying foam to the underside of the roof deck will stop air infiltration and minimize the amount of moisture traveling to the attic ceiling along with the potential for condensation.

You do not have to condition the attic in this type of application. Just spraying the foam to the underside of the roof deck will prevent your interior temperature from escaping to the outside and the attic space will naturally be within 6-8 degrees F of the interior space.

You do not want to spray the floor of the attic. It would not help energy efficiency and would separate the interior temperature from the attic space.

You would not want to use fiberglass on the underside of the roof deck without venting the attic space. That would cause moisture problems.

Hope that helps.

Posted: Nov 12, 2007 10:30 AM

If i did have unvented attic, and had foam in rafter bays, I would have to cover it wtih an ignition barrier correct, I was going to use 1 1/2 of fiberglass(no paper) since i have alot of it. Also like you stated foaming my ducts and then wrapping them. Also, your saying not to do anything to drywall ceilings that are 8' and not in vault areas, and side walls. (Not to exterior they would be foamed) I know that heat would escape into that unvented attic space, isn't that defeating the purpose of trying to keep in down in my living space, or is that a by product of not venting attic, but still getting some form of heat into the space. My Hvac is having a hard time with that, because I loose heat to attic, and for me its a great air barrier between my conditioned space and the exterior. I understand that an air gap in the walls between foam and drywall is good, is this the same principle, or I am wasting heat to an unused area? And should I try to stop it? I really aprreciate you input, thanx!

Posted: Nov 12, 2007 06:48 PM

Your previous statement said not to use fiberglass under roof decking , is that fiberglass only, or if I use foam can I still use fiberglass, under foam as an ignition barrier and still not vent attic? I think I got it, just want to be sure raters bays foam and fiberglass, and ceilings, nothing at all. anywhere ther is foam in attic I have to cover it with fiberglass, but never insulate with any product the ceilings at 8' or interior exposed to attic only side walls. All exterior surfaces with have foam and fiberglassin attic space if exposed, if not exposed then vault areas with drywall cover foam for ignition? Correct?

mason

Posted: Nov 13, 2007 08:14 AM

It is a very easy concept. You want the temperature inside the interior of the house to "leak" into the attic. By spraying the foam to the underside of the roof deck, you have separated the interior environment (temperature and humidity) from the exterior environment. It takes much less energy to cool or heat the attic space this way, than trying to have your furnace or AC maintain a consistent temperature inside the house with the attic temperatures much colder or hotter (depending on the time of the year)

Tests performed by Oakridge National Labs show that when you insulated the floor of an attic with 14 inches of blown in fiberglass that with outside temperatures of 0 degrees F, the attic temperature will be around 6-8 degrees F. At outside temperatures of 110 degrees F, the attic temperature would be well over 100 degrees F. However, after spraying sprayfoam to the underside of the attic the temperatures maintain within 6-8 degrees of interior temperatures.

(note: in an unvented attic or cathedral ceiling application, insulating your attic ducts would not be a great energy saver, but if you insulated the floor of the attic and let the attic stay hot or cold, then insulating the ducts would save a great deal of energy, but not as much as spraying to the underside of the roof deck without venting.

For more information go to sprayfoam.org and in the news and media section scroll down to an article titled SPF Outperforms Fiberglass in ORNL Attic Tests.

mason

Posted: Nov 13, 2007 08:33 AM

Yes, when you spray foam in the attic, you need to cover it with an ignition barrier and unfaced fiberglass will suffice. I was only mentioning that if fiberglass was the only insulation used against the underside of the roof deck, that it would have to be ventilated to minimize condensation

Posted: Nov 13, 2007 04:56 PM

THANK YOU VERY MUCH!!!! :)
I am going to have an unvented attic, and will probably not cover ducts as it is not worth the money as you stated. I will seal them of all holes and voids , but thats it. I guess alittle temperature hot or cold coming off ducts will help my attic space. Thank your again for your input.

Darren Gomez

Posted: Dec 19, 2007 11:36 PM

Mason,

WHere can I get a copy of the study that you mentioned by Oakridge?

mason

Posted: Dec 20, 2007 07:50 AM

Contact SPFA at 800 523 6154 or visit their website at sprayfoam.org. An article on the attic testing is downloadable from the website.

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