I am planning my house now, 1/2 the house will have a cathedral ceiling w/ sissor trusses, Here I plan to use spray foam and no ventalation . the other 1/2 will be 2 story w/ regular roof trusses . The question is can I just spray cellulose on the bottom of the attic floor ( a thick blanket, cheaper than foam ) and go nonvented. Or should I spray foam on the top of the roof decking and go non vented. Either way I will I still have to have HVAC vents running up there? Now the Hvac will run through the floor trusses.

Or should I scap the nonvented attic and go vented
Sorry about so many ?
any inf owill be helpful thanks,
dc

Depends on your climate malibud - if you're in a hot humid climate then non-vented is definitely the way to go. There's a lot of good information about this at:

http://www.buildingscience.com/index_html

i am in a mixed humid area Western NC. I am just worried about the summer/spring / fall when we often open up the windows. Where would the moisture go then ?

With non vented attic do I have to a HVAC vented into the attic ?
Thanks for the replies.

 I have look a lot on the building science web site but they don't talk much about opening windows during nice weather or is this a no no

I'm not a pro, so hopefully someone more knowledgeable will contribute. But if you use a construction method which creates a tight building envelope (like SIPS, ICF, or various concrete methods) it is usually recommended that you use a mechanical ventilation system. But I don't see any issues with opening your windows in nice weather.

If you intend to open your windows a lot, you might look at installing a 'whole-house' attic fan system, but instead of venting it into the attic, it would need to be ducted to the outside. However, you should be very aware of the outside humidity when planing to open windows. Dumping humid outside air into your house will cause your hvac system to work much harder to remove this excess humidity, or you could wind up with moisture related problems inside your home. When the humidity is high, you are better off with the windows closed, and your air exchanger bringing in fresh air.

Here's a great article about unvented attic spaces.  It's quite informative and will answer many of your questions.

I'm way up in Maine and I'm still going to go this route.


John

Thanks for the info although I was not able to open the article. I am a bit confused ..
Approximately, what is the relative humidity outside need to be so that I can open windows and if I go with a Vented attic will it be any different ?
Lastly would closed cell foam help the matter is it is sprayed underneath the roof decking?
thanks

Have someone help you download the article and open it up.   It's a PDF file.   You'll need to download a free copy of Adobe Acrobat if you don't already have it on your computer.    You'll probably find many of the answers you seek by reading that article.


John

Malibud, if the weather feels nice enough to open the windows, then I would bet the humidity would be at a moderate level. Surely you could just turn off the HVAC/ventilation system and go with some natural ventilation when the weather is conducive to that, like in the spring or fall. I think the only issue would be, for example, if you were to run the HVAC *and* open the windows in the middle of a hot and humid summer. But you wouldn't be likely to do that anyway. When you should open the windows wouldn't be any different if the attic was vented or not.

Spraying the foam under the roof decking I think is the way it's usually done in a non-vented attic constructed with trusses or rafters.

I believe that an indoor relative humidity level of 50-60% is considered good. Anything much above 70% would be too high. I suspect that in your area, a great deal of the summer months will exceed the 70% threshold, especially the overnight and early morning hours. Check with your local weather service.
As to the insulation and your attic, spray foam on the underside of your roof decking. I find that this is the best for our area which has similar weather conditions to your area. Never introduce warm, moist air into your attic. This is true whether or not you have hvac equipment in the attic. The key is to introduce some small amount of conditioned air into the attic, either way. This can be accomplished in various ways. Be creative.

Lamb I got your article thanks , Thanks to everyone for their replies. Last questions on the non vented attic subject:

Can i just add vents or open space in the ceiling going to the attic or do I need duct work going up there ?
Can I not open the window if the outside humidty is above 70% and the temperature is below 60?
Thanks Again I am going w/ sprayed closed cell foam on the roof deck of a seal attic and crawlspace w/ an ERV.