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-   -   Rafter insulation Q. (http://www.diychatroom.com/f19/rafter-insulation-q-35138/)

Alan 01-05-2009 09:58 AM

Rafter insulation Q.
 
I'm trying to wrestle this thing in my own mind, but i'm confused if the purpose of having airspace above the insulation when insulating the rafters is to have actual 'airflow through the entire roof' or just allow air to get into the area?

The problem i'm having is with the way the building was added on to, even if i leave a space above the insulation for air, the rafter bay deadends into the side of another rafter going the opposite direction no matter which way i go.

So, is it sufficient to leave the airspace for air to get in even if it doesn't flow, or would I be better off trying to seal it up completely? Maybe there's another option i'm not thinking of?

Let me know if a drawing is needed here. Thanks.

Alan 01-10-2009 02:30 AM

Still looking for an answer on this topic if anyone has any input. Thanks.

Bob Mariani 01-10-2009 06:40 AM

this air space is needed to vent the moisture that will accumulate due to the cold outside air reacting with the warm air. That is why you also need a vapor barrier on the warm side of the insulation. This vented space should pull air from the soffit vents and extract it out the ridge vent. or out the side vents. If you are missing these vents, you may need to add them.

Chemist1961 01-10-2009 07:23 AM

to best understantand attic ventilation issues here's a great website
Buildingscience.com

Alan 01-10-2009 12:33 PM

Quote:

Originally Posted by Chemist1961 (Post 210248)
to best understantand attic ventilation issues here's a great website
Buildingscience.com

Great link, thanks!

I'm still confused as to what my options are here. Obviously it would be best to get air to flow from one end to the other, as i'm working with a flat roof, and there is no ridge to ventilate.

Without drilling holes in the rafters, however, i'm not sure how I should accomplish this.

:(

Bob Mariani 01-10-2009 02:59 PM

Quote:

Originally Posted by Alan (Post 210361)
Great link, thanks!

I'm still confused as to what my options are here. Obviously it would be best to get air to flow from one end to the other, as i'm working with a flat roof, and there is no ridge to ventilate.

Without drilling holes in the rafters, however, i'm not sure how I should accomplish this.

:(

A flat roof would not have a ridge vent, but if you have the typical parapet walls, these would have vents above the snow line.

Alan 01-10-2009 04:06 PM

I do not have parapet walls.

Maybe i'm a little unclear on whats going on here. I'll try again.

I'm going to be attaching sheetrock to the rafters. I need to insulate the rafters. Due to the construction of the rafters on the flat roof, if I use a soffit vent for "attic ventilation", each rafter bay deadends into a rafter going perpendicular to that bay. Therefore i'd have airflow INTO the space, but not necessarily out of the space. I've seen the rafter baffles to keep the insulation from blocking the airflow, but i'm not sure if doing that is useless if the air can't flow all the way through the entire bay.

Should I make up a quick drawing? :huh:

Alan 01-10-2009 04:17 PM

1 Attachment(s)
This is just a quick mock up, not to scale, there are more boards than this involved. On the long side of the image is where the existing house is, so without drilling through some of the boards, it'll be nearly impossible to ensure airflow. They're all 2x6, so am i safe drilling a small hole (5/8 or 3/4) or two every couple of feet in the middle of the rafter?

What's my best option here?


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