Does my Shed Roof need to be vented?


Old 10-22-18, 10:27 PM
Thread Starter
Join Date: Oct 2018
Location: United States
Posts: 7
Received 0 Votes on 0 Posts
Does my Shed Roof need to be vented?

Hello I recently purchased a house and I am going room by room trying to update. In the process of upgrading the bathroom, I found that my Ceiling has very thin insulation (like one inch). Behind that insulation is a vented cavity. I believe the rafters are 2x10. Above the roof sheathing are 2, inch foil faced rigid insulation, covered by a epdm roof.

The roof is a shed roof (low slope) and the inside ceiling follows the slope, and is essentially a cathedral roof.

Since the cavity is vented, I feel like that negates the added insulation. Furthermore, since the vents are on the fascia, on both the low and high side, I feel like the actual airflow is minimal.

Can I add insulation to fill the cavity and close up the vents in the soffit? I would do this room by room.

I attached a cross section of the house.
Sponsored Links
Old 10-23-18, 12:48 AM
Marq1's Avatar
Join Date: Sep 2016
Location: USA MI
Posts: 6,541
Received 368 Votes on 343 Posts
I guess my only question, if it's a shed why worry about insulation? It's not being conditioned is it?
Old 10-23-18, 01:01 AM
Thread Starter
Join Date: Oct 2018
Location: United States
Posts: 7
Received 0 Votes on 0 Posts
I'm sorry the title is misleading. This is for a house. The roof type is "shed" (low slope).
Old 10-23-18, 03:57 AM
Forum Topic Moderator
Join Date: Mar 2005
Location: USA - N.E.Tn
Posts: 47,649
Received 319 Votes on 283 Posts
If there is a cavity between the insulation and the roof - it should be vented.
Old 10-23-18, 11:05 PM
Thread Starter
Join Date: Oct 2018
Location: United States
Posts: 7
Received 0 Votes on 0 Posts
Is there anything I can do to increase insulation? Doesn't the vented cavity negate the added insulation below to epdm membrane? My insulation is literally like an inch thick. I have large heating bills and I wonder if this is one of the reasons.

For example, can I install plastic rafter vents and fill the remaining space with insulation?
Old 10-24-18, 03:18 AM
Join Date: Oct 2001
Location: USA
Posts: 963
Received 3 Votes on 3 Posts
You are correct that the insulation above the vented air space does little, if any good. I would definitely not fill the rafter space or eliminate the air space entirely. You could add insulation in the rafter space and leave a 2" air space, but I don't know how that could be done without removing the ceilings or the roof and sheathing. The only way I can imagine adding insulation now, leaving all existing materials in place, is to add more rigid insulation below your existing finish ceiling. And even then you're limited by the length of drywall screws.

BTW, I think it was back in the early to mid 80's, one of the big name insulation manufacturers was advocating completely filling the rafter space of vaulted ceilings. That didn't last long, maybe 5 years, before they found it was a bad idea.
Old 10-24-18, 06:13 AM
Join Date: Oct 2008
Location: New England
Posts: 10,514
Received 37 Votes on 34 Posts
Adding rigid insulation above the roof deck is a standard method for building an unvented roof assembly which of course eliminates the venting, good so far.

The concern might be the thickness of the rigid in relation to the amount of added insulation. The concern is that the first surface above the air permeable fiber insulation needs to always be above the dew point. Now, I have no specific guidance for a roof assembly but there are referenced for walls with rigid on the exterior and they are also unvented. Here is that reference. What we need with that reference is your climate zone.

Check the article on unvented roof assemblies and don't look at the initial picture, well you can look but read why it happened.

Old 11-06-18, 09:05 AM
Member In Good Standing
Join Date: Jan 2008
Location: usa
Posts: 159
Received 0 Votes on 0 Posts
The only way I see the insulation as correctly installed is if you are in a hot and sun intense area and they were insulating the home from solar gain through the roof and there was no A/C. If this is so and now you intend to cool the room then I would add insulation nearest the living area and keep a ventilated area in between. This way you are insulating form solar gain through the roof, insulating the conditioned side, and ventilating between.
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: