split insulated roof
Ogden2024
| Posted in Energy Efficiency and Durability on
Hi ,we are planning a insulated roof over a conditioned attic. We are zone 4C .starting from the bottom R 19 bat insulation 1/2 inch plywood deck, tape the seems for air barrier, roofing under layment for rain protection until we finish the roof ,3 inches rockwool , 1/2 inch plywood , roofing under layment , metal roofing. Will the two layers of under layment trap moisture .
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Ogden2024,
You could use a permeable roof underlayment for the lower one.
https://vaproshield.com/products/roof
upper roof underlayment should be from manufacturer of the roof panels so that you have a water-tightness warranty. if they don't have one, then suggest grace ultra or henry PE200HT or the like.
Rigid mineral wool is not the same as rigid foam insulation, it is not air impermeable, so it is still fluffy insulation which means you need to vent above.
If you swap out to rigid foam insulation, your assembly will work.
Akos,
I had the same thought, but these threads seem to disagree:
https://www.greenbuildingadvisor.com/question/unvented-cathedral-ceiling-with-all-mineral-wool-above-and-below
https://www.greenbuildingadvisor.com/question/rockwool-on-roof-sheathing
The BSI info shows a vapor diffusion vent in Fig 3 here for this type of assembly:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-096-hot-and-wet-dry
Moisture can still get between the two roof decks and since it can move right through the rigid mineral wool it can condense on the upper roof deck, thus need for venting.
My guess is that it needs a lot less venting than a typical roof and that is why the diffusion vent can work.
Even with a diffusion vent, this is still not code compliant except in zone 3.
Akos,
I was surprised at the comments in those discussions. I understand how mineral wool can be substituted for foam on the exterior of walls as they can dry to the outside, but roofing is largely impermeable, so I don't see how it works there. I hope someone can shed some light on why continuous permeable roof top insulation works in an un-vented assembly - or doesn't.
Thanks for the replys, would leaving off the bottom underlayment allow the roof to dry to the inside?
In 4C the winter is still too long so anything that can dry to the inside, can also accumulate moisture.
I don't see a way of building an unvented roof with fluffy insulation only. If this in-between roof deck would somehow work with rigid mineral wool, than you could build the same assembly with loose fill and we know those can fail in cold climate.
Since you are getting close to zone 3 where a diffusion vent over a standard roof can work, I would guess a diffusion vent as per the BSI link would work since not much moisture should make it up there.
The one you want to avoid with fluffy insulation (rigid mineral wool is still fluffy), is a vapor barrier sandwich with a vapor barrier on the cold side. Without some form of venting that assembly is guaranteed to build up moisture over time.
If you want something that is guaranteed to work, swap out the mineral wool for rigid foam. In zone 4 you don't need all that much for condensation control plus it is significantly cheaper.
If you must have mineral wool, you can simplify your build and save a bunch by using regular batts. I would install either rafters or purlins over the 1st roof deck and fill those with regular batts. You could use 2x6 rafters with R14/R15 batts, the extra space above the fluffy can now form your vent space. Above this install the 2nd roof deck. You now have a nice vented assembly that is pretty easy to build, doesn't need any expensive materials or long screws. Bonus, it is also code compliant.
Doesn't matter which assembly you pick (unvented exterior foam or vented with semi rigid mineral wool or batts), I would detail the 1st roof deck as the main air barrier and vapor control layer.
The 2nd roof deck can be covered in whichever underlayment your metal roofing manufacturer prefers. With foam, the permeability here doesn't matter and moisture won't make it to the underside of the 2nd roof deck. With the vented option, you have a vent space bellow the deck to help with drying.
How about 1x4 vertical battens over the stone wool vented top and bottom then plywood underlayment and roofing. Then the thing is we are in a high fire risk zone and vents become a problem. The vapor diffusion port mentioned earlier sounds good but not sure if i understand the details of how it works. Thanks again for the help.
1x4 is not a compliant vent space. Most likely it will work but you are still opening your roof to ambers. 2x4 on flat would be code compliant, you'll have to put amber screens on the vent openings though.
With an unvented roof with rigid insulation, if the heat gets through your metal roof, the class A underlayment, the roof deck, finally to the foam, you have much bigger issues. You can also use polyiso which chars instead of burning.
A diffusion vent works similar to a roof vent. In a regular roof, you have some airflow to move moisture out. In case of the diffusion vent, it relies on a highly permeable material (ie house wrap or denseglass) to allow the moisture to diffuse from the roof cavity to the outside. It is not as effective at moving moisture but can work if you air seal well bellow.
We have decided to go with polyiso thanks again