Insulating a Vegetated Roof
My project is on urban property in Toronto (zone 7), and I’m looking for advice about roof assembly and insulation regarding a vegetated/green roof. The roof over this portion of the house is going to be a low slope non vented cathedral ceiling with 2×12″ LVL joists. Mainly I’m wondering if the green roof has a bearing on the unvented assembly recommendations from this article: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs
In particular the assemblies I’m looking at are either closed cell spray foam and mineral wool batts underneath the sheathing, or rigid foam above the sheathing and mineral wool batts underneath.
My finished green roof height is limited by zoning rules, so a thinner roof assembly with ccsf insulation inside would allow me to maximize the ceiling height inside. But I’m worried about the possibility of cracks in the foam due to shrinkage / building movement which could lead to condensation problems with the sheathing down the road. If I had to replace the sheathing in the future due to moisture, that would be quite expensive considering the green roof on top.
It looks like a safer alternative is to have rigid foam on top of the sheathing, which in zone 7 would need to be quite thick at 7.5″ of EPS to achieve the R30 minimum. I understand with this kind of inverted assembly the root barrier is typically installed under the rigid insulation so the insulation gives extra protection from aggressive roots. The downside for me with this is the extra thick assembly comes at the cost of reduced ceiling height inside the building because of zoning constraints. Is this the better way to go?
And for rigid foam under the green roof is either EPS or XPS more suitable? Setting aside the benefit of EPS over XPS for the blowing agents, I’ve seen manufacturer marketing materials on both sides touting that each one is superior for green roofs due to their compressive strength, moisture permeability, etc. Is one type actually better here?
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Replies
Robin,
You may find this article useful. My suggestion would be to do a conventional flat roof with pavers, and limit the greenery to as many raised planters as you want.
https://buildingscience.com/documents/insights/bsi-052-seeing-red-over-green-roofs
I have a green roof on my house here in DC. Joe Lstiburek notwithstanding, I am in an environmentally sensitive zone where I have to catch and process all of the rainwater that falls on my property. A green roof slows the rainwater down and allows me to have a smaller dispersal system.
I have a flat roof deck, four inches of polyiso above the sheathing, then sloped polyiso above that to give the roof enough pitch to drain. Above that is TPO roofing, and the root barrier is above that. I have 12" i-joists below. The polyiso seems to be pretty standard, the roofing guy had no problem with it.
If the R24 that four inches of polyiso gives isn't enough to prevent condensation in your climate I'd recommend closed cell spray foam below the deck to get you where you need to be.
DC,
I didn't so much link to that article for Joe L's views on vegetative roofs as the details he provides.
His details all presume a poured concrete roof. It's possible to do it with I-joists.
Thanks for the replies. Agreed that a conventional flat roof with pavers would be easier, but due to zoning quirks around here the only reason I can get approval for a rooftop deck and penthouse space is because of the green roof!
Joe L's article did have a bunch of useful info for me. Seems XPS is the way to go for inverted assembly where the roof membrane is below the foam.
DCcontrarian your tapered foam insulation to get the slope for drainage is an interesting option, I'll look into that.
Have you checked the exact language of your zoning on how height is measured? We have a height limit as well but it doesn't include the roofing material.
Robin,
Probably worth doing then. That rooftop deck and loft looks like a nice space to spend time in.
>DCcontrarian your tapered foam insulation to get the slope for drainage is an interesting option, I'll look into that.
Around here at least this is a normal way of doing flat roofs. Here's a link:
https://www.gaf.com/en-us/products/energyguard-tapered-polylso-foam-roof-insulation
IRMA roofs have the primary drainage plane at the structural deck. tapering the insulation above isn't the way the roof is designed. It is almost impossible to get tapered XPS from Dow/Dupont or Owens Corning since there isn't much market for this hybrid product.
Wow. I know Joe L. gets a lot of play around here.
After reading that post...hard pass.
Ryan_SLC,
If you do that you are making a huge mistake. Joe L. has forgotten more about building science than most of us know. I think his reasoning around what makes a building sustainable in this article is one of the best articulated defenses of beauty in architecture I've ever read.
I'd suggest going with rigid foam on top of the sheathing. Yes, it reduces ceiling height a bit, but it's more reliable for moisture control. About EPS vs XPS, both are good, but EPS is more eco-friendly and cost-effective. It's a trade-off, but the top-layer foam is a safer choice for a green roof https://www.roofersofarlington.com/sterling-va.php.
10 foot ceiling
slope the framing
IMO more durable assembly since it is naturally draining
most of the ceiling height remains visually unchanged
Thought I'd throw in a picture of my roof:
wow. nice
DC,
It's beautiful!
Thanks. Here's a shot from today.
The water in the background is the Potomac River. There is a 180 mile long national park along the north bank of the Potomac, the National Park Service land begins about 6' off the edge of the roof. All those trees belong to the Park Service. We're not allowed to dump any stormwater onto Park Service land or really anywhere off of the property.
Behind the house there is an infiltration tank which is basically like a septic system, except it's buried deeper than the basement so it can't leak into the basement. All of the downspouts go into that tank. The yard is landscaped so that all water drains to the lowest spot, where there's a drain that goes to the tank. The driveway is made of permeable material.
By having a green roof we were able to have a smaller infiltration tank. We were barely able to accommodate the one we have. The Park Service takes the position that they own 100% of any tree when any part of the tree is on Park Service property, so we weren't allowed to dig in the root zone of Park Service trees.
What Lstiburek doesn't account for in his article is how major cities are now turning their attention to stormwater runoff. DC is in the middle of a multi-billion dollar upgrade to their storm sewer system, part of it is they're requiring most new residential construction to contain their rainwater on site.
Commercial flat roofs almost universally use polyiso here, which is what "roofing" polyiso is made for. You can even get tapered sheets so that you can taper the roof a bit for runoff if needed. An EPDM membrane could be used over that. This can be tricky at the edges though since you need to control water that can potentially pool up and work it's way around seals.
Bill
We have a 2' overhang all the way around. The drains are flush with the side of the house, and then the overhang slopes toward the drain.
I know you've gotten some links to related articles already but I'll add a few more:
Lessons Learned on a Living Roof
Do Green Roofs Temper Urban Heat?
There Are Many, Many Green Roofing Choices
Design a green or living roof system
GardenScapes Vegetated Roof System
How to Insulate a Flat Roof
How do you include an eco-roof into a home energy audit?
generally the best approach is to get an integrated system with a total system warranty. that means for the next 20 years, if the roof membrane leaks, then the over-burden removal is included with the leak repairs.
so the question of poly-iso; xps; or eps is secondary to the question of which manufacturer system are you using. I've seen lots and lots of IRMA green roofs with XPS; none with EPS. in either case air layer above the insulation is required to maintain long term thermal performance. drainage layer below the insulation is good practice to avoid water build-up.
As a note: In an IRMA green roof, the insulation is not thought of as root protection. Root barrier is either taped or has really really long overlaps with adjacent.
conventional roofs with green roofing above are becoming more common. mostly I see TPO or PVC roofs over poly-iso. The advantage of these systems over EPDM is that the seams are hot-air welded rather than glued. and these plastic roofs have better puncture resistance than epdm.
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Thanks for the feedback everyone, lots of useful info for me to chew on here.
Roofer,
Good start. Your first post is spam.