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REMOTE vs. PERSIST approach for a flat roof?

I'm not sure I fully understand, but the REMOTE system states an advantage over PERSIST by allowing more insulation in the roof and not building a second roof structure.

Does this advantage apply for a flat roof?

I'm also trying to understand whether a vapour permeable barrier is needed for the roof and walls on the outside of the sheathing, before the exterior insulation is installed. REMOTE promotes vapour permeable, and PERSIST uses impermeable.

Also, do the current weather conditions in Toronto suggest using a vapour permeable layer over my sheathing so the plywood and framing can dry to the outside before I install my insulation (which are insulated metal panels so a vapour barrier).

Asked by Jerry Chwang
Posted Tue, 02/11/2014 - 19:41

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12 Answers

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1.
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Jerry,
As you probably know, if you use the PERSIST system, all of the insulation (usually rigid foam) is installed on the exterior side of the sheathing.

The REMOTE system is a variation of PERSIST that substitutes a vented, unconditioned attic (usually, with deep cellulose on the attic floor) for a conditioned, unvented attic (the PERSIST approach).

If you are planning to build a flat (low-slope) roof, the PERSIST approach would require all of the insulation to go above the roof sheathing. This is the standard approach for most commercial roofs, and it is a good one.

The REMOTE approach would require you to install the insulation between your ceiling and your roof sheathing, and would require you to include a vented space between the top of the insulation and the roof sheathing. I'm not a fan of this method when it comes to low-slope roofs, but Joe Lstiburek says that it's possible to make this approach work, as long as you follow certain guidelines. To read more about Lstiburek's recommendations, see Insulating Low-Slope Residential Roofs.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/12/2014 - 05:44
Edited Wed, 02/12/2014 - 11:40.

2.
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Jerry,
Q. "I'm also trying to understand whether a vapour permeable barrier is needed for the roof and walls on the outside of the sheathing, before the exterior insulation is installed. REMOTE promotes vapour permeable, and PERSIST uses impermeable."

A. What you need on the interior side of your insulation (in other words, on the exterior side of your wall sheathing and roof sheathing) is an air barrier. The PERSIST system uses a peel-and-stick membrane like Ice & Water Shield, which also happens to be a vapor barrier. Since this layer stays warm during the winter, you won't have any condensation problems.

If you substitute housewrap for peel-and-stick, you need to do a good job of air-sealing -- either by taping the wall sheathing, or by taping the housewrap -- an option which saves money, but risks air leaks.

When it comes to roof sheathing, it makes no sense to choose a vapor-permeable roofing underlayment, especially for a low-slope roof. Since your roofing will be some kind of membrane (typically EPDM or PVC membrane), your roof assembly clearly has no ability to dry to the exterior. The roofing is a vapor barrier.

Q. "Do the current weather conditions in Toronto suggest using a vapour permeable layer over my sheathing so the plywood and framing can dry to the outside before I install my insulation?"

A. Your wall sheathing will dry to the interior, even if the sheathing is damp when your insulation is installed. All of the insulation is on the exterior side of your wall sheathing, so the sheathing stays at interior conditions (warm and dry during the winter).

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/12/2014 - 05:51
Edited Wed, 02/12/2014 - 06:00.

3.
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Thank You. I planned on treating the roof in a similar manner to my walls so would be more like REMOTE. I did, however want to put additional batt insulation (mineral wool) between the joists (keeping at least 2/3rds of the R-value exterior of the sheathing). May do that in the future with the walls as well.

For the air barrier, I was debating between vapour permeable vs. Impermeable liquid spray-on. If there is no additional value to help the Assy dry before the final layers are put on, is there any disadvantage? I.e. Is permeable more expensive, less robust / durable, more difficult to detail (plan was to use tapes for the window openings...do they stick to liquid membranes?).

I would treat my walls the same as roof because the insulation is an insulated metal panel that is impermeable or should be as well. That is, unless there is a failure at a caulking joint, in which case would a permeable air barrier like Aquabloc's 21 perms help with drying to interior?.

Thx again for your help.

Answered by Jerry Chwang
Posted Wed, 02/12/2014 - 09:28

4.
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Jerry,
You wrote, "I planned on treating the roof in a similar manner to my walls so would be more like REMOTE."

Nope. That would be more like PERSIST.

Q. "For the air barrier, I was debating between vapour permeable vs. Impermeable liquid spray-on. If there is no additional value to help the Assy dry before the final layers are put on, is there any disadvantage? I.e. Is permeable more expensive, less robust / durable, more difficult to detail (plan was to use tapes for the window openings...do they stick to liquid membranes?)"

A. You can use either one. If you have questions about pricing, you should talk to the supplier from who you are buying your liquid-applied air barrier. I have no idea which products you are considering, or which ones are cheapest in your area.

Q. "The insulation is an insulated metal panel that is impermeable or should be as well. That is, unless there is a failure at a caulking joint, in which case would a permeable air barrier like Aquabloc's 21 perms help with drying to interior?"

A. The best way to keep rain out of your wall assembly is to create an assembly with a rainscreen gap between the siding and the rigid foam. I have no idea whether your metal panels include a rainscreen gap; but if they don't, then your wall system will be less resilient than wall systems that include a rainscreen gap.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/12/2014 - 09:49

5.
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I want to make sure I understand this and am not misinterpreting:
My walls are drywall, 2x6, plywood sheathing, air barrier, insulated metal panels, rainscreen. Roof is the same, with the addition of the waterproof membrane. Stud bays currently empty but may add batt later. Roof joists could be empty but I wish to put batt in now above my drywall ceiling. I didn't think venting in the roof was needed but can you confirm? Your earlier stmt that REMOTE has all insulation outside of sheathing and PERSIST requires ventilation above insulation inside the sheathing is what confused me.

PS. Yes, we do plan on having a rainscreen outside of the insulated panels.

Answered by Jerry Chwang
Posted Wed, 02/12/2014 - 10:49

6.
Helpful? 0

Jerry,
Here is an article on PERSIST that you may want to read: Getting Insulation Out of Your Walls and Ceilings.

The original PERSIST approach puts all of the roof insulation (usually rigid foam) on top of the roof sheathing.

If you follow the PERSIST approach, your attic will be conditioned (unvented).

The REMOTE approach puts all of the ceiling insulation (usually cellulose) on the attic floor. If you follow the REMOTE approach, the attic is a conventional vented attic. That means that the attic is unconditioned.

REMOTE walls -- just like PERSIST walls and PERSIST roofs -- are insulated with rigid foam. On the outside of the rigid foam, it's a good idea to install strapping to create a ventilated space (a rainscreen gap).

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/12/2014 - 11:10

7.
Helpful? 0

So was this just a typo:

"If you are planning to build a flat (low-slope) roof, the REMOTE approach would require all of the insulation to go above the roof sheathing. This is the standard approach for most commercial roofs, and it is a good one."

Jerry

Answered by Jerry Chwang
Posted Wed, 02/12/2014 - 11:15

8.
Helpful? 0

Jerry,
Oops! You're right; that was a typo. Sorry for the confusion -- my fault.

I have corrected the typo.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 02/12/2014 - 11:39

9.
Helpful? 0

Follow-up to this low-slope roof. So going with unvented (PERSIST) but now I am unsure whether to have it outboard of roof membrane or inboard, i.e. PMR vs. Conventional roof. Related decision is whether to install a green roof (we have structure - engineered joists 2x16 at 12" OC). The 2 options appear to be:

1. Conventional - plywood deck, air barrier, 10" insulation, cover board (optional), roof membrane (likely 2ply modified bitumen). I originally was going to have similar construction to my wall with insulated metal panels and mineral wool or XPS on top as the insulation layer. However, my architect and building science consultant are recommending against using the insulated metal panels on the roof as sharp metal edges may damage air barrier, difficult waterproofing details at transitions between panels and panels to parapet, possible change in metal panel dimensions due to temperature changes (depending on how much additional insulation I put on top), difficulty in detecting leaks. Potential green roof in future on top.

2. Protected Roof Membrane (how is this different than an Inverted Roof Membrane Assembly?) - plywood deck, roof membrane as air barrier/vapour retarder/rain control, drainage layer, 10" XPS foam, another drainage layer, green roof as ballast so would have to be installed now. I am worried about rainwash (cold rainwater reaching membrane), and most PMRs are installed over a concrete deck so would there be potential excess condensation / rot issues from the rainwash? Does green roof help by slowing/storing water before it gets to lower layers?

I have attached roof plan. Its very simple square with NE corner cut out for a deck one storey below. The roof has a single 2 deg slope from south to north side where there is no parapet (essentially a large scupper) with water falling into a large eavestrough below. Sloped insulation (shown) is required to get water away from the east side to the 'middle' so it can run north.

So, I think the main issues I am worried about are: PMR durability on a plywood deck? Ice dam potential on north side 'scupper'? High structural load from Snow (which won't melt?), and green roof (pros and cons for these roof designs)?

AttachmentSize
Roof Plan.pdf 17.38 KB
Answered by Jerry Chwang
Posted Tue, 04/22/2014 - 11:45

10.
Helpful? 0

Jerry,
These are commercial roofing systems that need to be designed by an engineer or architect. You also need to get your builder and roofer on board with your decisions. If this group of professionals doesn't want to proceed with your plan, it hardly matters what GBA readers think.

There are no building science or roofing advantages to vegetated roofs -- only disadvantages. If you really want a vegetated roof, go ahead; but you need to understand that it will be expensive, and that the soil will have a lower R-value per inch than conventional insulation materials.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/22/2014 - 12:13

11.
Helpful? 0

Ok. Then if I take their advice and omit the insulated metal panels, what is GBA advice on PMR vs. Conventional Roof on a plywood deck?

Answered by Jerry Chwang
Posted Tue, 04/22/2014 - 12:56

12.
Helpful? 0

Jerry,
I understand the principles underlying the Protected Roof Membrane approach, but I have no direct experience with it. It is rarely used for residential construction.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 04/22/2014 - 13:02

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