I have a client who needs your help. In the early 80's, he and his wife built a beautiful off-grid home on a small island near Fort Myers, FL. It currently serves as a convenient getaway, but as they approach retirement, they want to install a limited amount of air conditioning in the form of a couple of small, high-SEER ductless mini-splits.

The aging 12V PV system is quite small -- the refrigerator is propane powered, and there's no well (fresh water supplied by rainwater collection). So he's going to need to upgrade the power system to support the mini-splits. To keep costs reasonable, some envelope improvements are warranted.

It turns out, the gorilla in the room is the uninsulated metal roof. It's supported by beautiful site-built beam trusses and the exposed roof panels serve as a decorative ceiling. You can only imagine how hot that surface gets, making the home virtually uninhabitable for 4 or 5 months a year.

I'm not sure what's the best approach to insulate this roof. The owner wants to avoid pulling up the existing roof, which is seriously bolted down, successfully weathering Hurricanes Wilma & Charley and numerous tropical storms. Moreover, the owners have a strong preference for preserving the aesthetics of the exposed metal interior, which means working from above.

I'm thinking 4" of XPS or poly-iso covered by another metal over-roof. Keep in mind the roof serves as primary collector for fresh water so shingles are a no-go (aside from blow-off risk from the inevitable storms). However, I don't know enough about this to advise on attachment and edge details. Nor do I have a feel for cost, other than it seems obvious that working from below would be less expensive since it wouldn't be structural and he could use a less expensive insulation product.

I've attached several images for reference. I know we have some savvy retrofit gurus among the membership. I'd appreciate your advice!

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FYI -- I ran a little test yesterday with three kinds of tape: blue masking tape; shiny black duct tape; and standard black vinyl electrical tape. I applied patches of each, about 2" by 4", on the underside of the ceiling, then compared the IR thermometer readings on the bare ceiling and each patch under partly sunny conditions when the ambient temperature was in the low 70s.

I couldn't detect any difference in the temperature readings on the bare ceiling or any of the tape samples. The readings varied within half a degree or so as I moved the red laser dot; that was true directly on the ceiling and over any piece of tape, and true whether I was measuring from 5' or 5" away. I watched the ceiling temperature rise from the mid-70s at 10am to over 100 degrees a couple of hours later under full sun.

The ceiling never reached the highest readings I've observed in the past (>115), maybe because the ambient temperature was in the low 70s instead of our summertime high 80s.

I'm a little confused about the necessity and purpose of using support rails if working on top, and if so, how that would work.

Can 4" of PIC not support a new aluminum roof lag-bolted to the purlins through the PIC?

If rails must be installed, I imagine they would run parallel to the purlins, although they wouldn't necessarily have align with the purlins, providing some flexibility. For added strength, the rails could be anchored to every top chords across the roof (although 1-3/4" blocking would be required for any rail that doesn't align over a purlin).

@Brad, if he lays down plywood over the PIC as you suggest, does this eliminate the need for rails?

I missed where the suggestion for rails came in. The plywood would not need any rails. 4+ inches is a long way way to go to screw the new roof. The plywood would only need a HeadLOC screw about every 2 ft. to secure it to the purlins. Then the new roof would use conventional roofing screws to screw it to the plywood. BTW, structurally, the cheapest PIC boards that I have encountered have a compression strength of 20 psi. The better question is can they span 2 ft. to support the dead load plus wind load. I think it can.

Brad wrote: I missed where the suggestion for rails came in.

Sean Lintow proposed rails and clips (re: first comment in this discussion), but in the context of using spray foam, which has an uneven surface.Then John Hatfield proposed rails in conjunction with PIC fill a few comments later.

I only drilled down on rails because Bill questioned that in his last comment on page 1 and I didn't want it to get overlooked. (I should make it clear I'm pretty much clueless on this. Just trying to manage the conversation a bit!)

Sorry I have been offline. The importance of the Rail material is to provide a complete structural fastening contact to the purlin underneath. So lets say we are going to install 4 inches of insulation then a 4 inch tall rail would be needed to attach the roof above the insulation surface to create a structural sandwich. In this case the rail would run perpendicular to the existing purlins and a standard number 12 or 14 roofing screw could provide attachment to the purlin underneath that would be structurally sound in most wind conditions. The rails could be on 4 foot centers to allow for a standard 4X8 insulation product to be inserted after EPS foam fill panels level the metal surface have been installed. Metal roofing could be installed over the ISO insulation if the surface is felt faced, it would not work well with EPS since the roof surface could easily exceed the melting point of the EPS insulation. Metal roofing could then be attached directly to the rails with shorter screws since the rail material is attached to the purlins underneath.

Rail materials can be lumber or metal and if metal could be a "Z" shape to provide a closure strip to lock in the insulation board. However it is critical to attach to the purlins and then to the rail material to resist structural uplift. Metal "Z" rails should be 20 gauge to tie the new roof to the old. Noting the need for boat delivery the Metal Z rail and the 4 inch ISO can be easily moved across the land by hand.


John wrote: "Metal roofing could be installed over the ISO insulation if the surface is felt faced, it would not work well with EPS since the roof surface could easily exceed the melting point of the EPS insulation."

@John, thanks for chiming in. I mentioned possibly using thin EPS boards to fill valleys between ridges on the existing roof. In that position, the EPS wouldn't be exposed to high temps seen at the top roof. Also, I don't think PIC comes in sheets thin enough for that space.

@Bill, check out R-Max Multi-Max FA-3, which has organic fiber (felt) facing on both sides: https://www.rmax.com/s/Rmax-Commercial-Product-Brochure-fs8l.pdf

You need to lay it down in at least two layers so joints can be staggered. Two layers of 2" would go down a lot quicker than four layers of 1" and would cost less (fewer facings). R-Max has a distributor in Tampa (IDI 813-621-2898). The 2" Multi-Max comes in 24 sheet bundles.

For the valleys, I don't know how tall those ridges are, but 1/4" and 3/8" EPS is available in rolls (http://thebarrier.com/under-concrete-insulation.php)

I do not see the need to be concerned with trying to fill the spaces between ridges of the corrugated roofing, other than to keep the warm humid outside air from getting between the PIC and the original roofing and condensing when the interior is air conditioned. I would just use a product such as Todol's Purstik (a one-part foam adhesive compatible with all rigid insulation products) and lay a generous bead or two around the entire perimeter of the original roofing as you lay the PIC down. It will set quickly in hot and humid weather, so only do a section at a time. We use that even on a smoother roof deck, shooting a bead around each panel of PIC and several lines in the middle. If you want belt and suspenders, go around after the 1st layer of PIC is down and shoot black one-part foam (UV resistant) around the perimeter of the original roof.

I'd work on the inside. 2.5" Thermax white finish polyiso can be left exposed and looks decent (better than bottom of metal roof), provides proper thermal/fire barrier (at least here it does), can be air sealed at seams and seams covered with wood strips to mimic existing look and feel. I think I'd try to leave the air space to existing roof and install across all existing purlins (not between). Air sealing will be tricky but doable and may need some trim work to hide it.

Hesitant to go outside and lay board foam on top of due to all the gaps that will exist between ribs and new foam but air sealing edges could allow it to work ... but then you need a new roof.

You could infill between purlins against metal roof with cheaper foil faced poly-iso in addition to the white finish Thermax inside of that to get better R-value if needed. Could be a better air sealing effort of you did this as canned foam can be used at joists and seams and hidden by the finish layer to avoid a bunch of half-round trim work.

I wasn't aware of this product; very interesting.

The manufacturer's literature talks about 'exposed interior wall applications' -- but doesn't mention exposed interior ceilings -- is that an issue, for instance as to fire safety?

That would probably go into the category of "ask your local code official". I think the same rule applies to walls and ceiling, which is equivalent thermal barrier to 1/2" drywall. We see it used here on foundation walls and ceilings in residential single family but code enforcement for that building type is next to nothing in most parts around here.

There are two issues rolled up in that: First, wall versus ceilings... When Thermax and R-max refer interior walls, that also applies to ceilings. It's just that it's not common to use PIC for interior ceilings.

Second, the ability to leave foam exposed to the interior depends on the facing material. For example, the Mulit-Max product has an organic facing and thus is intended for exterior applications. Any product that is rated for exposed interior walls can also be used for exposed interior ceilings.

R-Max has two products with a white un-marked facings rated for interior exposure: Thermasheath XP (residential) and TSX 8510 (commercial), so whichever one is easiest to obtain or least expensive. I had not considered that you might find this acceptable. I believe the white facing is also foil, just not the reflective type.

I don't recommend leaving inter-purlin cavities empty, so a 1.5+ layer between the purlins. As Matt said, that layer can be foil faced if it saves money. (R-Max TSX 8500 has foil on both sides). All joints should be taped (presumably both companies sell white tape for this purpose).

If you run the 2.5" layer parallel with the purlins, your main seams would be perpendicular to the trusses so covering those seams with wood would preserve the current look. You could either leave the short seams exposed (I don't know how obvious taped seams would be) or cover with wood to get a grid pattern.


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