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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You might consider a commercial grade roof foam spray (higher density than normal CC Foam) with a coating over that - if they want metal over that, that's fine as there are some special clips & rails they can use but the clips need to be installed first, the rails afterward

Interesting idea. Commercial roof on a slope!

Typical density for roofing spray foam is 2.5 to 3.0 lbs/ft3, depending on expected foot traffic, with R-value of about 6.5 per inch.

On 'flat' roofs, the mix is normally set for a slower set-up to allow the foam to self-level. But on a sloped roof, the set-up would need to be faster. I'm not sure how much that would impact surface flatness. Also, faster set-up means shorter lifts -- probably 3 lifts for 4 inches.

The coating layer is designed to protect the foam (especially from UV), so a steel over-roof would be overkill. It would prolong the life, but a periodic re-coat would obviously cost less. It may come down to aesthetic preference.

This is Bill Spikowski – David Butler has been helping me figure out how to make my getaway place more comfortable during the endless southwest Florida summers. I’m overwhelmed by the helpful responses here on how to insulate my roof!

I hadn’t ever considered spray foam for this application. In the mid-1970s I remember looking into it for a customer who had a leaky geodesic dome, which had some slopes steeper than my roof. The dome owner rejected spray foam on aesthetic grounds, regrettably never coming up with a better solution.

Spray foam has some appeal to me if a pro could apply it using equipment that would fit in a small boat and run off my limited power supply.

John Hatfield said the foam surface wouldn’t be acceptable for collecting drinking water. That’s a pretty big negative for us since we have no other water source, but not necessarily a deal killer because we could haul drinking water and use the water we collect off the roof for everything else.

I’d be more interested in spray foam if a second metal roof could be (VERY) firmly attached on top of the foam. Hurricane Charley showed us what 155+ mph gusts do to building envelopes. Our existing roof was one of the few in this community that survived intact, which is amazing since it’s really thin aluminum sold as barn siding. I credit the roof’s survival to extra-extreme fastenings along the edges and merely really-extreme fastenings everywhere else. I had figured on replicating that fastening system if I end up putting a second metal roof over insulation.

The second metal roof would probably have to be aluminum, not steel. Aluminum that’s painted really well holds up OK in our salt air. The only steel that has survived so far has heavy hot-dip galvanizing; there may be lesser coatings that would let steel survive but I’m reluctant to experiment with something as important as the roof.

The solution maybe installation of foam fill panels between the existing raised metal ribs within the existing metal roof, then support rails with a polyiso fill with a new metal roof. Working previously in the roofing field I found that not all roof surfaces are acceptable for human water collection. Spray foam roofs are and application that would not be suitable for drinking water as wells as the shingles, EPDM or other single ply products. Collection of water for human conception has a whole different set of roofing requirements and not all materials meet those thresholds or potability.

Hi John, thanks for pointing out my faux pas: I made clear in my original post that rainwater collection is a critical requirement, thus my follow-on comment that a steel over-roof would be overkill is clearly wrong.

I'm sure the owner would prefer rigid panels over spray foam since he could do that work himself. He and his wife physically built the house, and he worked as a carpenter earlier in his career.

How would you trim out the edges? I'm thinking the support rails could be arranged so the end rails are recessed an inch to hold a 1x5 fascia board ripped to fit. OTOH, I think it would look better make the top roof slightly larger than the original with end rails flush with original roof, then a larger fascia board would extend down over the original fascia. Does this make sense? (disclaimer, my carpentry skills are amateur rank at best!)


I think the finish edge application is critical but several un-knows exist. Such as how far does the edge overhang and what does the enclosure need to look like in relation to the rest of the house? What is the wind uplift condition to determine fasteners. I have used a screw that was 8 inches long but very stout #14 roof screws to accommodate roof uplift. Larger fascia boards are a problem I have yet to find a great solution when using SIP panels so yes the fascia is a critical field design element depending on other materials used in the house.

The existing metal roof probably overhangs the outer edge of the lowest purlin about an inch, maybe a little less.

As to aesthetics, the existing roof looks nice from below, but from its edges and above, it’s merely functional and doesn’t need to look much better. So for instance, I could remove the lowest row of fasteners and slip a painted aluminum U channel below the existing roof. The ‘bottom’ of the U would become the exposed fascia; one side of the U would be sandwiched against the purlin and be held in place with the screws that hold down the existing roof; and the new roof would be screwed to the other side of the U, probably overhanging it by an inch so rainwater would drip off the edge into the large gutters.

Or, if I used support rails and they were aluminum, maybe one of them could be painted and replace the U channel I just described and serve as the fascia.

John Hatfield has the right idea here for a new roof deck. Polyiso boards always need a good stout nailer on all roof edges. Whatever the final roof height here you have clobbered the elevations on the upraised roofline. 

And any work on the interior to raise the window heights requires delicate interior work. But the owner only has to please himself. :)

Is there an issue with screws distorting the existing metal roof with backing compression only provided by polyiso? This could possibly mess up the nice even lie of the existing metal ceiling. I guess the carpenter will be using properly sized nails with threads only in the last 1" and carefully aligning screws with the existing purlins?

As to the elevations on the cupola – the existing windows up there might survive adding 2" of foam, but not 4"! I never did trim out those windows either inside or outside, and they’re crappy windows anyway, so I’m going on the assumption that they’ll need to be replaced with something a little shorter and a lot less breezy than jalousies.

As to distortion – yes I’m a little concerned on several points. The existing roof is already somewhat ‘wavy’ because the purlins aren’t dead flat -- because the roof trusses were constructed in place (read, me sitting on a 20' high scaffold in the woods with few power tools and just enough river-recovered heart pine timbers to build them – no replacement timber available!).

I guess I’ve been assuming that foam panels would be flexible enough to follow the wavy contours of the existing roof, but stout enough to support themselves on the twin raised ridges without crushing the foam and without the foam collapsing into the spaces between the ridges. I don’t know foam at all, so if those assumptions are unrealistic, it would be better to learn that as soon as possible....

As to fastenings – the existing fastenings are stainless steel screws that go into heart-pine purlins that sit on the trusses. The purlins are spaced about 24" apart. I had assumed the new roof would be fastened the same way as the old one (with screws that are 4" longer). I don’t see how this method would have any effect on the existing roof, although I do worry about hitting an old screw with a new screw. An unknown for me is the strength of the foam panels, because the new screws would be going through them BETWEEN the twin ridges on the existing roof panels. Unless the foam panels were extremely stiff, they’d distort the foam into the void between the ridges, creating gullies in the new roofing. Maybe now I understand why John Hatfield suggested putting thin pieces of foam in those spaces!

It shouldn’t be that hard to fasten thin foam panels between the existing ribs, but if I put 4" of new foam over the entire roof, how important would the extra thin foam be? The existing screw heads would probably keep the extra foam from sitting flat, so I’d have to carve out a spot on the underside for each screw.

What kind of ‘support rails’ are you thinking of? Would they be run parallel to the existing aluminum panels, or cross ways? Would they provide a flat enough surface for a new metal roof that the new foam between them could be sprayed in place instead of using foam panels? (Clearly, I know very little about roofing.)

Filling those shallow valleys with rigid foam not only prevents primary foam panels from 'collapsing into the valleys' but fills that gap. You want to minimize air pockets in your foam sandwich to reduce potential for R-value robbing air currents. That's another reason to seal the ends as Brad noted on page 3 (more than just covering with a fascia).

Hopefully someone with field experience will comment on this, but my sense is that when you screw down the sandwich, the thin foam boards should mash onto the bolt heads. You could easily test this. If PIC doesn't mash down flat, then EPS certainly would.

It kind of depends on the PIC. Not all insulation is made the same. Here in VT (and all of New England I hear), Dow no longer distributes their PIC. Lumberyards here have been carrying PIC from R-Max, Atlas, IKO, etc. There is a huge difference between the Dow Tuff-R and the IKO Enerfoil. The latter might compress more to conform to screw heads, but I doubt that even the IKO will deform enough to fill the ups and downs of the corrugated roofing. EPS has half the R-value per inch of PIC => you need twice the thickness.


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