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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Taped seams would be pretty obvious. I have seen vinyl 'T' strips used at seams which look decent.

For a glimpse of what this product looks like on foundations check this out,

https://youtu.be/5XFVdUC5upM

I agree that taped joints would be too obvious.

I could make wood moldings to cover the joints. But Rmax makes ‘T’ strips, maybe like the ones you’ve seen, that would look a lot better than taped joints and wouldn’t call attention to the joints the way wood moldings would; Rmax also sells some J channels that might work at the edges:
https://www.rmax.com/s/Rmax-Joint-Closure-Installation-Instructions

I watched the video – very interesting. I was wondering what these companies mean when they say the aluminum facing has an “embossed surface.” In the video, the panels look like they have a really extreme pattern of some sort, like someone experimenting with a big trowel on stucco! Is that what they mean by “embossed”?

I looked at the literature from R-max and Thermax; trying mainly to figure out if the aluminum facing is more like thick foil or thin sheet metal .

If I put these panels on the inside of the existing roof, they’d be completely out of harm’s way -- but in full view, thus showing any damage from hauling and installation (and future cleaning).

The R-max datasheet for TSX-8510 (the commercial product) doesn’t say how thick the aluminum facing is, but claims it’s pressure-washable up to 1000 psi.

The Thermax datasheets report on their ‘light duty’ and ‘heavy duty’ versions of this product. The facings on the light duty panels are 1.25 mil on both sides; the heavy duty panels are 1.25 mil on one side and 4.0 mil on the other. They claim both versions can be pressure–washed at 2000 psi.

Around the edges, I’d probably make some small wood moldings from leftovers I saved while making the trusses.

You can get a sense of what the Thermasheath XP surface looks like in the installation guide (zoom to 600%).

According to the R-Max technical evaluation report (section 4.1), the 8510 has a stronger facing (glass fiber reinforced) compared to Thermasheath XP (residential). I don't know how that might affect appearance, but based on what I've seen in the field, I recommend that you request samples if you're not planning to add a finish ceiling.

The residential stuff is thicker than normal foil but not as rugged as sheet metal. It could certainly be damaged if scraped or dented in transport. I've never seen the commercial grade stuff with 4 mil surface. "Embossed" might be more marketable way of saying "wrinkled". Zooming in on the install manual, like mentioned, is a good way of seeing its surface. A sample would be a good idea if planning on going this route to make sure.

When reading the introduction to this topic I didn't hear a goal of bringing the building to a deep retro-fit level of performance and I have no idea what the rest of the envelope condition is. This idea is based mainly on getting an effective yet moderate improvement to the ceiling considering the logistics of bringing materials to the site and the affordability of options.

Building a new roof on the outside certainly has many advantages and would perform better thermally. There are commercial grade roof panels that have foam embedded in them and they snap together and perform wonderfully. The cost of either of those things and the logistics of getting the materials to the site seem too much for your purpose but that is only for you to decide. If now is the right time for a new roof or if you want to be bringing this building closer to a super insulated level then going to the exterior with higher R-value may be the right choice. 

I wish you luck in your endeavor - I'll try to keep an eye on this blog for my own curiosity but feel free to touch base any time.

I suggest  that you revisit the spray foam roof with a silicone coating.  There are several on the market that are safe for rainwater catchment (NSF approved) and a silicone/foam roof would have a very long service life while being very easy to repair.  It would change the look I don't believe that would be a bad thing (my opinion)   As far as I know some foam/silicone roof systems are rated for high wind applications.

Just this morning I was trying to track down information on rainwater catchment from spray foam roofing, with little success.

Is it the silicone top layer that makes the difference for rainwater? Or are there other coatings that would work also?

At first I didn't think spray foam would be suitable here. I was concerned about losing the ability to collect rainwater that we could drink; and I didn't see how I could easily put another metal roof over slightly irregular spray foam. And how could I attach PV panels to a spray foam roof?

But if we wouldn't need a second metal roof, that reduces the complexity considerably. I'd still need to find a top-notch contractor who wouldn't be fazed by working on an island without his truck and heavy equipment.

Is there a good source for me to learn more about spray foam roofing, particularly wind-resistance, and different surface treatments?

The silicone top layer has the NSF certification (check with the manufacturer)  Silicone roofs have a very long service life and the new ones can be recoated and repaired easily.  I'm not sure about other coatings but I am sure about silicone.

I suggest that you see if you can install the PV attachment system before you spray.  I am sure that there are options for that but I suspect that you would install the attachment first. and then install the panels after the coating has been applied.

A foam roofer will need his equipment but I would talk to them.  Many are accustomed to adverse conditions and challenging applications.  There are several great contractors in Florida.

There is some info on line about this.  As far as I know spray foam roofs have performed very well over the past 15 or so years in hurricanes.  The Super Dome in New Orleans comes to mind.

SPF has very good adhesion and silicone adheres very well to SPF. 

You would also be getting a new roof out to it..........

I am enjoying the thread.  I cant wait to see how your project turns out. 

@Bill, for a specialized application like this, I recommend first contacting an application engineer and/or sales rep at one or more foam system manufacturers. Beyond discussing your technical requirements, a regional sales rep would be able to steer you to some good installers in Florida. Getting a reasonably flat surface on a pitched room requires skill & experience. You would want to use 2.5 lb density or higher (closed cell) to support light foot traffic for PV installation and maintenance.

Aside from the majors (Johns Manville, Dow, BASF, etc), I can think of several companies that specialize in spray foam insulation: Icynene, Demilec, Accella... See https://polyurethane.americanchemistry.com/Members/ for more.

Feel free to contact me if you would like additional info.  I can think of at least one good foam contractor in your area.

Bill, Most foam applicators use plural mixing machines with generators 25KW or bigger I believe logistics and cost will rule this out.

You did a great job structurally with your screw pattern to purlins and deck that's why you haven't lost any roof!

I think this is the best roof that would avail itself to your skill sets and time therefore this is what I recommend....

1) power wash existing roof  then roll out over Aluminum roof Ames Liquid Rubber 2 coats.

(this seals screws and stiffens the aluminum) and becomes a permanent barrier without redoing prior work.

2) Overlay thin 1# density EPS to match height of metal ribs between ribs. Then install 2-3" PolyISo held in place by 1"x4" purlins directly over existing purlins screw together purlin to purlin staggered pattern 8"OC proper length 5" or 6" countersunk wood screws. 

3) Overlay new metal roof. Same screw pattern you did on Original roof.

The salt spray from Hurricanes has most likely eroded existing roof.

4) Install sheet EPS between purlins interior 1 1/2" thick? Same as purlin depth. Then a layer 1" thick EPS, new interior finish prefer metal again with purlins. I would use metal c channels all edges.

EPS in a fire will produce smoke similar to wood fire, urethane Polyiso produces thick black smoke very acid please bear this in mind for personal safety.

I know this is labor intensive but interior can be done over longer span of time. Rain water capture, do you check the acidity? Just curious.

I’ve put in an inquiry to a reputable local spray foam contractor and am waiting to hear if he has some way to spray this roof without his truck and rig on site. If there’s some way to do that, I think spray foam would be the easiest way to insulate this roof – although but my wife is not convinced about the aesthetics, and I don't know yet about the cost.

If that’s not possible, I’m intrigued by your method. Adding a second layer of purlins would be less cumbersome than putting a sheet of heavy plywood over the sheet foam. Another method that was suggested was to screw rails to the existing roof and install the foam between the rails. However, that would create a lot of places where the foam has to be sealed to the rails; also, if the rails were mounted perpendicular to the purlins, then I’m not sure how the roof would be fastened. I know very little about standing seam metal roofing but there may be hidden attachments that could span the rails and still withstand our high winds.

With your method, I’d end up with a space between the foam and the new roof (the thickness of the new purlins). I don’t know if that would be a problem as to the insulating value, or if it would harm the wind resistance.

If I were to install this system on the outside, why add insulation to the inside as well? Couldn’t I just make the outside insulation thicker and skip the inside completely?

As to the quality of the water collected off the roof – I have the local health department test it occasionally, but I’ve never asked about acidity. I have mechanical filters in each downspout; the water is then routed to two large polyethylene tanks that sit on the ground. I add a bit of chlorine to those tanks, then filter the water again before using it. All pretty simple, and it seems to work pretty well.

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