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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Brad, I was only talking about using EPS between the ridges, if necessary, to conform to bolts. Would still use PIC for the 4"!

I'd consider a reflective roof.   Depending on their expected usage of this building, a reflective elastomeric roof paint such as "Kool Seal" might be sufficient without adding another roof surface or insulation.   Here's a write-up in GBA.   

http://www.greenbuildingadvisor.com/articles/dept/building-science/...

Kevin,

Is the "Kool Seal" product certified for potable water collection?

Good question.  I don't know the answer.   I'd love to know whether there is a reflective roofing paint that is safe for potable water.   Sounds like I need to do some research! 

@Kevin, that article is behind a pay wall. Here's the original posted on Allison's blog:

https://www.energyvanguard.com/blog/the-best-way-to-cool-your-attic

No doubt Kool Seal or similar would reduce radiant gains but nothing in that article suggests this would be a suitable substitute for insulation at the ceiling, especially in an off-grid scenario where the levelized cost of powering an air conditioner is somewhere north of 50 cents per kWh.

Regarding suitability for rainwater collection: the owner is a member of this forum so hopefully he'll weigh in on this. For example, if he hauls all his consumed water and only uses rainwater for toilets and shower, then a reflective paint may be acceptable (in addition to the insulation of course).

In an earlier response, I noted that we COULD haul drinking water and use the water we collect off the roof for everything else, if a new roof surface couldn’t safely collect drinking water – but that would be a step backwards in self-sufficiency, which we’re reluctant to take.

The existing roof is bright white (well, not so bright after 35 years). I’m sure it was never as reflective as new reflective paint; a new coating would probably help, but I don’t think it would get me where I need to go.

After David Butler enlightened me about the effects of MRT from my exposed roof, I bought a simple non-contact infrared thermometer and played with it on hot days. The temperature of the walls, floor and furniture are nearly identical to the indoor air temperature -- but the underside of the exposed roof fluctuates all over the place. When there's cloud cover, the ceiling is often 5 degrees hotter than the room; but once the sun breaks through the clouds, the ceiling can reach 115 degrees and more in a hurry, and I’ve seen 125! This must explain why the house itself is too hot to enjoy during summer days, while my workshop below the house feels almost comfortable even when its 90 outside.

Gentlemen, and ladies,

So, as I read this, I see a perfectly good roof that works in serious weather to do what it needs to. It has proven itself. The owner seems to like the interior metal exposure, but that doesn't preclude working from the inside.

How big are the beam trusses? How much room is there to insulate from the inside as well as necessary, then cover with metal?

Or am I missing something?

Walter

The purlins are 1 3/4" thick. I believe the top chord of the trusses are 10" deep (but possibly only 8").

I had earlier considered filling the spaces between the purlins with foam and covering the foam with panels (metal or wood?), leaving most of the purlins exposed. The purlins are all-heart pine and are quite beautiful, but now I’m thinking that amount of insulation wouldn’t be enough to do the job. Unless there’s some type of foam that would insulate well with thinner panels?

Filling the entire space between the purlins AND between the trusses could provide ample insulation, but we’d lose the entire aesthetic effect of our ceilings, which we’re pretty attached to!

If you insulate master bedroom and family room from below, you definitely need more R-value than what can fit between the purlins. Also, the purlins themselves are thermal bridges. The method I described would leave the bottoms of the top chords exposed (as well as diagonals and bottom chords of course). Not the same look as you have now, but we're only talking two rooms, not the whole house. And it would surely be a lot simpler than working from the top.

In the other rooms, you could install PIC between the purlins as you describe. If you cover the insulation with metal panels, flush with the purlins, you'd preserve the look you have now while making the rest of the house more tolerable in summer. 

Of all the types of rigid foam insulation, PIC has the highest R-value per inch (6+). Considering ceiling material thickness and purlin true measurement, you could probably go with 1.5". If it would work, Johns Manville sells a 1.65" PIC product that's rated at R-10.

So this building has a few issues with air leaks and air leaks have so much to do with discomfort in a home, and adding more... enough electrical power to make this little place comfy? 

Is there any way to hack a separate secure space which you could seal and insulate and secure, and then hack a propane powered AC to keep this particular space cool?  You could vent the heat pump to the exterior to get rid of heat and be cool in a space created for comfort. I guess everyone would have to get together well in a small space... :)

@Stuart,I don't think there's any concern about impact of 4" build-up on elevation aesthetics, certainly not to the extent that they would consider changing window heights. But your caution re: potential for connectors to distort metal ceiling is well taken.

The owner intends to isolate the family room and master bedroom for cooling. Any upgrades to windows and floor insulation would be done in those rooms first. The plan is to go with two small single-splits. That way he could opt to run one at the time to save on inverter cost.

Re: propane A/C... Robur makes an absorption chiller that's powered by natural gas. Presumably it can be converted to propane, but it consumes more electricity than the mini-splits by the time you add a pump and hydronic fan coil! Also, the owner has to haul propane to the site by boat.

@Walter, I'm not sure how deep those double top cords are but looking at the 2nd image, they appear tall enough to accommodate 8" R30C batts with enough remaining depth to slightly recess a new ceiling, thus keeping the beam bottoms exposed. It hadn't occurred to me to suggest making the new ceiling metal, to keep the current look. Thanks for the suggestion. I'm in no position to judge, but this approach may be appealing in light of the challenges involved with working from the top. Moreover, working from below would allow the roof insulation work to be done in just the rooms with A/C.

Well, @David Butler, you have seen the property more accurately than I have, but my impression of elevations for flashing material under the windows of the copula makes a 4" lift to the existing roof deck problematical without structural issues there. I don't see that great and I am probably distorting what I see from the images. exterior - roof & cupola.jpg, and  exterior - roof bolts.jpg but the window frames do not look that high above the existing roof. 

It looks like you have the cooling products down quite well. :)

Walter's idea of working on the underside and adding new metal roofing for a ceiling sounds great. I would even suggest a couple layers of polyiso here and sealed well to stop air leakage. 
It is a tough decision what to do inside because of the great look of the hand built trusses, but the bedroom space makes these issues more isolated; both limiting the insulation and air leakage issues, but also isolating the degradation of esthetics. 

I think you have this well in hand, David. 

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