Insulating an existing metal roof from the outside

I am rebuilding an existing house. After removing the ceilings I decided I'd like to leave the roof beams and trusses exposed so the entire metal roof is visible. I will need to insulate from the outside. I'm thinking insulated roof panels (probably 4") with new metal over. Does anyone have any experience with this, or know of a source for used insulated metal roof panels? If anyone has a better idea or sees a problem with my idea, I'm all ears. This house is located in Florida.

Chris

where do you live? IE what climate.

Hi Chris R.,

Since you live in Florida, the thermal load on the living space is probably more from cooling than from heating in the winter. You can do what you are thinking, but you will require a very experienced contractor, unless you are planning to do this work yourself. If that is the case, you have some research ahead of you.

To Do List:

1. I would start with determining the age of the structure.

2. You must determine the load limitations on the existing roof trusses, and understand they may require augmentation to take additional loads.

3. Source a stress skin panel company or prepare build your own panels. You can also lay the insulation in place on top of the roof, with additional rafters and strapping to take the new roof. I would recommend a new metal roof as this will be the least heavy. I would also recommend a standing seam, or "hidden fastener" type metal roof, as the other screw down type fail to quickly.

4. Determine load path of walls and trusses, and ascertain the capacities they can take.

5. Find a reputable building inspector that can determine if you have any wood destroying insects or rot in the truss assembly. This can be difficult and must be done by some one that has experience in both renovation and pest control.

That should get you going. Photos of your project would help folk here understand you plan. Feel free to ask more, and be prepared for a hit to the wallet.

Regards,

jay

It's worth it to note that there are still Fed tax credits for metal roofs. http://www.energystar.gov/index.cfm?c=tax_credits.tx_index

If you decide to go that way, read the instructions carefully. Most require a certified installer to install the roof, a warranty to be available over a certain lifetime, specific reflective coatings (a boon in your hot area), and a special form to fill out for tax purposes.

There is a ceramic spray on product I've read about called supertherm. The numbers look good and it is specifically used to insulate seacan homes in your state. It has impressive numbers for resistance against sun energy and marginal against cooling R28 vs R10 or something like that. It looks really interesting, you'll have to ask around for real world results. I live much further north then you so I doubt I'll ever have use for it.

That said if the numbers are good and stand true here is what I would do. I'd spray foam the underside of your steel roof with 2lb spray foam 3" thick so you'll have a consistent durable insulation, vapor and air barrier and then spray to top with the supertherm. It will give you more then enough heat retention at cool times and lots of heat resistance on the hot ones. You could carry the spray foam theory through into the walls if you like and have the cavities sprayed with 1/2lb through drilled holes. 1/2lb foam is a great air barrier and insulation retrofitting product and also allows moisture to diffuse. I would be reluctant to use a densepack in your situation.

If its all done correctly you could have a near passive house performance without having to tear things apart. If you are gonna gut the interior of the house anyhow then definitely spray the 2lb in the walls connecting the whole envelope 100%, this will require and HRV system though so beware the rabbit hole.

Do you really want to insulate over the existing metal roof? Or will you be pulling off the metal and Insulating just above the sheathing? Whichever method you take, look at and address air leakage (infiltration) into your building at the roof. So make sure that the new insulation is firmly connected to the underlying layer. Consider sealing this joint to prevent air leakage just below the new insulation layer.

Hurricane wind rating is probably your biggest hurdle.

What you want to do is/was pretty common with flat roof renovations in the midwest. Remove gravel from the tar roof, add foam insulating and use long screws to hold it down, then tar or membrane over the top. The same thing should work with metal.

I think you are in for a serious battle for proper weatherization to achieve the aesthetic look you are after. It would probably be easier to insulate from the inside and add false architectural elements if you have the headroom of course. Or ripping off the existing metal roof and adding the insulation on the roof deck. At any rate, the problem is creating your air and thermal barrier at the existing metal roof plane.

Sean, what's the deal with the ceramic spray coating you keep suggesting? Those products are a scam.