Thanks for the reply Dana1! The house will be located in central Florida. The house is two story so I planned on building a deck over the first story roof. As well the house will be shaded most of the day. It's in a fairly old wooded area so there are 50'+ oaks with a large canopy and no underbrush. During the day pretty much no direct light hits the ground. On the second story roof I planned on building a false roof to block sunlight as well as catch rainwater. I am looking at painting the exterior with light colored ceramic IR blocking paint.

I hadn't even looked at the operating temp of spray foam. I wouldn't think it would get any hotter than when they spray it on the inside of the roof deck.

What are your thoughts on condensation on the interior steel?

Posted By jonr on 25 Jun 2010 12:30 PM
Whatever buildup you use, I suggest using rot/moisture/termite proof materials in extreme or questionable conditions - but I just came back from a place where they don't use any wood in construction. I agree with painting it a light color although I've never seen any credible support for "insulating ceramic paint".

Insulating ceramic paints have largely been debunked.  I'm not sure if any of them even meet the CA Title 24 spec (although SuperTherm appears to have met the Energy Star's more relaxed standard, and outperforms many white paints.)  I wouldn't pay any extra for nano ceramic BS coatings- hold out for CA Title 24 compliance.

CA Title 24 2008 calls out a minimum initial solar-reflectance (>0.70)and a minimum initial thermal emissivity (>0.75)  as a means of specifying a product as "cool roof".  Those that meet teh spec will result a reliable reduction in peak temps compared to products that don't.   Many light-colored paints meet the spec, but many others fall short on the emissivity end despite being highly reflective. Some of the aluminum stuff is VERY low-E and runs hot despite solar reflectivity above 0.9.

Bottom line, don't count on the  visible-spectrum color alone to make the difference- an all-important half of the sauce takes place in the not-so-visible infra-red.

You can find the products that meet your minimum reflectance & emittance criteria (verified by third party testing) using this search engine.

Posted By Quarantine on 25 Jun 2010 12:47 PM
Thanks for the reply Dana1! The house will be located in central Florida. The house is two story so I planned on building a deck over the first story roof. As well the house will be shaded most of the day. It's in a fairly old wooded area so there are 50'+ oaks with a large canopy and no underbrush. During the day pretty much no direct light hits the ground. On the second story roof I planned on building a false roof to block sunlight as well as catch rainwater. I am looking at painting the exterior with light colored ceramic IR blocking paint.

I hadn't even looked at the operating temp of spray foam. I wouldn't think it would get any hotter than when they spray it on the inside of the roof deck.

What are your thoughts on condensation on the interior steel?

Under even a minimally sloped wooden roof deck you have both some convective cooling and about R1 of  roof deck between the hot exterior & the foam.  Under a flat steel top to a  cargo can you'd have very little to no slope, and effectively zero R.  But if you're shading it from direct sunlight, you're good no matter what.

See my previous post regarding ceramic paints vs. other cool-coatings.  (Some are better than others, some aren't nearly up to CA Title 24 standards.)  Some are basically useless, despite the hype.

I'm a bit confused by "interior steel".  Is the steel the exterior skin of the structure, or are you building an insulating shell over the cargo can, with some other siding on the exterior? 

If the steel is on the exterior you won't have condensation happening on the interior surface in FL.  It typically has to be under 40F outside for that to happen, and when temps rise the condenstion will re-vaporize.  But with either spray foam or cellulose snugged up against it there is no air against the steel for the water to condense FROM (but there may be some inconsequential micro-condensation in some of the closed cells of the foam.)  With cellulose up against the steal any liquid water is wicked away into the insulation as it condenses, and redistributed.  When temps rise, the water re-vaporizes.  Cellulose can absorb water to as much as 20% by weight without damage- if the interior wall reasonably air-tight you'll never run into saturation issues with the cellulose.  In FL there's no need for an interior side vapor retarder of any type with cellulose and a vapor-tight steel exterior (but latex paints would be fine.)

I'm a bit confused by "interior steel". Is the steel the exterior skin of the structure, or are you building an insulating shell over the cargo can, with some other siding on the exterior?

I was meaning the steel skin of the container. I wasn't sure whether I should worry about condensation but from what your saying I shouldn't. So I can have the cellulose up against the steel without issue. If thats the case then I will most likely skip the spray foam altogether.

Also you said the XPS or EPS sheets would be ok. I planned on taping and sealing them, then staggering the drywall and taping/sealing it as well. Do you think it would still be permeable enough or should I not tape/seal the foam?

Posted By Quarantine on 25 Jun 2010 02:20 PM

I'm a bit confused by "interior steel". Is the steel the exterior skin of the structure, or are you building an insulating shell over the cargo can, with some other siding on the exterior?

I was meaning the steel skin of the container. I wasn't sure whether I should worry about condensation but from what your saying I shouldn't. So I can have the cellulose up against the steel without issue. If thats the case then I will most likely skip the spray foam altogether.

Also you said the XPS or EPS sheets would be ok. I planned on taping and sealing them, then staggering the drywall and taping/sealing it as well. Do you think it would still be permeable enough or should I not tape/seal the foam?

If the steel skin of the container is on the outside of all the insulation, the condensation potential on the interior face of the steel is inconsequential in your climate.  (In cold/very cold climates it could become an issue.)

As long as the XPS is less than 2" thick it should be fine on the conditioned-space side of the cellulose.  If EPS, be sure to use only unfaced goods (it comes with poly or foil facers), or you run the risk of creating a moisture trap.  With unfaced EPS you can go thicker (up to ~4-5").  DO tape/seal the seams & edges to make it an air-barrier for best performance.  You want to be air-tight, but water-vapor semi-permeable.  Don't feel obligated to go for the max-thickness on the rigid foam- the stackup would work just fine if you skipped the foam altogether from a vapor diffusion moisture control point of view. (I'm not quite sure why you're using it at all.)

Take extra care with the flashing & sealing of any window/door cuts, electrical & plumbing penetrations, etc. on both the interior & exterior interfaces.  A square inch of air leakage is worth a whole wall's worth of vapor diffusion through latex-on-wallboard, (and worth a whole house's worth of vapor permeation through 2" of XPS.)  Spray foam sealant is your friend...

If cost is an issue, in your climate fiberglass batt insulation would be the lowest cost install. And since you have the steel shell there is no exterior moisture issue. Use steel studs... and drywall that is paperless. That would be a great build in my opinion and safe from all issues.... mold, voc, termites etc.

What do you thiink of that build Dana?

fiberglass batts work well when air can't move and with Florida delta T... and with no wood for condensation trouble or infestation food.  Use formeldyhide free glass and no VOCs.

Next question, thoughts... what would be a great design for cold climate... Northern NY?

Exterior steel frame set a foot off container infilled with an insulation, cells or cells and fiberglass... strap siding off of exo studs for airgap... to stop exterior moisture... no wood, no mold, no rot... no termite issues

Inside, glue paperless drywall to steel.

Posted By adkjacUpstateNY on 25 Jun 2010 04:25 PM
If cost is an issue, in your climate fiberglass batt insulation would be the lowest cost install. And since you have the steel shell there is no exterior moisture issue. Use steel studs... and drywall that is paperless. That would be a great build in my opinion and safe from all issues.... mold, voc, termites etc.

What do you thiink of that build Dana?

With fiberglass the convection currents within the insulation would result in occasional condensation against steel, and has no capacity to wick it away.  Fiberglass also has less than 10% of the thermal mass of cellulose, it's R-value goes down with temperature difference, and batts inevitably have gaps & compressions that increase convection, even further lowering it's as-installed performance.  Steel studs transmit many times more heat than wooden studs, and would need foam-board as a thermal break.  Condensation wouldn't necessarily cause a problem for the steel, but the damp layers of the fiberglass would have almost no insulating value until it dried.  Wet-sprayed ultra-fine fiberglass like JM Spider would probably work if installed at at 1.8lbs density or higher though (a density at which it's convection is comparable to 2.5lb cellulose.)  If you're going with fiberglass, use blown fiberglass.

Dry blown cellulose is a fairly easy DIY project even using a beater rent-a-blower from a box store, at similar or lower cost as batts at equivalent R,  but with measurably higher as-installed R value.  Using 2x3 studs (steel or wood) and leaving a 2" or greater gap between the exterior edge of the stud and the steel, then blowing the whole thing full of cellulose would provide a significant thermal break at the studs.  When condensing conditions occur in the cellulose it retains it's insulation value until it's almost 50% water by weight. (But that level of humidity would cause it to sag- keeping it under 20% is usually recommended- and would  always be the case FL with a vapor-tight steel exterior.)  Dry blowing wet-spray "stabilized" cellulose would be good on a couple of fronts- wet spray goods only use borate fire retardents, which also deters ants, and should it ever gets more than slightly damp from condensation it'll glue itself in place rather than sag as other goods might if blown at low density.  It's also pretty easy to "dense pack" dry cellulose to 3lbs+ density, at which point settling won't be an issue for at least 50 years (or even centuries, with minimal humidity cycling.) 

Either as DIY or contracted out, do specify "borate only" fire retardents, since some fire retardents contain sulfates, which can corrode copper under high humidity conditions.  The stuff sold at big box stores in my neighborhood are all sulfated, but a distributor for National Fiber isn't very far out of the way, and none of their goods use sulfates.

adkjacUpstateNY:

Peak Florida delta-Ts are much bigger than you think if it's anywhere but in a jungle. If any side of that steel can gets ANY sun even indirect light (like line of-sight off the sugar-sand of Panama City Beach 2 blocks away), it can see a real temperature rise on the siding. Something on the order of half the solar gain of a wall or window is from scattered indirect light in many areas. With a hot steel exterior the outer 2" of standard-density fiberglass is rendered neutral, since a significant amount of radiated heat will get into the fiberglass.

But I s'pose if the exterior is coated with a true CA Title 24 compliant cool-roof type finish that aspect would be much reduced.

To insulate a cargo-can in a cold climate, calculate how deep into the R value 35-40F air is on design-day with a 70F interior- any insulation to the exterior of that point should be air-tight foam. eg: In northern NY where the design-temp for heating systems might be -10F, the 40F point occurs halfway through the total R, so half the R needs to be in exterior-side foam, the rest can be anything you like. So something like 3-4" of iso roofing insulation (R20-R25) against the steel (seams sealed), with an interior 2x6 studwall using high-density "cathedral ceiling" batts or spray cellulose or whatever (no vapor retarders) would work for an R40-R45 wall, if that's where you're going with this. As long as air can't get past the interior face of the foam insulation, you won't have frost/condensation issues of any consequence against the steel. (If you use iso you can even paint it black and park it in the sun and still might not blow past the operating temp of the insulation. :-) )

I originally was looking at using glass batts for the insulation for cost reasons, but from what i've seen of the blown in cellulose doing it myself won't cost much more and I won't have to worry about getting it just right. I just have to blow it all in.

But I s'pose if the exterior is coated with a true CA Title 24 compliant cool-roof type finish that aspect would be much reduced.

I will definitely be coating the the exterior with something. I originally was looking at the Supertherm product, then saw some others advertised that you can mix in with normal paint. Like you said the Supertherm product is the only one that seemed to backup it's claims, the others seemed kind of flaky.

For the doors and windows I'm using 1/8" steel plate to make a boxed frame welded into the steel skin for them to sit in. Then I will bolt wood to that and box it out with OSB to the interior wall and stud out the edge.

Depends on conditions, but anodized aluminum is about 9C cooler than a reflectance = 0.70, emissivity = 0.75 paint. But better paints get it down to a couple of degrees C difference (ie, special paint can be almost as good).