closed cell spray on foam insulation

[quicksandfarmer]

I don't know about suitability for this application, but I had a good experience with a spray foam kit from FOMO that I bought from Amazon. It was 605 board feet and cost $618 delivered last summer, although Amazon has discontinued it. It comes in two tanks each about the size of a propane tank, about 120 lbs total. Spraying is easy, you just have to be mindful not to put it on too thick, it expands after you spray it, and you want to plan your work so you can spray continuously. If you stop for more than a minute you have to change nozzles on the spray gun. And you have to mask anywhere you don't want it to get -- including on yourself, which means Tyvek suit, goggles, respirator and gloves. I don't think it took an hour to spray out the whole kit, and I did it all with two nozzles.

I understand that keeping the tanks warm is important. I sprayed on a summer day, which was a joy in all that protective gear.

 

[JFoy]

We recently had a 24x60 metal building erected that we're turning into a home. First thing I did was have it sprayed with 1in closed cell foam. Three reasons. First, it shut down the interior condensation. Second, it air sealed the building. And third, it added some peace of mind on the water tightness of the building.

Now, we have some window ledges that are 2.5in steel square tubing to frame out the windows and those were not sprayed. The house isn't finished yet and when it gets cold, the exposed metal around those windows sweats.

A note about the air sealing - the building is so tight, we've had to add an ERV (ventilation system) to the building as it doesn't breathe well. That also means it traps moisture really well.

The building is on a cement slab and even when the outside temp has gotten down into the 20's, using a 900watt heater, the building stayed at 50degreesF minimum.

So yeah, *closed* cell spray foam works great for metal buildings.

Hope that helps,
JFoy

 

[RSR]

RNeumann, you are close but missing a key ingredient. The condensation is caused by a condensing surface that is colder than the dew point of the air. Dew is condensation. The reason condensation forms on the underside of the roof is the same reason it forms on the top side. It is because the metal is colder than the dew point of the air. Here's the really cool thing.

On a clear night, the roof metal is colder than the air. This is caused by the metal radiating it's heat to the night sky. Everything radiates heat at some rate. If you and I are facing one another then we don't notice it because we are about the same temperature and have a similar emissivity. If instead you are sitting and talking to a snowman you would feel cold. The snowman isn't radiating cold. That would be against the law. The second law of thermodynamics teaches us that cold doesn't radiate. What is happening is that you are radiating more heat than the snowman so what you are feeling is a net heat loss. The roof is facing deep space. It radiates it's heat but deep space doesn't have much heat to radiate back in return so the roof experiences something called radiant overcooling. The roof may be as much as 10 degrees colder than the ambient air temperature.

If you check the roof of your car there may be condensation in the morning but the sides are dry. That isn't because dew falls. Dew forms. It is because the sides of the car see other things that are also radiating heat like walls and plants so there is little or no heat loss. The roof of the car is colder than the sides. Another thing that is caused in a same way that you may have noticed is when the temperature is above freezing but there is frost on your roof. The air may be 35 degrees and the roof is 27 degrees because of radiant overcooling. Frost doesn't fall either. It forms in the same way dew forms. Cool stuff, eh? On a cloudy night the bottom of the clouds are significantly warmer than deep space so radiant overcooling isn't a factor. Almost no dew on a cloudy night. There may be drizzle but not dew.

Here's how this matters to the question asked by MFWD. If he sprays closed cell foam on the underside of the roof the topside will still be cold and form dew but the surface that the air will see on the underside will be the bottom of the foam. It will be facing the things in the carport and won't over cool by radiation. It will stay warm enough to not condense out the water vapor in the air because it resists the movement of heat to the cold topside.

There is however a condition when things can still get wet in that carport. That is when the temperatures have been cold and a warm humid mass of air moves into the area that has a dew point that is higher than the temperatures of the surfaces in the building. We see that happen when our porches and other things under cover are soaking wet after humid air moves in. It isn't the fact that the air is warm that causes it to condense. It is the fact that it has a dew point that is higher than the condensing surfaces that we are concerned with. Warm DRY air won't condense on cold surfaces until the surfaces are colder than the dew point of the air.

If you spray closed cell foam on the underside of the roof it will be like putting a coozie on you soda can. At least with the temperatures we deal with. Now if you fill the beer can with liquid hydrogen and call it a fuel tank and hook it to a rocket booster it won't work as well. The difference in temperature is too great. At <-423 degrees f liquid hydrogen is so much colder than the air in florida that the vapor pressure is powerful enough to cause water molecules to move through the foam on the tank and form ice between the tank and the foam, separating the foam from the tank. On takeoff the vibration shatters the ice and the foam and ice fall off and strike the space shuttle, eventually dooming the brave astronauts inside. The second law is to be always respected.

Ray - while what you have said is correct, RNeumann is also correct. As long as the proposed air circulation is sufficient that the convective heat transfer from the ambient air to the roof is about equal to, or greater than, the radiative heat losses, the temperature of the roof will be maintained at the ambient temperature and condensation will be avoided. The practicality of this approach, which would undoubtedly require forced airflow, is another question.

Of course, if that seems impractical, the other option is just polish the roof up to a nice smooth metallic shine so the emissivity drops to almost zero.

 

[RayfromTX]

Ray - while what you have said is correct, RNeumann is also correct. As long as the proposed air circulation is sufficient that the convective heat transfer from the ambient air to the roof is about equal to, or greater than, the radiative heat losses, the temperature of the roof will be maintained at the ambient temperature and condensation will be avoided. The practicality of this approach, which would undoubtedly require forced airflow, is another question.

Of course, if that seems impractical, the other option is just polish the roof up to a nice smooth metallic shine so the emissivity drops to almost zero.

Of course radiation is more effective than convection so moving air in the amounts that could be practically done would likely just increase the available moisture to the underside of the roof without raising it's temperature significantly. The heat radiated to the underside of the roof from the slab is more effective but it is just passed through and radiated to space. It would be like keeping the metal roof of your car cool while parked in the sun by running the vent fan. The sun is obviously more radiant gain than the radiant loss to space but the same principles apply.

 

[3Ts]

One more real life report. I have a 40 x 80 building - it is 40 x 60 enclosed with a 20 x 40 "covered porch", on the humid days where I get condensation dripping from the metal of the covered porch, I do not get any condensation in the 40 x 60 enclosed area that has been spray foamed with about 3" of open cell foam. (open cell is all the contractor would use).

 

[ovrszd]

Following for the education.

 

[RayfromTX]

One more real life report. I have a 40 x 80 building - it is 40 x 60 enclosed with a 20 x 40 "covered porch", on the humid days where I get condensation dripping from the metal of the covered porch, I do not get any condensation in the 40 x 60 enclosed area that has been spray foamed with about 3" of open cell foam. (open cell is all the contractor would use).

If condensation occurs in your 1/2 lb foam application it would happen at the interface of the foam with the metal where you would not see it but it is likely to be too transient to create an issue if it were to even occur. I would doubt that it is ever an issue unless you were in a cold northern climate and had a source of moisture beyond what is in the outdoor air. Now if you close the building in a cold climate and heat with a salamander then you will build up moisture between the metal and the open cell foam but I'd be more concerned with the carbon monoxide. Both would be a problem.

 

[oosik]

WoW!!! Well, once again - I met my daily goal. I learned something new.

 

[mikester]

From experience I would add that 1. yes it works and 2. if you can afford to hire a contractor, do it - doing it yourself isnt worth the headaches, mess, or potential health risks dealing with the chemicals.

The commercial guys use heated 2 part systems and will give you better, uniform controlled coverage than a home spray kit ever will.

 

[EddieWalker]

Yes, close cell foam will reduce or even stop condensation from forming under your metal roof. The reason that most people don't do this is because it's so freakishly expensive. The more common, and cost effective method is to install a ridge vent and vent your eaves. Heat rises, even if it's just one degree warmer, the air will rise. Moving air dries out the condensation. It doesn't stop it from happening, so there will be times that you still get some moisture, but overall, the simple movement of air from your eaves to your peak will eliminate most condensation issues in most places around the country.