closed cell spray on foam insulation

[RSR]

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.

It's not quite that simple. Convection is a function of the temperature change (delta T), given by q" = h(T_surf - T_air), where h is the convection coefficient, which can easily reach values of 10-100 W/(m^2 K).

Radiation is a function of temperature to the 4th power given as (under some approximations) q"=eps*sigma* (T_surface^4 - T_surrounding^4), with eps being the emissivity of the surface, which varies between about 0.3 - 0.8 for painted metal. (For highly polished metal it's <0.1, hence my earlier comment).

While this would suggest radiation is more important, sigma is the Stefan- Boltzmann's constant, which is 5.67 x 10^-8 W/(m^2 K^4) (very very small). In practical use, radiation is only more important than convection if there are large temperature differences.

It wouldn't take much of a fan to move enough air across the underside of a roof to offset the radiative heat losses and avoid condensation. The question is how practical it would be to access power, etc. and set it up.

 

[pmsmechanic]

I have a 30 x 60 storage building where the dirt floor is always damp. Someday when I get around to it I will put some plastic on the floor and then cover it with reject sand. That will stop the moisture migration out of the ground.

In the meantime I was at a lumber yard sale and I found two 14" wind driven ventilation fans. ($5 each new. I couldn't go wrong). Since they are turning more or less continually they have removed a significant amount of moisture from the building in the last few months.

Meanwhile I will learn something from this thread.

 

[ovrszd]

I have a 30 x 60 storage building where the dirt floor is always damp. Someday when I get around to it I will put some plastic on the floor and then cover it with reject sand. That will stop the moisture migration out of the ground.

In the meantime I was at a lumber yard sale and I found two 14" wind driven ventilation fans. ($5 each new. I couldn't go wrong). Since they are turning more or less continually they have removed a significant amount of moisture from the building in the last few months.

Meanwhile I will learn something from this thread.

I've got a machine shed that needs a couple of those. You scored big time!!!!

 

[RayfromTX]

It's not quite that simple. Convection is a function of the temperature change (delta T), given by q" = h(T_surf - T_air), where h is the convection coefficient, which can easily reach values of 10-100 W/(m^2 K).

Radiation is a function of temperature to the 4th power given as (under some approximations) q"=eps*sigma* (T_surface^4 - T_surrounding^4), with eps being the emissivity of the surface, which varies between about 0.3 - 0.8 for painted metal. (For highly polished metal it's <0.1, hence my earlier comment).

While this would suggest radiation is more important, sigma is the Stefan- Boltzmann's constant, which is 5.67 x 10^-8 W/(m^2 K^4) (very very small). In practical use, radiation is only more important than convection if there are large temperature differences.

It wouldn't take much of a fan to move enough air across the underside of a roof to offset the radiative heat losses and avoid condensation. The question is how practical it would be to access power, etc. and set it up.

RSR thanks for the refresher equations. I was trying to keep it in simple terms, especially because other factors play such a significant role such as the temp of the ground, dewpoint of the air, emissivity of the roof metal top and bottom, air tightness of the building, etc.

In the end, the foam will have several benefits that may be greater than eliminating condensation and a risk as well. In my garage, I put up radiant barrier paper and I have found the building significantly cooler than my shop in summer and free of condensation in the winter. It was quite cheap at the time and 25 years later it is mostly still intact.

I enjoyed your comments. Thanks very much.

 

[Jchonline]

I have a similar question. I have a stand alone garage that has a standard shingle roof with a ridge vent. For now heating is with a few kerosene units when we are up there. I have R-23 insulation on the lower 12 feet up to the eave (rockwool). I am considering closed cell foam on the gable walls and the underside of the roof (ie gable style) to insulate the entire building. I am assuming the spray foam will not cover the ridge vent along the top, but wondering if this is just going to let all of my heat escape anyway if left open? I really want to keep the ridge vent open if possible for circulation. Thoughts?

 

[ovrszd]

I have a similar question. I have a stand alone garage that has a standard shingle roof with a ridge vent. For now heating is with a few kerosene units when we are up there. I have R-23 insulation on the lower 12 feet up to the eave (rockwool). I am considering closed cell foam on the gable walls and the underside of the roof (ie gable style) to insulate the entire building. I am assuming the spray foam will not cover the ridge vent along the top, but wondering if this is just going to let all of my heat escape anyway if left open? I really want to keep the ridge vent open if possible for circulation. Thoughts?

There has to be a separation between the "heated/insulated" area and the atmosphere.

It will be non productive to add insulation to the underside of the roof and leave an escape hatch open at the top.

For this added insulation to be of value there needs to be a ceiling added. Then insulate the ceiling. Then let the condensation etc. vent thru the ridge vent.

Closing this vent at the ridge and foaming the underside of the roof could cause serious ventilation problems which will make you wish you hadn't. This happens when buildings are sealed too tight. Then an ERV has to be added to ventilate the "too tight" building. Kind of counter productive.

 

[quicksandfarmer]

There has to be a separation between the "heated/insulated" area and the atmosphere.

It will be non productive to add insulation to the underside of the roof and leave an escape hatch open at the top.

For this added insulation to be of value there needs to be a ceiling added. Then insulate the ceiling. Then let the condensation etc. vent thru the ridge vent.

Closing this vent at the ridge and foaming the underside of the roof could cause serious ventilation problems which will make you wish you hadn't. This happens when buildings are sealed too tight. Then an ERV has to be added to ventilate the "too tight" building. Kind of counter productive.

This is good advice. To add on, shingle roofs need to be vented to keep the shingles cool, they last a lot longer. You want vents at the eave to let the air in and vents at the ridge to let the hot air out. The traditional way to do this is to have a ceiling below the eave that is insulated and an attic that is unheated. If you want to insulate the roof itself you can put these channels:
https://www.homedepot.com/p/ADO-Pro...l}:rt+dln:{542332}+qu:{insulation+channel}:qu

between the roof and the insulation to allow air flow. You have to take great care at the eave to make sure the vent flows into the channel and nowhere else. They always struck me as kind of cheesy.

I guess you could use the channels with spray foam, but that brings me to another point: spray foam is the most expensive form of insulation. It's biggest advantage is that you can just spray it in places where insulation is difficult. Generally on roofs you want a lot of insulation, heat rises. If you go with an attic, insulation is easy -- just layer the batts or pour in insulation. By way of comparison, spray foam is R6.5 per inch and the kit I used cost just about a dollar per board foot (which seems to be the going rate). At Home Depot I can get R30 fiberglass insulation, 15 inches by 25 feet, for $29.51. The get the equivalent R-value would take 4.6 inches of foam. To cover the equivalent area would take 145 board feet of foam, which would cost almost five times as much.

 

[ovrszd]

Good stuff farmer.

In my studies I learned that spray foam has an R value of 6.5-7 per inch. Okay, I can deal with that. What caught me by surprise was that it gets 90% of it's R value in the first 2". Wow. So you basically get 13-16 R value regardless how much you spray...

Properly installed batt or blow-in added to a properly constructed building exceeds that considerably.

My shop has R19 walls and R45 ceiling for .37 cents per square foot.

I could have gotten spray foam at R value of R16 with foam at $1 per square foot.

Decisions, decisions.

 

[Jchonline]

Great advice. I will rockwool the ceiling of the main garage. Then enclose the 300 sq ft attic space in the same. I have a 10/12 pitch with attic trusses sonI can make that upper level into a room. I will enclose it on all sides with rockwool. That will leave air between all insulated spaces and the roof underside as well as the ridge vent in tact. Thanks all!