QUESTION.

[pi71]

Ive got a metal roof (r panel) on a metal framed cabin. I am going to have the spray in insulation . this is an open ceiling roof. can i just have theinsulation sprayed in and then install my ceiling without venting this? what would be the point in venting this space when it is full of icenyne insulation?

thanks.

[VALENT]

In reality, if there is no air space, the venting would be a waste. However, you may want to consider adding an air space between the insulation and your actual ceiling and allow for venting as this will remove a lot of heat and cold that will be getting through the insulation and r-panel.

[bkrahmer]

Hey Valent, didn't you mean to say between the insulation and the roof, not the insulation and the ceiling?

[VALENT]

I guess it would really depend on where the air is able to enter at the soffit and where it would leave at the gable or ridge or vice-versa.

[bkrahmer]

My theory on venting metal roofs is that the metal is such a good conductor of heat that you want the airflow going right next to it. If air can freely flow between your insulation and your ceiling, you may as well not have insulation. Anywhere air can freely flow, heat can freely flow as well.

[VALENT]

My theory is in agreement with yours bkrahmer. However, I consider the venting as being an ability of the air flow to carry the heat away(that is right next to the roof) before it continues downward toward your ceiling. In my theory, I think it is best to allow the venting to remove as much heat before it comes in contact with the "conditioned living space."

[Sweep]

Venting doesn't have much effect on heat transfer to the living space since it is mostly by radiation. It's purpose is to cool the bottom of the sheathing a bit and possibly extend the life of the roofing/sheathing material (I've seen no proof of this) as well as allow fiberglass insualtion to dry if it gets wet. With foam insulation it shouldn't be necessary. The whole concept of a vented roof is being challenged by researchers and may disappear from the building code some day. I am more concerned if the insualtion manufacturer has a maximum temperature limit for its material and if your location and roofing might allow it to be exceeded.

[VALENT]

sweep, how do you figure that most heat transfer is by radiation and not through conduction. If this were true, then almost all insulation would have zero positive effect.

[Sweep]

The dominant form of heat transfer in an insulated roof system is radiant because the insulation prevents so much of the conductive heat transfer. Venting the bottom of the metal roof will do little to further reduce heat gain inside the house since it will not reduce radiant heat transfer.

[danh]

I further the argument that the dominant heat transfer mechanism is radiation. You need something that will prevent the heat absorbed by the metal roof from being emitted to the living space....

...something with a low emittance....

...like a radiant barrier?

And VENT THE HECK OUT OF IT.

Texan attics are too big to do a conditioned attic system. Of course if you live anywhere but the northwest of texas, and you have HVAC up there, flooding your attic with humid air is a mold problem waiting to happen. If you live in Northwest texas the humidity is pretty low...so even if you do have HVAC up there, you might want to go radiant barrier anyways.

[rbisys]

Greetings,
Sweep and Valent you made my heart go pitty pat.

I have been selling and install'g radiant barriers for 30 yrs. Mostly sales now.

You're rt about down heat by radiant. Actually it's over 92%.

The reason you need venting with bulk insulations is that you can't VB the ceiling without pooling condensation. The moisture that passes thru the ceiling has to be removed. With radiant barrires the moisture passing thru is almost nil.

I have written a paper describing the process of these materials along with a simplfied enegy calculation chart. If you would like a copy e-m me your address. I also have a manul for a simplified method of install RB along with sources for materials. I'll include samples too.

As a start, from a mech engineer'g handbook, glass, wood (paper) less than 10% eff. Foam, from petro, about 20%. Alum 97% eff.

FG, CEL, FOAM about 1% mass, 99% airspaces. Rad energy travels at about the speed of light. And we wonder why our bills are so high.

[VALENT]

ok, i'm missing something here. We agree that the metal roof conducts the heat right through. Then the heat is transferred to the air which is right next to the metal(inside the attic). Then the air continues to heat further away from the metal roof and closer to the ceiling insulation, the ceiling drywall and finally right into the living space. My point or theory is that if you allow that initial heated air(right next to the metal roofing) to vent upwards (as heated air supposedly does) it would be taking much of that heat "out" of the attic before it continues down into the living area. Where is my misthought with my logic?

[rbisys]

Greetings,

Not there yet.

Depending on the metal it will EMIT as radiation small or larger amounts of energy. It is this radiated energy that is absorbed by the insulation and then conducted or radiated to the drywall and then to the room. Hot air does NOT RISE DOWN, to coin a phrase.

If you ck Marks Mechanical Engineering Handbook (emissivity chart) you can determine the % of energy materials absorb and re-radiate.

If I remember correctly galv and paint have a high absorbtion and emissivity rating. How ever galvalum should have a lower rating. I haven't been able to ck it though.

If you have FG as your insulation then the material will absorb and re- radiate over 90% of the energy. That's why FG mfgrs do not talk about radiant energy. FG is about 1% material and 99% airspaces so the radiant energy passes thru at about the speed of light. On a 95deg day, no shade, a FG insulated ceil'g will get up to about 110 Degs. If you have a 75deg floor the ceiling will radiate about 37 btu/hr/sf, ( engingeer handbook formula). That's why an a/c will run all day and half the night.

However if you have a RB in the airspace above the drywall, no FG, the RB will radiate about 2-3 btu/hr/sf to the drywall. This is why RB insulated homes use so little a/c and and considerable less heat'g.

I have written a paper on this subject, not technical. If you want a copy e-m me your address and I'll send it along with a manual on an advanced method of install'g RB so there is 99% reduction in conductive/radiated surfaces, studs and joists, of walls and ceilings. I'll include the names of the RB mfg's so you can try them out if you wish.

There are many different types of radiant barriers. If you want know which is best for a job, ask me.

[rbisys]

Geetings,

I missed answering one of your questions.

Vented attics CANNOT remove the energy faster than it is absorbed and RE-RADIATED to the insulation. Consider the amount of surface compared to the venting abilities.

If this was possible then you would not need insul'n in summer.

[pi71]

ok my original question, metal roof (r panel) bolted onto a metal cabin frame. no decking, i want to spray icynene from inside and fill the whole cavity of space and then put the inside trim. no air space,no vent,, cathedral ceilings, im talking about 4 to 5 inches of space filled with icynene insulation and then wood interior planking