Thermal Value

[BSVBD]

ASCE 7-10, Table 7-3 states the following:

“Structures kept just above freezing and others with cold, ventilated roofs…”

I’ve understood this to refer to a typical wood frame building with pre-fabricated roof trusses with insulation on the bottom chord. The “attic” space is the ventilated area that is kept “just above freezing”. Therefore, the thermal value for this roof is 1.1.

In a typical metal building with insulation packed to the roof deck, between purlins, and an unventilated roof, the thermal value is 1.0.

If the ventilated roof is kept “just ABOVE freezing”, wouldn’t that mean that the roof surface would be warmer than the unventilated roof and the unventilated roof slightly colder?

Shouldn’t these values be reversed?

Notice that the warmer greenhouses are at 0.85 and the "structures intentionally kept BELOW freezing" are 1.3. Therefore, why are "All structures except as noted", assuming no apparent heat loss, at 1.0 and a slightly warmer, "just ABOVE freezing", implying a slight heat loss, at 1.1?

What am I misunderstanding?

Thank you!

 

[26sig]

I have always interpreted this table to mean, for the Ct = 1.1 entry, that there are two separate cases: 1. "Structures kept just above freezing" and 2, "Other (structures) with cold ventilated roofs....". Also, the "kept" at a certain temperature, I take to mean chilled or refrigerated to that point.

The coldest roofs are, naturally, the "Structures intentionally kept below freezing". These would be industrial freezers, heavily insulated, and would bleed very little heat, even if there was any to bleed. Most of the time, it's going to be warmer outside than in. It might even draw heat in, even in a snowstorm.

The next coldest, are the "unheated" and "open air" structures. No heat to bleed here.

Then there are the Ct = 1.1 structures, which should logically be a bit warmer than the below freezing roofs. Here also are the "cold, ventilated roofs", which they're assuming would bleed less heat (since it has an airspace to act as last resort insulation, even if vented), than the non-ventilated roofs, with no attic airspace, and the cold air adjacent to the insulated surface.

The "all other structures", being the next cooler set of structures, gets the 1.0 factor. It is assumed that a not-insignificant amount of heat gets out, to melt a bit of snow.

Then, of course, the heated greenhouses, which let out a lot of heat, and in turn melt a lot of snow.

As I interpret it, the "base" category for no heat loss would be "unheated"/"open air", increasing the heat loss as the structures get warmer.

 

[jittles]

Structures kept just above freezing.
Cold, ventilated roofs.

One case is a nice toasty 70 deg F inside the building with a cold, well-ventilated roof.

The other case is a building where the interior temperature is just about at 32 deg F, with whatever roof insulation situation you'd like. The roof itself is not kept just above freezing.

 

[Guastavino]

The 1.1 applies to structures like you indicate that have a ventilated attic (or similar). This is almost like an 'unheated structure' on top of a heated structure. The unheated structures have Ct=1.2.

I'd argue that the PEMB building with insulation bags on the decking roof would be warmer than the attic space where the insulation is down low and the space is vented. When the heat leaves the PEMB it hits the roof directly. WHen the heat leaves Ct=1.1 structures, it goes to a ventilated space and then the roof.

Does that make sense?