U-factor alternative 2009 IECC

[Coder]

Just looking for some feedback. I have a set of plans here showing a 2x12 rafter roof assembly insulated with fiberglass R-38 batts. According to the IECC, (all other sections of the code considered) The prescriptive table calls for R-49 here (insulation r-value only). However, if you go to Section 402.1.3 U-factor alternative (entire roof assembly u-value) and work the numbers for this same assembly, R-38 will work. Am I wrong in saying that even though the prescriptive requirement is R-49, The U-factor from Table 402.1.3 of 0.026 ( inverse of R-38 + the R-values of the other components) for a U-value of equal to or less than 0.026 will suffice?

My post here sounds more confusing than it really is. apologies in advance to those who attemt to decipher it. :cowboy

 

[tmurray]

Are you taking into account the 2x12 to develop a composite u/r value or just doing a straight calculation. I'm not going to be much help as we don't use the IECC here, we have our own energy efficiency provisions. To develop a composite r value using ASHRAE methods you'd need to know the percentage that each component (batt vs wood) takes up within the cavity and use the following formula;

100/((%a/Ra) + (%b/Rb))

also does it specify two different depth for insulation; one for slopped roofs and one for flat roofs. heat flow will, for the most part, go straight up, so if it is a sloped roof the insulation typically shouldn't be measured perpendicular to the framing members but straight up and down.

 

[lunatick]

Are you taking into account the 2x12 to develop a composite u/r value or just doing a straight calculation. I'm not going to be much help as we don't use the IECC here, we have our own energy efficiency provisions. To develop a composite r value using ASHRAE methods you'd need to know the percentage that each component (batt vs wood) takes up within the cavity and use the following formula;100/((%a/Ra) + (%b/Rb))

also does it specify two different depth for insulation; one for slopped roofs and one for flat roofs. heat flow will, for the most part, go straight up, so if it is a sloped roof the insulation typically shouldn't be measured perpendicular to the framing members but straight up and down.

And that would then make sense why the code would permit a vaulted ceiling to have less inches of insulation. As the pitch would create a deeper cross section than the amount of materials installed.

Mr. Inspector. Othewise you could look at different types of insulation in the cavity or atop the deck.

 

[Coder]

 

[ICE]

heat flow will, for the most part, go straight up, so if it is a sloped roof the insulation typically shouldn't be measured perpendicular to the framing members but straight up and down.

I haven't heard that before. Does that hold true for all insulation types?

 

[Daddy-0-]

Do you have the reduction exception for 500 sqft in your jurisdiction? If not ask for an alternative using rescheck. They could over insulate in other areas and get rescheck to work.

 

[tmurray]

I haven't heard that before. Does that hold true for all insulation types?

It's a pretty big generalization, but in heating climates it essentially holds true because it's a function of heat and not the insulation. If we are talking about conduction and omit air leakage the only reason you will have heat loss through any of the building is because you get to a sufficient quantity of heat that it becomes better distributed along the other building envelope components. Remember, air is a fluid. Hot goes to cold (It's the law!). Warm air has a lower specific gravity than cooler air so it will be more buoyant and will collect at the highest possible point (this is why sprinklers work). So, the building envelope assembly that is located at the highest point in the building will have the highest heat loss since the heat is basically being forced up against it.

 

[jeffc]

Heat loss can occur through one or all of the following methods: Conduction, Convection, or Radiation. These methods rely on heat going to cold, with no particular direction. Heat loss is determined by ua (conductance of the assembly x area) x temperature difference= heat loss. The stack affect occurs when the air heats up, rises and develops a temperature difference. Stack effect only plays only a minor part in the total heat loss of the structure as the temperature difference is only a couple of degrees.

 

[tmurray]

Heat loss can occur through one or all of the following methods: Conduction, Convection, or Radiation. These methods rely on heat going to cold, with no particular direction. Heat loss is determined by ua (conductance of the assembly x area) x temperature difference= heat loss. The stack affect occurs when the air heats up, rises and develops a temperature difference. Stack effect only plays only a minor part in the total heat loss of the structure as the temperature difference is only a couple of degrees.

Depending on where your are. In the original post I see a mandatory R-49 this is roughly the same as where I am and our design temperature difference is 43 degrees Celsius (22 indoor and -21 outdoor). This method only works when you assume that the temperature at the floor is the same level as at the ceiling. While you are correct in the calculation procedure for determining heat loss (q=U*A*(To-Ti)) this is still just a method for determining approximate heat loss (+/- 30%) and if the assumptions for this method were true the minimum r-values in walls and ceilings would be equivalent. Even though the calculation method is flawed without an actual building this is the most accurate way of determining heat loss.

 

[Coder]

Do you have the reduction exception for 500 sqft in your jurisdiction? If not ask for an alternative using rescheck. They could over insulate in other areas and get rescheck to work.

Yes i do. The current language in the 2009 IECC remains unchanged. Although I am still having a tough time grasping the concept of the allowance of 500 sq ft or 20% to be less insulated. To me this rationalle doesn't make any sense. Complex roof framing designs could benefit from this I guess but good luck trying to enforce this provision in the field. I can just see it now " hey it looks like you have 510 sq ft of R-38 there" Telling someone that their entire roof has to be R-49 except for 500 sq ft is silly IMO

 

[Daddy-0-]

We see the exception often on additions or room over garage. They are trying to match the existing roof line in most cases. This is also used when finishing an attic. The existing rafters are not deep enough for the new required level of insulation. If it doesn't work....rescheck.

 

[Daddy-0-]

BTW. You cannot mix prescriptive with alternative methods. You can use the u factor alternative but you must then follow that path or other alternative methods for compliance. You cannot use prescriptive tables for r value in the walls and then say oops I need alternative methods for the roof because the chart doesn't work there.

 

[Coder]

Not trying to start a riot here but, unless I am missing something, the code Does not say that anywhere though (no mix and match)? Just that it is an alternative other than R-value for an assembly.

 

[Keystone]

2009 IECC 401.2 Compliance. Projects shall comply with Sections 401, 402.4, 402.5, and 403.1, 403.2.2, 403.2.3, and 403.3 through 403.9 (referred to as the mandatory provisions) and either:

1. Sections 402.1 through 402.3, 403.2.1 and 404.1 (prescriptive); or

2. Section 405 (performance

 

[Codegeek]

The way I read 402.1.3 is that an assembly, in this case the roof, can comply with Table 402.1.3 and be permitted to do so as an alternative to the R-value in Table 402.1.1. In my opinion, this would mean the entire roof assembly, not just a portion of it, would have to meet the U-factor alternative. The rest of the house could then comply with Table 402.1.1.

 

[Coder]

The way I read 402.1.3 is that an assembly, in this case the roof, can comply with Table 402.1.3 and be permitted to do so as an alternative to the R-value in Table 402.1.1. In my opinion, this would mean the entire roof assembly, not just a portion of it, would have to meet the U-factor alternative. The rest of the house could then comply with Table 402.1.1.

I am with you.