Anyone familiar with Florida Building Code or something similar, regarding R-19 roof insulation?

FBC 13-404.1A Prescriptive Requirements Specific to Method A - Efficiencies determined by the FLA/COM Method A performance calculation to demonstrate compliance with this code shall be minimum performance level installed in roofs/ceilings. Multiple-family residential roofs/ceilings shall be insulated. The insulation shall a R-value of at least R-19, space permitted.

This is for a highrise condo project, which called for 4-ply built-up on tapped LWC requiring minimum R-19, which meant the lowest roof drains to have minimum 2" light weight, with 1/4" per foot pitch up.

All local roofers quoted this project qualified as minimum average R-19. That's how they interpreted the code and have done so on many projects. This minimum average R-19 is to reach average R-19 not minum R-19. Architect insists on minimum R-19 but roofers won't price it that way because they never have problem with building depts before.

Any advice?
Original Post
I'm not sure how building departments will interpret that requirement.

However, if the architect specified a minimum R-value of 19, then the contractor is required to provide it, regardless of what the code permits. Code provisions are minimum requirements and the architect (or any design professional) can exceed those requirements if he wishes or at the direction of the owner.

Something to keep in mind is that mechanical systems may be designed based on minimum design insulating values, and providing less than what the systems were designed for may diminish any energy savings intended.

I suggest you provide the Owner with an analysis that shows the cost difference incurred in complying with Architect's direction. The Architect's language may have been imprecise, or he/she may not appreciate the cost differential. The Owner may have a different take once he/she has the two cost proposals in hand.

The Architect may indeed have good reason for insisting on R-19 minimum, with 2" minimum light-weight concrete. There may be factors in play that have just not been clearly explained to you.

As rlga points out, if you have a contract that calls for R-19, then that is what you have to install unless you obtain a Change Order.

I am a bit baffled by your statement about subs that "...won't price it that way...". Since when do the subs get to tell the GC, the Architect or the Owner how the project should be constructed or bid out, based on what a building inspector has allowed them to do in past?

Riga and Hil, thank you,
I have a non-binding code interpretation from Florida Building Official Asso. and they interpreted the same as Riga did.
When I checked with recent roofing jobs for our company and others by three different roofers, I were surprised that 90% of roofing job was done based on average R-19, not minumum R-19.
So it is obvious that building inspectors or other A/E considered it non-difference.
Cost-wide, additiona light weight for this project is about $30k out of total $150k roofing work, or 20%
Thanks alot
it's not just the matter of cost...
the difference in concrete thickness changes dead load...
was the building designed for the thinner application (as your roofer says is 'normal' in his way of thinking) or was the building designed for the heavier load?
... and, yes, there is a difference, whether the roofers realize it or not, in the overall energy performance of the building...
for example, if you had a long sloped run on the roof, and you were looking for 'average' rating, and the rating at the 'lower end' went down accordingly, you could easily end up with portions of the roof acting as 'energy drains', which result in much poorer overall performance...
also depending on climate and the other building components, you could be looking at unintended condensation issues, etc.
the designer is expected to have the 'whole picture' in mind when he designs, whereas the sub in not expected to understand all of the designer's reasons for specifying certain components... that is why the GC is obligated to ensure the subs are providing what is called for in the plans and specs (not to allow the subs to re-design the project to what they think is proper)... and that includes building inspectors who also do not have the right to modify, or suggest modifying something in the plans and specs that exceeds code minimums.

Hil, thanks, we spoke to a mechanical engineer who is familiar with energy calcuations, and he explained that, if code requires R-10 or R-19 insulation, minimum or not, he would use 10 or 19 in his calculations. He won't consider the lowest point as 19, up to highest point (lets say 27) and then took an average in his clas. This is the only way complying with so called minimum R-19 with actual and higher average R value into cals. In another words, his use of 19 is an average figure with safety factors considered into it just like every other engineering cals.
If this auguement is correct, then there is nothing wrong for roofers or building inspectors to interpreet minimum R-19 as minimum average R-19
Your thoughts?
you may have missed my point, or perhaps i was not clear...
engineering calcs may work using 'average'...
that does not mean that this is what the architect specified, and does not reflect on what the structural engineer may have designed for (if you were using thicker concrete as part of the system, as I think you indicated)...

the contractor must comply with the plans/specs and contract for the Work... if these documents require minimum R-19, and not 'average' R-19, then that is what must be installed... what I suggested was that instead of relying on what a subcontractor said, or a BO, or a different engineer, you need to discuss this with the Architect of Record whose documents form the basis of the Work.

With respect to using 'average' insulation as the method, what I suggested is that when you examine how buildings actually perform, this may not result in the energy efficiencies that were actually intended. Once you get past a certain R-value, the incremental improvement falls off... i.e. R27 does not improve building performance 50% over R19.

However, when you use 'thinner' insulation, building performance can be affected significantly. Localized performance depends on more than just the insulation value. It is affected by the air volume, use of the space, occupany levels, humidity level in the space, amount of air cirulation, concentration of windows or other factors that change the exterior envelope performance in a particular area of the building, etc. etc...

If an engineer assumes R-19 for an entire building, but some areas have different 'internal' or envelope characteristics, a review of the performance on a space by space basis might show that areas with 'less' insulation would perform well below the 'average' expectation... but the ones with 'more' insulation might perform only slightly better than the calculation assumes. It is possible for a building, analyzed area by area, to differ substantially from what an 'average' approach anticipates. For this reason, a designer may require that the entire building use the 'minimum'.

There are many ways to deal with the practicality of achieving uniform insulation values, so you are not necessarily looking at having R-19 at the 'low' end and then increasing the R-value to achieve roof slope... but since I don't have information about your particular building, it would not be possible to understand all of the operative factors.

Where is the Architect in this instance... have you had a dialogue with him/her to understand the parameters used... have you in fact determined if the designer wants 'uniform/minimum' insulation, or 'average' insulation as your roofer says is 'standard'?

Hil, thank you again, you made good point about minimum R-19, it is pretty much what AOR and EOR have told us. The auguement point is that developers, roofers and many A/E have used average R-19 for so many projects with the consent of local BO, they don't want to backtrack to these old average R-19 projects, and redid it. That would be a mess.
WHH, code is only a "minimum" this is a case where the architect appears to "know" what he is doing and the contractor, who is not the design architect, is trying to second guess him, contrary to the specifcs of the spec; for what financial gain?
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