ASHRAE 90.1 requirements

There is no requirement for SHGC <= 0.4. That's just for the baseline building... so your building is measured against the baseline building.

Like Jean said, it's a "prescriptive" requirement. If you're doing LEED, chances are you're following the performance path and don't need to meet all prescriptive requirements (though you still need to meet all "Mandatory" requirements).  Having said that, you can still get pretty good daylight but also with low SHGC with coated glazing (Low-E). LSG (light to solar gain ratio, could also have other acronyms) is a measure of how much visible light you can get to how much solar gain is let through. In 90.1 2016 the minimum is 1.10, which ensure good visible light while blocking solar gain. Some products have LSG up to 2.30 and even more. For example, Viracon VNE13-63 has a VLT of 67%, which is pretty good, and a SHGC of 0.29, also pretty good when compared to 0.40 applicable to most moderate climate zones like Paris. Also note that the SHGC allowance increases if the glazing has permanent external shading on it, which reduces the solar gain on the glazing.  I'm not familiar with HQE, does it require VLT that's considerably more than 65%? 

Hello Jean, hello Waleed,  Could you tell me if it's if it is mandatory that the thermal coefficients of my building comply with table 5 - 5.3 of ASHRAE 90.1  which governs the performance of each type of envelope? I'm lost, I'm on a project outside the United States.

Sara, Table 5-5.3 is a "prescriptive" requirement, not a "mandatory" requirement.  All buildings wanting to comply with 90.1 must comply with ALL MANDATORY requirements, regardless of compliance path. There are two paths for compliance in the standard: Prescriptive Path and Performance Path. 

• For projects following the Prescriptive Path, both MANDATORY (sections numbered X.4) and PRESCRIPTIVE (sections numbered X.5) requirements (or provisions) MUST be met. 
• For projects following the Performance Path, MANDATORY requirements MUST be met, but PRESCRIPTIVE requirements can be traded off, which is the point of having a performance path, to give some flexibility to design teams. 

In your post above, you identify Table 5-5.3 as "Mandatory" when it is in fact "Prescriptive", so if you're following the performance path, you don't need to follow all of them. Needless to say, following them are minimum best practice, and if you don't meet too many prescriptive requirements, you won't be able to comply with the standard using the performance path. 

Many thanks walid,
You saved the whole design team. We thank you all for your help.
I understand that I am confusing the normative and the prescriptive ... could you tell me how I can clearly make the difference?
Can I consider as mandatory provisions only the points below? 5.4 6.4 7.4 8.4 9.4 10.4 there are others points?  Many thanks

Yes all the mandatory provisions are in Sections X.4 as you indicate above.

What about sections 5.8.1.1 through 5.8.1.9 ? It says in mandatory provision 5.4.1 that insulation should comply with those when prescribed by 5.5 or 5.6.

Hi Eduardo, Section 5.6 defines the options for envelope performance trade-off with other building components. When using the performance path from Appendix G (energy modleing), the referenced Appendix C calculations are accounted for in the modeling.

Sections 5.8.1.1 through 5.8.1.9 address characterisitics of the insulating materials. So Section 5.4.1 means that when insulation is prescribed in Section 5.5 or 5.6 it must comply with those characterisitics. So these mandatory provision are associated with prescriptive requirements. I suppose they are a cross over of sorts.

That's right Sara and Guillaume,  As a matter of a fact, the credit requires the following: 

1. compliance with the mandatory provisions of ANSI/ASHRAE/IESNA Standard 90.1–2010, with errata
2. inclusion of all energy consumption and costs within and associated with the building project
3. comparison against a baseline building that complies with Standard 90.1–2010, Appendix G

As you can see, the projects need to comply ONLY with mandatory provisions of ASRHAE 90.1, and be more efficient than an energy model that follows the Appendix G. Consequently, you have plenty of flexibility to design your envelope and MEP systems. Some of the U-values might be more efficient than the baseline (other U-vaues may not be more efficient).... The same applies with MEP systems, for example some spaces might have a LPD be more efficient than the baseline (other spaces may not be more efficient)...etc. The energy model will serve to verify that the WHOLE design is more efficient than the baseline building, but you do NOT need to be more efficient than the baseline on every single item. In your example, your project may perfectly have a glass with a SHGC less efficient than the baseline (greater than 0.40). I hope that helps!

Hi,  as far as I understood, only previsions provided in chapter 5.4 have to be considered mandatory, while, for instance, building envelope performances represented in chatper 5.5, and connected tables, are prescriptive.
So, for instance, although 5.4.1 references value in chapter 5.5 for thermal insulation, I do not mandatory have to respect those minimal values, but I can consider the whole building in order to achieve targetted energy consumptions reductions.
I pose this question because I am designing the refurbishment of a historical building in Rome and I cannot add thermal insulation to external walls, so, if I am right in reading this conversation, I can pursue LEED certification even if I cannot reach minimum insulation values for external walls of conditioned spaces stated in chapter 5.5.
Thanks in advance for your attention and support 

That is a correct interpretation. 

Hi Marcus, in continuation to the discussions had before, I want to confirm whether it is mandatory to have an external wall insulation in proposed design. I am using PRM as per ASHRAE 90.1 2010.

The amount of insulation is not a mandatory provision.

So, if we are not proposing insulation at all for the external wall, it is accepted as per PRM ASHRAE 90.1 2010 right?

Yes, it is possible to not install any insulation and meet the requirements.