Greenhouse exemption?

[steveray]

Is it me or is this one of the worst written code sections ever?

C402.1.1.1 Greenhouses.
​

Greenhouse structures or areas that are mechanically heated or cooled and that comply with all of the following shall be exempt from the building envelope requirements of this code:

1. 1.Exterior opaque envelope assemblies comply with Sections C402.2 and C402.4.5.
   Exception: Low energy greenhouses that comply with Section C402.1.1.
2. 2.Interior partition building thermal envelope assemblies that separate the greenhouse from conditioned space comply with Sections C402.2, C402.4.3 and C402.4.5.
3. 3.Fenestration assemblies that comply with the thermal envelope requirements in Table C402.1.1.1. The U-factor for a roof shall be for the roof assembly or a roof that includes the assembly and an internal curtain system.
   Exception: Unconditioned greenhouses.

So it is exempt from the envelope if it complies with the envelope requirements?

Not exactly what commentary says, but i'll hold on to that for a minute and see what people say...

 

[steveray]

No energy code folks hunh....?

 

[Mech]

steveray - Did you mean poorly worded to convey their intent or the dumbest requirements?

Just a warning - I am not sure the last time I read the energy code envelope section and I am not on the 2021 version yet. If I pass ComCHECK, I assume I'm good to go, but I'll take a stab at this.

I think item 1 is exempting the greenhouse from 100% envelope compliance provided the design complies with the two listed sections. I suppose they could have made it easier by stating something like "Greenhouses are only required to comply with sections . . ."

Compliance with section 402.5 is not required, so the greenhouse can be as drafty as desired.

Compliance with 402.3 is not required for those certain limited climate zones, and I suppose that's a good thing. Imagine installing a glass roof only to be limited by solar reflectance requirements.

Is 402.2 saying if the greenhouse walls and roof are not glass, they must be insulated like a normal building? That's kind of dumb but not dumb. If I install all glass, then the U-value can be 0.7 and the winter time heat loss is huge compared to an opaque wall! For an opaque metal framed wall in Zone 5, it must perform 12 1/2 times better than the glass! If I choose to install an opaque wall, must I build the walls per Table C402.1.3 using continuous insulation and batt insulation for a metal framed wall? Table 402.1.4 would allow all batt or all continuous insulation as long as I meet the required U-value, but Tablce 402.1.4 is not mentioned. The only not-dumb reasoning that high U-value glass will reheat the building (assuming there is daylight) much better than a high U-value opaque wall; so the opaque wall should be built with better thermal resistance.

Item 2 - I'm not reading all of those sections, but I'm guessing they are requiring almost but not quite full compliance for separating conditioned space from unconditioned space.

Item 3 402.1.1.1 Glazing maximum U-factors are relaxed and there is no maximum solar heat gain, but since we're trying to heat the structure with solar, there should not be any maximum solar heat gain value anyway.

Did I get anywhere close?

 

[Mr. Inspector]

Greenhouses are exempt from permits in PA except when open to the public.

 

[steveray]

Is 402.2 saying if the greenhouse walls and roof are not glass, they must be insulated like a normal building?

This is exactly what I am saying....If they meet the wall requirements (per Ex 1), they are exempt from the wall requirements......

 

[steveray]

And commentary isn't much help...

C402.1.1.1 Greenhouses.
P​

Greenhouse structures or areas that are mechanically heated or cooled and that comply with all of the following shall be exempt from the building envelope requirements of this code:

1. 1.Exterior opaque envelope assemblies comply with Sections C402.2 and C402.4.5.
   Exception: Low energy greenhouses that comply with Section C402.1.1.
2. 2.Interior partition building thermal envelope assemblies that separate the greenhouse from conditioned space comply with Sections C402.2, C402.4.3 and C402.4.5.
3. 3.Fenestration assemblies that comply with the thermal envelope requirements in Table C402.1.1.1. The U-factor for a roof shall be for the roof assembly or a roof that includes the assembly and an internal curtain system.
   Exception: Unconditioned greenhouses.

❖ The 2021 code added new provisions for greenhouses, which can use substantial amounts of energy. The provisions identify parameters for mechanically heated or cooled greenhouses to be exempt from the building thermal envelope requirements of the code. This includes fenestration U-factors falling below the maximum values in Table C402.1.1.1 Low-energy use greenhouses structures are still exempt if they have a low energy usage per square foot in line with Section C402.1.1.