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Is exposed concrete a thing of the past with the 2018 energy code?

HolisticDesign

Our city has adopted the 2018 IECC and we are struggling with a number of aspects of the continuous insulation that now seems mandatory. 

We were planning to use exposed concrete on the exterior of a number of areas of a 15 story condo tower, namely stair towers, but are receiving a harsh lesson that this is just seemingly not possible with continuous insulation. The only method we can think of is a concrete panel cladding secured with clips on mineral wool insulation on a structural concrete wall. However, metal clips themselves are thermal bridges and thiner, say fiber reinforced concrete panels, could be prone to cracking as they settle over the 160' tall wall we want to use them on. Building a second 8-12" concrete wall on the exterior of the insulation is just a waste of space that our client won't accept. Transitioning the cont. insulation from exterior to interior at the concrete walls would double the required concrete thickness as theyre now free standing (cant be tied back due to thermal bridging). 

Were  stumped and depressed at the thought of losing a rich material.

How are all of you addressing exposed concrete with continuous insulation?

 
Feb 26, 20 12:11 pm
flatroof

I'm trying to become Tadao Ando and ask for an unlimited budget. 



Hasn't worked out.

Feb 26, 20 12:46 pm
archanonymous

In this case you could make the thermal envelope at the back of the stair - essentially have the stair be outside. 


Otherwise, yeah, it is very difficult to pull off with prescriptive CI in exterior wall assemblies. Even precast sandwich panels don't meet the letter of the code.


Feb 26, 20 1:14 pm
SneakyPete

Try a performance based approach?

Feb 26, 20 2:02 pm
archanonymous

This is the way to go. Last time I did I was sorta surprised... Some of the areas I thought we had bad thermal bridging weren't actually that bad and the rest of the wall was quite high performance so it balanced well.

bowling_ball

I don't use that particular code, but I live and work in one of the coldest cities on the planet, and we have an energy code that (at least prescriptively) calls for effective R27 walls. That's a lot of insulation. I think the last time we did a cavity insulated building was 2014. CI does restrict some design options, but it can be done well.


I'm doing a high rise apartment building now (well under construction) that uses a precast sandwich panel for all exterior walls. The ties between the inner and outer concrete wythes are not accounted for in our code, unless we wanted to model everything (and we rarely do). This building has been a huge pain in the ass to coordinate, but it does perform as intended, in terms of insulation. I'm more concerned with all is the joints that have to get sealed (and those aren't all that insulated...)

Feb 26, 20 2:04 pm
archanonymous

Are there interior finishes or is the concrete exposed there too?

archanonymous

I've (only) been practicing 10 years and have never done cavity insulation.

bowling_ball

Mostly drywall finishes, with some exposed concrete in public areas. Unlike cast in place, there are gaps everywhere that have to be sealed up.

Wood Guy

Considering your screen name--Holistic Design--I'm surprised that you don't seem to be on board with climate-friendly building codes. My state is finally moving to the 2015 IECC this year and I couldn't be happier. Find another way to make the building look good. It's encouraging to hear that the newer energy codes are doing what they were designed to do--prioritize energy efficiency over architectural vanity. 

One approach that might work for you is a thermally broken standoff, such as this: https://www.cascadiawindows.com/products/cascadia-clip. I don't know if off the shelf versions could support thick concrete cladding, but they easily support fiber-reinforced cement board. 

Feb 26, 20 9:10 pm
b3tadine[sutures]

I love that clip. A friend in Hoboken used this clip on the first luxury Passivhaus condo there. Awesome stuff.

Wood Guy

Nice. I haven't used them but they seem like a smart solution to this problem.

anaisziwei

Im a student here, but may I know why is Exposed Concrete Walls a bad thing for energy code? May I know what is the reason?

Feb 27, 20 2:41 am
Wood Guy

Concrete is a terrible insulator. It's actually a conductor of heat. R-0.08/in (IP units), vs. R-1.2/in for wood or R-4.0 to 6.0 for insulation materials.

Additionally, the production of the Portland cement for concrete is responsible for about 8.9% of all global warming emissions--that's more than any other single material. 

bowling_ball

All materials are conductors of heat. The insulation value is the other side of the same coin. I know you know that, but not everybody might.

Wood Guy

Good point, BB. Said another way, concrete moves heat 15 times faster than wood, and 50-75 times faster than insulation.

mightyaa

And to add to those points, concrete is a good structural material. Before we worried about energy performance, your exterior walls could be load bearing with the floors bearing on top; you would just insulate the interior side and ignore those bridges.  Now you want continuous insulation protection.  So any concrete exterior you do, or I'd say any exterior finish, isn't going to be a structural component.  Anymore, a building exterior is like a bubble around the building with minimal attachment and thermal bridges to the supporting structures and interior spaces.

Chad Miller

Concrete is a good thermal mass for passive solar heating.

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