Controlling Heat Flow and Cracking in Concrete Slab

CONTROLLING HEAT FLOW AND CRACKING IN SLABS

ONE BUILDING, TWO SLABS

Two of the five concrete slabs at the off-grid homestead are now complete. The barn has two completely separate slabs for one building, with a two inch gap between them, filled with two (2”) inches of rock wool insulation to control heat flow and cracking. Most often that is called a construction joint, as opposed to a control joint, because it is a physical separation between two slabs.

construction joint

Having the construction joint here is helping prevent cracking at what would otherwise be an inside corner, if the slab were all poured at once. Instead of two inside corners, we have reduced that to one on the main slab area.

durable concrete slab construction

durable concrete slab construction

It’s at that inside corner, that we started two of the 1-1/2” deep control joints. These control joints help ensure that cracking, if it’s to happen at all, happens at those straight joints, instead of randomly. These random cracks can be unsightly.

durable construction concrete slab

The other purpose of this construction joint is to minimize and/or prevent heat loss or gain from happening between the two slabs. The main slab is in the part of the barn serving as the workshop, and is, technically, “unconditioned” space, and the smaller slab the floor of the tower, where the power room (batteries, inverters, etc.) and owner’s office, and is mechanically conditioned. The barn will have a wood burning stove the owners can use, but the tower will be heated and cooled with a mini-split (VRF) heat pump system.

The two stories of the tower will need to maintain comfortable temperatures, whereas the main area, or workshop, can be less controlled.

The insulation between the two slabs is minimizing the amount of heat transfer between the slabs, especially in the winter, since the workshop slab has the potential of being colder in the winter, and hotter in the summer.

construction joint

CONTINUOUS EXTERIOR INSULATION

In an earlier video, I explained how we are insulating all surfaces, except for the interior finished surface, of all slabs.

In the barn we have two inches under the slab and around the entire turn-down portion, and three inches will be installed at the outside face. This three (3) inches will continue up the exterior of the walls, around the entire building, and we will increase to four (4) inches on the exterior of the roof.

insulated slab edge

We will be doubling all the insulation for the community house. Two (2) inches for the slab will be four (4) inches. The three (3) inches on the walls will be six (6) inches, and the four (4) inches on the roof will be eight (8) inches. All on the outside of the structure.

insulated slab edge

The two inches between the two slabs in the barn are more than adequate, since both slabs are going to be fully wrapped with at least two (2) inches, and up to three (3) inches at the edge, where most of the heat loss occurs.

Since the workshop area will be “unconditioned”, and its temperatures not regulated by a thermostat, like the tower will be, the insulation between the slabs adds that insurance that heat transfer will be kept at a minimum, or eliminated.

SUMMING IT UP

This construction joint between the two slabs of the barn will,

  1. Minimize or prevent cracking at what would normally be two (2) inside corners.
  2. Control heat flow between these two slabs.

These increase the durability and lifetime of the slabs, as well as the comfort of the buildings occupants.

While a control joint is better than a random crack, how beautiful is it that we don’t need a single joint in the smaller slab? Due to the six (6) inch thickness, the added reinforcement, the size of the slab (18’-0” x 18’-0”), and the over six (6) inches of well compacted gravel underneath, the slab is more than capable of not cracking. So, control joints are not necessary. Not even as a belts and suspenders, which we love so much!

Since the workshop area was 50’-0” wide by 40’-0” deep, even the added reinforcement, thickness (6”), and compacted gravel is not adequate to avoid the need for control joints. So, they are cut in to the slab between 12’-0” and 15’-0” apart, based on the configuration and the areas that are most susceptible to cracking.

The next step for our foundation crew is to pour the footers for the columns of the courtyard roof structure at the House, and then we’ll move on to the slabs. The main living pod of the House is going to be eight (8) inches thick and the smaller pods will be six (6) inches.

The goals have been, and always will be, to build for 500 years or more. Controlling heat and focusing on durability, are just two ways of doing that. In this example, we’re insulating every surface of the building on the outside, and building in such a way as to prevent structural failures. Ultimately, we want to maintain the integrity of the materials, the structures, and the comfort of the people and pets living in them.

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