When planning to build Underhill House, we chose concrete floors because we want the most energy-efficient heating and cooling.  Concrete is a great place to store heat.  Our house is designed so that in the winter, the sun will pour in on our floors, and warm them all day long.  At night, that heat will be released to keep a steady, comfortable temperature. 

We added a foot-thick concrete wall between the main room and the den positioned to catch the afternoon sun and provide more thermal mass.

Both concrete and tiled floors are great heat sponges.  We opted not to tile to contain costs.  And, I happen to think treated concrete has an agreeable, honest look.

We considered acid etching and cream polishing.

We decided to go with the simpler polishing and sealing because it seemed to require fewer chemicals and was the most economical.

Howard Grote and Sons just completed the job.

Everyone we spoke with said you need a densifier to fill all the pores in the concrete and harden it when buffed.

Initial cleaning of the floor

Wickipedia says Concrete is by nature a porous material, with pores formed by water evaporation during curing. These pores interfere with surface uniformity, and make the slab more susceptible to staining from spilled liquids. The additional cementitious material formed by the densifier and lime tightens these pores for better surface hardness and durability. Most densifiers can react within 1-2 hours with concrete surface, however the chemical reaction with the calcium and free lime in the concrete will continue for up to 2 months after the application of it to the surface of the concrete.

Densifiers may use various carrying agents to accomplish the hardening process, potassium, sodium, lithium, or other agents.

Our floor finishers used a sealer on top of the densifier to keep the floor from staining if something is spilled on it.

We’ve been told that we need to keep applying the sealer every 3 or 4 months to maintain the surface by spraying it on and mopping it around.

In our case, we did not want a very shiny floor because we want the sun hitting the floor to sink in instead of reflect away.  Also, on sunny, winter days, a very shiny floor could be blinding.

To keep the floor less shiny, the flooring crew did no buffing after applying the sealer.  Even so it has quite a gleam.

We are pleased with the soft, mottling that naturally occurs when you seal concrete.  But there were a few unpleasant surprises that we could have avoided.

With all the building processes begin conducted on this floor, it’s taken a bit of a beating and had a few materials dropped on it (like some unfortunately leaked chainsaw oil right in front of where the refridgerator will go) that soaked into the surface and may be there forever.

Serious oops!

Perhaps the most startling stain is a line left where we used duct tape last summer to seal the edges of a tarp we placed over the opening where the stairway goes between floors.  The duct tape adhesive soaked into the concrete, and the floor finishers were not able to extract it.

They said that if they had ground down far enough to grind it out, they would have made a divot and exposed the stone in the concrete.  Some people have their concrete ground down to expose the stone, but in that case they use some kind of decorative stone in the mix.  Our stone is simply the mix our concrete team, led by Mike Flynn, felt would make a strong material.

We are going to try a few techniques to see if we can draw some of the stain out of the floor, but I’m not very optimistic.

At the moment all the floors are covered with paper to protect them while we paint the trim boards that will edge the floor and the doors and windows.

The idea of polished and sealed concrete seemed simple at first – minimal chemical and energy expenditures to achieve a durable, floor without compromising its thermal mass action.

I’ll report further as we see how it wears.