How an uninsulated concrete mass wall system from the 1940’s is holding up
We are renting a house right now, in between the house we sold and the one we are hoping to buy. There are all sorts of interesting things about this house, the most material to us being its size: almost exactly half the size of the house we moved from, which was the real reason we decided to try this experiment - could we actually live with half of the stuff in half of the space..but that’s not the most interesting thing about the house. The most interesting thing is that it is a concrete and steel mid-century modern home built as a project house in 1941 by the UT Arlington school or architecture.
The house is 947 square feet, and its walls and ceiling are built out of masonry and steel. structural clay tile is the heart of the system, modular terra cotta honey-combed blocks with ribbing on the exterior so plaster can adhere. These blocks were stacked vertically to form a chanel, like CMU is today, and rebar is run through the channel and concrete is poured inside. On our house, plaster is applied directly to the ribbing on the interior - about 1”. The outside wall has a stone veneer, and the eaves, which are 24” deep and parallel to the ground (mid-century modern) are plastered (more on that later).
I’d really like to know how the ceiling went up. My guess is that some type of wood substructure was used to lay up the tiles horizontally, run the steel through, and then pack them with concrete, and parge the top with concrete, and then waterproof; all of which seems incredibly time consuming. Probably they would do this in sections, as it cures. I suppose you could lay it up one at a time over the rebar, then pack each one with concrete, then slide the next one up to it, and repeat. I’d love for someone to educate me on this.
There are some very interesting parts to this system. The interior sills are all sloped about 10 degrees. I’m not sure why, I get sloping the outside for drainage, but why the inside? The floors are oak, so no reason you’d want the windows, which were probably steel / single pane, to sweat and drip on the wood. I’m sure there was a reason.
A few months ago, we heard a large crack, and I went outside to find a 2 sq ft piece of the plaster busted off and on the ground. It looked like a piece of granite - quite thick, and nearly as heavy. They had used steel mesh to reinforce the layer, but it had rusted from water entering above into a crack in the seal. We have a two-year old, so I inspected the rest of the property, and the landlord, who is awesome, came right over to repair it.
The interior wall and ceiling finish is a semi-gloss paint over a smooth plaster finish, which looks really beautiful and is super durable - by far the most elegant and practical finish I’ve seen. It has a subtle unevenness which gives is a warm, unpretentious feel, and makes the window light that is cast across it dance.
The problem is: the system is completely uninsulated, and probably has an R-value of around 8. Basically, during the day, the wall stores up heat like a clay kiln, and then as it cools off, the wall is releasing its heat into the cool house, fighting with the air conditioner. The landlord had to change out the ac unit because it just couldn’t keep up - it would be 90 degrees outside and 76 in the house, and not getting any cooler until 5-6 am. So just as we are waking up, the house is starting to creep down to 75, 74 degrees. We weren’t waiting until a 100 degree summer day to find out what the max would be.
These systems are wonderful in those rare places with mighty diurnal swings, like the High Plains & High Desert. The structure stores up heat in the hot daytime and releases it as the cold night takes over - the heat comes on just at the right time. But in hot humid climate, you just get a humid kiln.
So the landlord has a dehumidifier set up near the main return air path that runs 75% of the time, set to 50%. With all of the rain we’ve had this year, the crawl space stays pretty damp I imagine - its way too thin for me to venture in. However, It is pretty cool on those days that get warm and then drop, to be warm and cozy inside while outside its 50 degrees, and the heater hasn’t come on yet.
Another cool thing - sound attenuation is crazy-good. This is the quietest house I’ve been in. We live about ½ mile from a major train yard, and inside we barely hear them groaning and clanging; its the same with thunder. And the house hardly creaks, when it does, its the floor system. And we don’t have to worry much about a tornado.
Other downsides: when the house moves with seasonal drying, there is a crack that opens up and water comes in. Flat roof and all. The crack runs along a hinge point where 3 doorways align through the length of the house, and they are located along the axis where the hillside dips and the groundwater is pouring out of the hillside. The foundation was no doubt under-engineered, so it moves pretty much all the time lately with the rain we’ve been having. I told the landlord just to figure on fixing it each year - the cost to repair it would be astronomical - exposing the foundation, underpinning, waterproofing, drainage system...major.
Oh, and the dust - or lack of it. Since the house is basically a plane of concrete coated with latex, the only exterior dust and allergens entering the house come via the air entering the floor system, or open windows and doors, and around the holes punched in walls - windows, doors. I’ve noticed we breathe much better at night when we sleep in this house - fewer stopped up noses and dry mouths. I’m guessing a combination of the cleaner air plus possibly higher humidity levels?
I haven’t been able to find out anything about the specific professor or the build team yet, but I’m still looking. I have found this page in the UT Arlington archives. Structural terra cotta tiles would have been a dying breed when this home was built, giving way to CMU block. Pretty interesting stuff from a building science perspective.
As for us, it looks like we will be moving on to our ultimate destination soon, but our time here has been educational in several ways.
More to talk about with this house in future posts.