Construction Design-Why Old Homes Work

I live in an old home, nearing 70 years old.  Some of it’s systems have been updated or changed.  The natural gas forced air furnace is around 15 years old.  The home, when it was built, probably had fuel oil or wood heat.  (The old concrete block chimney is still present.)  Most of the siding is original, asbestos shakes.  There isn’t enough insulation in either the attic or walls, but the house stays dry and is in decent structural shape given it’s age.  What makes the old houses work?  Why does it seem there’s more damage caused by moisture in houses built in the 1970’s through the early 2000’s that use materials that are supposed to be better?

To answer some of these questions, we need to look at how we used to build.  Centuries ago we built with rocks.  A rock doesn’t care if it’s wet or dry, hot or cold.  The only building we really do with rock anymore is cladding on very expensive houses and interior finishing (marble, slate and granite).  It’s used for the pretty stuff we see, not the structure of the home.  Rocks worked, but we no longer build homes from them for obvious reasons.  They’re cold when it’s cold outside, wet when it rains and the wind blows through them when stacked together, and they are heavy and hard to work with, but they last forever.

An old coal burning furnace, converted to fuel oil. (Still in use today.)

So we moved to building with wood, lighter weight, easy to work with and abundant.  Up until about a hundred years ago, we built with big, old trees.  Old trees made for really good lumber.  They were more rot resistant than younger trees.  They were able to store and release more moisture than young trees.  They were stronger because they grew slow.  We used them everywhere in the home.  Framing, sheeting, siding, flooring, wall finishes, even as shingles.  There were lots of gaps and cracks in the home where the wind would blow through all the individual boards nailed on the outside of the house.  In a northern climate, we needed to pump lots of heat into the home to maintain some comfort.  We burned lots of coal and wood, and later, fuel oil.  All this heat energy and air moving through the building materials helped dry things out if they got wet.

Individual boards used as roof sheeting, wall sheeting and floor sheeting.

But then, someone came up with this great idea.  Let put something in the wall and roof cavities to keep the heat in the home.  Along came insulation.  More of the heat that used to be lost to the outside was kept inside.  It also meant less heat was drying the structure.  The outside walls became a little damper.  Some painters started to refuse to paint houses with insulation because sometimes the paint would peal after only a couple years.  The painter’s blamed the insulation for attracting water, which would move through wood products and blister the paint.  Early building scientist came up with a solution.  A vapor barrier, or plastic sheeting that is attached to the inside of the exterior walls.  (We now know vapor movement through building assemblies is much less of a worry than air containing moisture moving through the assemblies.)

Old insulation in an old house.

Around the same time as vapor barrier’s introduction, builders were looking to speed up the building process.  Plywood sheet goods were introduced.  We no longer had to use individual boards for roof, wall and floor sheeting.  Four foot by eight foot panels could substantially speed up construction of a home.  The 1960’s and 70’s brought us oriented strand board (OSB), which reacts to moisture a little different and is cheaper than plywood.  The 1970’s also brought the first energy crisis.  More insulation was being installed in homes to save heating costs.  Putting all this together without a good understanding of the science behind the building led to major home rot and mold issues.

So, back to my original question, why did old homes work?  Lots of heat loss in uninsulated walls and roofs helped dry building materials when they got wet.  Heat was cheap.  Lots of natural air exchanges through all the cracks and gaps in individual board also allowed materials to dry.  When a house got wet, it could easily dry.  Old growth lumber was more rot resistant and could store more moisture, a lot more than todays younger lumber and manufactured building materials.  I live in a house like that, and it’s uncomfortable, cold in the winter and hot in the summer, but it has the ability to dry.

Are we going to go back to the way we built at the turn of the twentieth century?  Old growth lumber, lots of heat loss and many holes in the building envelope that allowed a house to “breathe”.  Won’t happen.  Building science has come a long ways.  We have a better understanding of what’s going on inside our homes.  We have an order of importance in dealing with moisture, we understand ventilation is needed, especially when building in a cold climate and we’ve learned how to slow the loss of heat to the outside with less worrying about the reduction of drying building assemblies with heat.  When done right, we can build a better, long lasting, heathier and more comfortable home.

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