How to Insulate Old Walls Without Wrecking the Plaster

So to understand why these homes hemorrhage energy, we have to look at what's actually sitting behind that beautiful plaster.

And in the vast majority of legacy construction, the answer is absolutely nothing.

Just air.

Literally just air.

You are dealing with a completely empty wall cavity.

You have the interior lath and plaster, the exterior wood sheathing, and between them is roughly three and a half inches of open air.

And according to the building science literature, this empty cavity is where Fourier's law of thermal conduction just goes into overdrive.

Oh, yeah, big time.

Because we know heat naturally moves across a temperature gradient, right? It's always seeking equilibrium.

But these old homes, the lack of thermal breaks and the sheer size of that temperature gradient in the winter means the conduction rate is staggering.

Heat just pours out of the living space.

It does.

But conduction is really only half the problem.

That empty three and a half inch void creates a perfect environment for convective air loops.

Fluid dynamics take over.

Exactly.

The air inside that empty cavity touches the freezing exterior sheathing, it cools down, and it sinks to the bottom of the wall.

Then it sweeps across the interior side, touches the warm plaster, heats up, and rises back to the top.

Wow.

Yeah, this establishes a continuous rotating Ferris wheel of air inside the wall cavity that literally scrubs the thermal energy off the back of your drywall and dumps it outside.

So it effectively weaponizes the empty space against your furnace.

That's a great way to put it.

Yeah.

And it gets exponentially worse depending on the home skeletal structure, right? Because a significant portion of these older homes utilize balloon framing.

Right, balloon framing is a whole other beast.