May 17, 2018
Spider web in a corner of a home.

You can probably think of a few places in rooms, drapes, and closets, or behind a radiator, where spiders make their webs. Under the wood porch is a thicket of webs. A dirt floor crawlspace can be haunted with webs. Yuck! 

We are about to explore the building science that our spiders reveal to us. Spiders maximize their hunting with two master's degrees in geometry — Pythagorean geometry to design gossamer architecture and Vector geometry to determine exactly where to build. Spiders know, with precision to rival NASA, exactly where their prey travel.

Next time you see a fresh gossamer triangle with radial spokes, approach it with a flashlight and carefully observe. With a pencil, point straight into the radial center (perpendicular to the web plane). What now are you pointing towards? Notice in the wall or ceiling finish any defect – a seam, crack, hole, peeling wallpaper.

What does it mean?

Your spider has sited her structure precisely aligned with the center of a tiny draft of air. The crafty spider knows the drafty air is a hot spot, or a cool spot, for flying food.

Web geometry forms a stunningly accurate target. Now we zoom in to follow the airstream back to its source:

Warm, humid air in the room moves through the spider web, then seeps into a plaster crack and a wall stud cavity. Following a drain pipe upward, the airflow cools and drops some moisture onto wood studs, a rusty pipe, and nails. Airflow turns a corner and gets turbulent, colder, and dryer. Finally, air flows outside through a tarpaper layer and exterior shingles.

A series of distinct microclimates are created along the air path, changing in temperature and humidity along the way. One spot is perfect for laying particular microscopic eggs or perhaps larvae. Further along, another spot is perfect for propagating fungi, again food for a passing microbe. A flying airstream traveler stops to nibble. It is like flying through a food market. The deft spider waits at the tunnel entrance for the biggest, fattest air travelers who have plundered these hidden food markets. 

Another airstream enters from outside, wafts up into the porch ceiling, and squeezes under the bedroom room baseboard, turning the wall-to-wall carpet sooty grey along the baseboard. You cannot see the cobwebs above the porch ceiling. 

Another air mass under the bathtub nourishes hidden travelers on seeping shower moisture and soft wood studs. You can see cobwebs by opening the tub plumbing access hatch.

Look in the immediate area for a crack, an open seam, holes, loose wallpaper. Could be trim molding, picture hanger hole, settlement crack. Close the opening with appropriate sealant. The most universal sealant for interior cracks is "Clear Paintable Silicone Caulk" which usually disappears when cured. Keep in mind air flow is very good at finding detours. So caulk similar openings anywhere you see them.    

Why should I care where spiders build their webs?

Building science indicates that a leaky house is not a properly ventilated house. Spider webs are flags marking building pores, holes where energy is coming and going out of control, possibly creating micro food chains.  

An old farmhouse is likely to have accumulated a unique set of odors that are hard to identify and may be hard to eradicate. Some of the scents are old air leaks between the creaky boards.    

What can be done?

If you have tracked the specific spider web areas, you have a head start on finding your air leaks. 

A Blower Door Test, part of any complete home energy audit, pressurizes the heated air and makes a calibrated measurement of the total holes in your house. The test helps identify where the holes are. Tell your home energy auditor where spiders build their traps.  

The next step after the energy audit is to contract for air sealing and insulation upgrades. These steps lower utility costs while improving comfort and air quality. BGE offers rebates, reducing the cost to you, the homeowner. The money you invest is back in your pocket in a few years of reduced BGE payments, a very high return on investment. Learn more at

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Frank Lee, Guest Poster

Frank Lee RA CEM BPI is an energy strategist with the City of Baltimore’s Office of Sustainable Energy.