Why Houses RotNovember 29, 2006
The fact is rot is not a SIPs house issue; it’s a modern house issue. As we strive to make houses that have tighter envelopes with less air infiltration and that are more efficient, as we strive to build houses that put less demand on HVAC systems, as we strive to build houses that cost less to live in and put less demand on the earth’s resources, as we strive to create house that are built green, we put more demand on our building systems and reduce our acceptable margin of error.
Here’s how that works.
This is my house.
It’s a 200 year old colonial where air flows in and out freely. Let’s just say there is plenty of indoor-outdoor transition because there is not a stitch of insulation in it. This is not a good thing for house efficiency. If I decided to insulate the entire structure, replace the windows, and seal all the doors I might have a warmer house but I might create big problems. Say I missed a spot. For argument’s sake, say I left a two inch hole in the corner of an upstairs bedroom. All the air that used to move in and out of the entire structure would now try to rush through that small hole in the bedroom wall.
This winter all the moist, warm air in the house would be trying to get out that hole. As it mets the sub-zero air coming in from the outside, the moisture in the warm air would condense. Water droplets would form on the inside of the wall. It would seep down the wall cavity and soak the insulation. It would pool on every horizontal surface and waterlog inside the wall. If, when I insulated, I added vapor barriers on the interior and exterior surfaces (not unheard of), the water would be trapped inside. Rot and mold would take hold. If I didn’t’ notice right away the problem would go unchecked and half my wall would rot away while I was playing in the snow.
Photo courtesy PDPhoto.org
This is a microcosm of the building efficiency evolution over the last thirty years: good intent and innovation applied without a complete understanding of bulk water management. SIPs are a flashpoint for problems because the envelope is so tight that small lapses, like an improperly sealed roof vent, create big problems like a rotten roof. The #1 weapon we have to combat problems in a tight house is a good HVAC system that is designed for the house (I will have more on HVAC in upcoming posts). But first we have to seal the house and this is where the techniques of SIP installation really hit the ground. This is what we learned today.
First and foremost, create tight joints.
Use an approved sealant on all the seams.
Spline or block all seams.
Use straps to pull the panels together tightly.
Don’t let things slide.
This joint went together easily and looked fine from the outside but when we looked at the bottom, we saw a gap from the overburn of the EPS. The thing to do here is mark the gap where we will see it later, then fill it when we go back to foam the joints.
I was exposed to so many good things today it would be impossible to cover it all. Here are just a few pictures I snapped along the way. Enjoy!
Charles H Byrd III, professional SIPs installer: “I’ve been installing sips for years and this is my second time through the course. I’m still learning stuff.”