Structural Revolution

ABOUT THIS BLOG

Follow Fine Homebuilding's John Ross on a gonzo journalism assignment (or as gonzo as we get in the annals of serious how-to) that chronicles what some are calling the next generation of residential structures.

Why houses rot

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, and as we strive to create houses that are built green, we put more demand on our building systems and reduce our acceptable margin of error.

Here’s how that works:

My house is 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. Water 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

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 I’ve learned:

First and foremost, create tight joints.

This joint is splined and injected with expanding foam. There is very little dead air space and no place for air to travel from one side of the wall to the other.

If you do get a gap:

Don’t continue without fixing it. 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 on the outside of the OSB where it can be seen. Then fill it with the same expanding foam that is injected into the joints.

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