Why Houses Rot

November 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.

winter in CT 

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.


Secure panels with the right screws.

 gimlet head

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!

mine homeplatehooks and strapstoyssill sealhot ballbig saw

speed square

Notch your Speed Square for easy marking.

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.”



  1. I’m enjoying the read so far, John. Some captions would enhance the photos, though. If they’re worth posting, it’d be worth a short explanation.
    thanks, steady on.

  2. This info on why houses rot is VERY helpful. We recently bought a house (we have had 4-5 yrs now). It is a post and beam and is about 15 yrs old. It has SIP panels and heating this house is awesome!!

    Yet we recently discovered a problem I am HOPING someone can help me out with. We have found that our window frames are rotting. (we are having a HVAC system put in as we speak). I wasn’t aware of why this could happen but after reading this I think I understand more. My BIG problem is tough that the panels exterior wood surface are rotted!! How on earth do I fix that??? I haven’t contacted my insurance company as of yet…. first I wanted professional advice as to how to correct this??

    Will I be scrapping off all of the old rotted wood and re-gluing new wood to these panels? How does the rot affect the foam? Please, please I really need some SOUND advice here!!!

  3. If it is true that the rot is extensive on the exterior skin of the SIPs then you do have a problem. One of the first things I would do would be to get in touch with the SIP manufacturer and the installers. Find out if they can help you with what went wrong and how to fix it.

    If it’s a timber-frame home you might have some more options as the SIPs won’t be structural.

    If you have time, let me know how it’s going.

    Good luck!

  4. What test can be used to determine that the proper amount of borate is contained in the EPS foam of a SIP?

  5. First of all, I recently open a structural Insulated Plant. My company is Energy Smart Panel Systen, 197 Gallintin St. N., Jackson, Ms. 39207. 601-960-0880.
    What will make my company different from all the others is the comcept. For over 20 years I’ve been building with SIP’s. In the past I would buy Jumbos ( 8′ x 24′) and fabricate from there. 1st of all I had to purchase the 8′ x 24′ jumbo panel (192 sq ft) @ the going price. If that wall section had 1 3’0 x 6’8″ door and (2) 3′ x 5′ windows. I just created over 50 sq ft of waste, plus added lobor.
    So I developed the first standardized panel system (SPS).Using this system allows me to work from 4 x 8′, 4′ x 12′ and 4’x 16′ stock. By creating
    the SPS I elimated ALL WASTE. I have a standing stock of enough fabricated panel at all time to ship 10 homes in 3 days if needed. When a person brings there plans in. I will have 95% of those panel fabricated ready to ship. I may have to modify 3-4 panel to coplete that order. That’s how I can ship in three days. I my business continues to grow, so will my in house stock.
    As you know we had a little wind called Katrina a couple years back. 95 % of the people on the Gulf Coast are still living in FEMA 10′ x 32′ trailors.
    I will be building a katrina cottage. It’s 16’x 36′, 2 story, with 2 side rooms. Total sq ft of 1,406 sf. compared to the 676 sq offered as a replacement to the trailors. My units will be Green Friendly. With I-Joist foundation, SIP’s for wall and roof, on demand hot water, whole house water and air purifacation system and much more.

  6. Good writing. Hope to visit once more

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  8. Very nice Article! In Australia we are starting to build complete houses from these insulated panels with amazing results. Australian homes have been designed and built basically the same way for many years. The need to protect our environment and to conserve energy has become the catalyst for revolutionary change. Some example can be by googling “Kit Homes Example”. Insulated panel kit home are the latest in prefabricated kit homes. The panels used to create these 21st century kit homes are manufactured with two skins of pre painted galvanized steel, which are bonded to an insulated core of Expanded Polystyrene (EPS) that is fire retardant treated. Other skin material can be used for specific purposes. Insulated panel roofing offers designers and builders long spanning wide open spaces, pre-finished with an attractive pre-painted ceiling along with a modern palette of Colorbond® roof colours

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  11. It is cool.. I like the house with this SIPs

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  50. Houses ROT simply because they are cellulose based for the most part. Stop building from cellulose and the problem is severely reduced.
    We now have plastics and metals, concrete and all kinds of alternatives…many new ideologies are designed to be water tight completely as well as thermal bridge and gap free at assembly.

    OSB sips should be considered dinosaurs already.

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