How Do I?

September 04, 2007
September/October 2007
A version of this article appears in the September/October 2007 issue of Home Energy Magazine.
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Q.After reading the article about your home (“Design, Construction, and Performance in Ohio,” Jan/Feb ’07, p. 36), I have some questions. (After all, imitation is the highest form of praise!)

Dryer vent. Is it the plastic one with the top cover that can be removed, or a different brand?

Ventilation system. Did you bring in your 6-inch duct and then go to a distribution box, and use smaller-diameter pipe to go to the bedroom closets, family room, and living room? Did you use all-metal ductwork or some flexible? Was there any problem with condensation occurring in the ductwork during the winter season?

SIPs. I am using structural insulated panels (SIPs; about R-24). I was wondering if it would be worth the extra time and expense to increase the size of the foundation by 1 inch and go around the outside of the SIP panels, with an additional 1 inch of rigid foam (taped at seams) to increase the R-value by about 5. Do you have a better idea?

If I did add the additional 1 inch of insulation, I would also bring the rigid foam up into the attic as a barrier for the cellulose insulation, as you did on your home. (I plan to use energy trusses.) How many inches are there between the top of the foam and the roof sheathing? Did you cut a piece of rigid foam insulation 1 1/2 inches to fill in that space where the truss is located?

Siding. I haven’t decided on the type of siding yet. If I have 4 inches of insulation around the foundation and the SIP panels are at the edge of the foundation, can I use metal flashing to cover up the top exposed 4 inches of rigid foam, or do you have a better suggestion?

Tom Wiprud
Hutchinson, Minnesota


A.Tom, first of all, I suggest that you consider the use of a home performance contractor. This type of contractor can provide assistance in terms of evaluation during the design phase; testing during the construction phase; and completing home rating checks that include energy efficiency, health, safety, and so on. I also suggest that (if you haven’t done so already) you consider getting your home qualified as part of the Energy Star or other certification programs. In terms of ventilation, your system will need to comply with the Minnesota state code requirement.

Here are my answers to your specific questions:

Dryer vent. I used a Heartland vent closure. I purchased it from EFI, which still carries this product (www.efi.org). You can also type the name into a search engine to find more information and sources.

Ventilation. I used 6 inches for the main and lateral ducts. I brought the outside air into the main duct via a tee and used tees for the laterals.

All the ducts are metal and all of them are sealed.

I assume that you are referring to the exhaust fan ducts when you asked about condensation, because the air in the fresh-air inlet duct is from the outside, and given its ambient temperature, it is not cooled as it travels to the interior inlet grilles, so there is no problem with condensation. Potential condensation of the warm, moist air in the exhaust ducts is a concern that must be addressed in any ventilation design. The key is surrounding the ducts with sufficient insulation. In my case, the ducts are buried below 10–12 inches of ceiling insulation.

SIPs. Given that the actual R-value of a SIP wall in total is close to the nominal value listed, while the actual R-value of a regular stud wall is considerably less than the R-value of the insulation; and given the inherent tight wall construction of SIPs, I question the addition of rigid foam.  You should do a heat loss calculation and compare the economics of the two options to evaluate the cost-effectiveness of adding foam. 
 
I used commercial vent chutes to maintain the spacing between the top of the foam and the roof sheathing. The top of the foam abuts the bottom of the vent chutes. I used scrap pieces of the roof peel and stick membrane to fill the space where the trusses are.

Siding. First of all, you can cantilever part of the SIP panel wall over the foundation to account for part of the insulation width. See Joe Lstiburek, The Builder’s Guide to Cold Climates (EEBA, 2006), p. 410. The 1 1/2 inches that I cantilevered my 6-inch stud wall took care of the situation in my house, because I ran the second 2 inches of insulation up the wall sheathing to the roof (see Figure). I haven’t personally dealt with a situation where the insulation may extend beyond the plane of the siding. Certainly metal flashing is a potential solution, if done correctly. Or perhaps a Home Energy reader who has experience with this construction detail can respond.  How about that, readers?

Allen Zimmerman is a professor of engineering technology and technical physics at the Ohio State University, Wooster Campus.
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