Log walls and IECC

[Sifu]

I am looking at a log structure that apparently will not meet the energy provisions of the code. While I do not pretend to understand most of ICC 400 as it relates to these structures it is pretty clear in table 305.3.1 that many of the log walls out there will not meet the code. Looking for the experienced wisdom of those who have been around log structures as to how to handle this. Much of the draw towards log homes is the look of the log from both the inside and the outside so requiring an insulated wall would be pretty unpopular would it not?

 

[Frank]

What climate zone?

May need thicker log walls or double log walls with insulation between or use rescheck and more ceiling insulation.

 

[jeffc]

Fiber glass insulation provides R-4 per inch and wood provides R-1.1 per inch so wood logs are not even close. You may need to look at IECC Section 405 for a "simulated performance alternative". They calculate the heat loss for a code compliant structure and compare it to what they propose. So in this case, less efficient walls but perhaps more efficient or less glazing so they end up with an equivalent amount of heat loss. I have seen a few of these and there is an infinite way to fudge the numbers. Good luck.

 

[DRP]

it is pretty clear in table 305.3.1 that many of the log walls out there will not meet the code

Probably not prescriptively using the table.

Section 305.2 (2) indicates that a program like reschek is still applicable, with some tradeoffs I suspect it can be made to work. The local manufacturer is still churning out 6" logs and one is under construction nearby. Previously I have built log homes that were approved using reschek. I haven't built one since the adoption of ICC400 and have one on the horizon so would like some feedback on how this goes.

From my read of table 305.3.1 at 8" or thicker in most species the wall has a good shot at making it through using the methods described in ICC 400. Below that thickness it would need to be a denser species, some companies and handcrafters use oak for instance. To the right of the line in either of the above scenarios the R value of the wood in combination with the thickness gets it above the threshold of 20 lbs/sq ft and the wall is viewed as an insulated high mass wall. Notice as the specific gravity increases the R value decreases while the specific heat increases. I haven't been through the gyrations of the math shown but I believe when it is all done the combinations of thickness and specific gravity will pass prescriptively. It looks like this would be pretty easy to put into a spreadsheet if it would help.

 

[Sifu]

These walls are 6", t&g. Going to confirm the species today but in any case they will not meet the presecriptive r-value for light wood frame. I am in zone 4(A), requiring r-13 for light wood frame or 5/10 for mass wall. If this is a mass wall then the way I read IECC 402.1.1 the log wall alone would need to meet the r-5 (without any other insulation) and in that case these walls would be acceptable. Am I reading the IECC 402.1.1 wrong? Agree or disagree? The logs are considered mass walls by three different criteria, from three different publications: IECC considers a mass wall simply if it is "solid timber/log", the IRC if it weighs 20 lbs/sq.ft. and the ICC400 by two complicated formulas for density and weight. The way I see it the simplest way to handle this would be through the IECC as a solid log wall exceeding r-5, next would be the IRC by actually weighing the material to verify 20 lbs/sq. ft. and last through the ICC400 formulas, which the way I see it I wouldn't be required to enforce since it is a standard and not a code. Thoughts?

 

[TJacobs]

When you use REScheck and don't install insulation, you insert the U-factor of the log wall instead of an R-value as the R-value applies to insulation only.

 

[Sifu]

Thanks TJacobs, ran it through rescheck. It asked for the log wall thickness and species but not the u factor that I recall. Anyway, it passed.

 

[DRP]

When you selected a species it input the correct data internally.

 

[TJacobs]

Thanks for the info Robert S and DRP...haven't had a log home in REScheck yet.