I am curious about how is it an antique erection if it has SIPs and tyvek. Do you mean the timbers are recycled antiques?
If you using #3/4 for siding, it would be a wate of time to caulk the seams, water will penetrate at knots and checks. I wouldn’t caulk anyway, it’d make painting impossible to do right and act as glue to cause more checks in the wood. You need tarpaper behind and definitely should backprime all sides of the boards. Did you say what climate you are in?
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You might try doing a search here on the tyvek topic because it's been beat to death already but to sum up my reasons-
Normally I don't take a stand on that. One makes me as happy as another. And I hate to tell you to change or overcoat what is already done. But Tyvek is not waterproof, only sligtly water resistant. You have chosen a siding that is appropriate in appearance for a barn style structure but is probably one of the least water resistant sidings I can think of. So you want to back it up. Especially since you live in a rainy climate.
If you were in a desert climate with less than 15 inches of annual percipitation, had large overhangs, and/or mucho vegetative wind screening, I might back off on that advice.
The furring behind is not a big deal to me, except maybe on the windward sides.
I agree with Piffin about the tar paper, and more to the point, if you are using SIP's, they are already more of an air barrier than Tyvek will ever be. Tyvek's main claim to fame is a barrier to drafts. Tar paper (especially #30) is much more waterproof, cheaper, and more appropriate to what you are doing.
As far as whether to seal the boards, sealing the back and sides is highly recommended. But only if you are sealing the front, also. Otherwise you will cause the boards to cup from only having one side sealed. If you want a weathered look, your best bet would be to look into a finish like Cabot's that either bleaches or stains the wood to make it look weathered. I am not an expert in these finishes, but I am sure that up in NE you will have no problem finding someone who is.
Edited 9/15/2002 11:15:59 AM ET by Nick Pitz