New siding for an old house
I live in a 1920 farmhouse in climate zone 4. Original plaster on the interior, original clapboards on the exterior covered by vinyl siding, and blown in cellulose that was installed years ago. I’m wanting to replace the vinyl with fiber cement siding but I’m also wanting to add 1″ of rigid insulation under the new siding. I’d like to leave the original clapboards so I can maintain the cellulose, apply a spray-on air barrier to the clapboards, 1″ of rigid foam/taped joints, 1/2″ furring strips and then the siding. Does this make sense? Or should a Tyvek type air barrier be placed over the foam instead? Or should I use foil faced rigid insulation? And if I do will this cause a vapor barrier issue? Although I don’t have a vapor barrier on the inside so moisture can still escape I’m thinking. Lots of questions, just trying to get his right. Thanks for any advise you can give.
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Replies
Michael,
In Climate Zone 4, 1 inch of rigid foam insulation is enough to prevent any moisture accumulation problems in your wall assembly. (For more information on determining the minimum thickness of exterior rigid foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.) So you are OK on that account.
You can use either unfaced rigid foam (for example, EPS) or foil-faced rigid foam (for example, foil-faced polyiso). Either product will work, and the foil facing won't cause any moisture problems.
I think it's possible for you to use a liquid-applied water-resistive barrier on your clapboards. However, that approach will be expensive compared to just using ordinary plastic housewrap (for example, Tyvek or Typar). That said, the liquid-applied product will almost undoubtedly provide a better air barrier than housewrap.
Martin,
Thanks for the quick reply. A couple more follow up questions. If I go with a housewrap, should it go against the clapboards, or on top of the rigid foam? I've read arguments for both. And secondly, do I need the furring strips under the new siding, particularly if I don't use a foil faced foam or can the siding be nailed directly to the rigid foam?
Michael,
Q. "If I go with a housewrap, should it go against the clapboards, or on top of the rigid foam?"
A. Either way can work. Here is a link to an article with more information: Where Does the Housewrap Go?
Q. "Do I need the furring strips under the new siding, particularly if I don't use a foil-faced foam? Or can the siding be nailed directly to the rigid foam?"
A. The only type of siding that can be nailed directly to rigid foam is vinyl siding (because it includes air behind it), and then only if the rigid foam is 1.5 inch thick or less. In all other cases, you need furring strips and a ventilated rainscreen gap. Here is a link to an article with more information: How to Install Rigid Foam Sheathing.