Lumber and Siding - Installing fiber cement siding over existing cedar siding
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heron
07-24-09, 03:50 PM
I currently have cedar siding that is failing, moisture from behind is not allowing us to keep the house painted/stained. The current setup is celotex, NO house wrap, 4" beveled cedar. Someone has suggested to apply the cement fiber siding over the existing cedar and adding house wrap/rain screen between the two. Any thoughts/advise would be helpful.
XSleeper
07-24-09, 04:10 PM
You didn't say how thick the foam was, but regardless, I don't think you would want to do that. Fiber cement siding must be nailed to the studs, and the Celotex on the house is already furring you out quite a ways from the studs. If you want to use James Hardie brand siding as a guide, they say it can be installed over 1" foam. (reading between the lines, they don't recommend it over 1 1/2 or 2" foam because of the weight that would be hanging on the "cantilevered nails".) Hanging the fiber cement on the face of your existing siding would be similar.
Additionally, to look the best, siding needs to be installed on a perfectly flat surface, or else you will have a wavy siding job as you look down the wall. Installing a new layer of siding on top of an old layer of siding which is on top of a layer of foam won't give you the flattest of surfaces because each layer has its ups and downs so to speak. And walls sheathed with Celotex are notoriously wavy already because Celotex doesn't keep wall studs as straight as plywood sheathing will. So I'd highly recommend tearing off the cedar clapboards. Adding more layers on top of existing layers also builds you out past window and door trim, which can make them appear sunken. If you are DIY, the tearoff will only cost you your time, and a trip to the dump or local burn pile. If you do tearoff, take care not to puncture the Celotex by always having a board along with you to pry against to protect the sheathing from damage. Hammers and prybars will tear the stuff up in nothing flat if you try to pry against it without having some protection.
I've seen other jobs where paint wouldn't stick to cedar over Celotex, so that's not a surprise. Backpriming and painting the cedar clapboards would have helped. A rain screen is a good idea, But nailing furring on top of your Celotex may not be. If the nails are overdriven, it will again make for a wavy surface. If you did nail on furring (1/4-1/2" thick), you'd have to be very careful about not overdriving the nails. The same would go for your siding installation. Overdriving nails will also cause a wavy looking installation.
One idea for a rain screen that wouldn't involve any furring is to use Tyvek Drainwrap, which is kind of like corrogated cardboard, but smaller. The grooves in it allow for a little breathability and drainage should any moisture need to get out. And you already know you have moisture issues with that Celotex.
Here's a great link (http://www.buildingscience.com/documents/primers/bsp-063-water-managed-wall-systems) to an article on drainage planes. You might enjoy picking up a few tips from it.
So those are my initial thoughts... I'm sure some others will chime in too. BTW, I see this is your first post, so welcome!
Additionally, to look the best, siding needs to be installed on a perfectly flat surface, or else you will have a wavy siding job as you look down the wall. Installing a new layer of siding on top of an old layer of siding which is on top of a layer of foam won't give you the flattest of surfaces because each layer has its ups and downs so to speak. And walls sheathed with Celotex are notoriously wavy already because Celotex doesn't keep wall studs as straight as plywood sheathing will. So I'd highly recommend tearing off the cedar clapboards. Adding more layers on top of existing layers also builds you out past window and door trim, which can make them appear sunken. If you are DIY, the tearoff will only cost you your time, and a trip to the dump or local burn pile. If you do tearoff, take care not to puncture the Celotex by always having a board along with you to pry against to protect the sheathing from damage. Hammers and prybars will tear the stuff up in nothing flat if you try to pry against it without having some protection.
I've seen other jobs where paint wouldn't stick to cedar over Celotex, so that's not a surprise. Backpriming and painting the cedar clapboards would have helped. A rain screen is a good idea, But nailing furring on top of your Celotex may not be. If the nails are overdriven, it will again make for a wavy surface. If you did nail on furring (1/4-1/2" thick), you'd have to be very careful about not overdriving the nails. The same would go for your siding installation. Overdriving nails will also cause a wavy looking installation.
One idea for a rain screen that wouldn't involve any furring is to use Tyvek Drainwrap, which is kind of like corrogated cardboard, but smaller. The grooves in it allow for a little breathability and drainage should any moisture need to get out. And you already know you have moisture issues with that Celotex.
Here's a great link (http://www.buildingscience.com/documents/primers/bsp-063-water-managed-wall-systems) to an article on drainage planes. You might enjoy picking up a few tips from it.
So those are my initial thoughts... I'm sure some others will chime in too. BTW, I see this is your first post, so welcome!