Shingles or siding over old T1-11 question
I have a puzzle that I thought would toss out to my fellow builders.
I have a house that was built in 1959 with a T1-11 type plywood siding that has an MDO like finish. I’ve owned it about 20 years. It has never held paint particularly well even though in one repaint it was nearly stripped down to the base, primed, and repainted. I’ve been thinking that rather than continue to paint over this plywood I would just apply a new layer of finish material over the existing plywood. I’ve variously thought of doing this in either shingles or some sort of shiplap redwood or cedar.
The thing that raises concern is that there is a layer of building paper under the existing T1-11. That, in my mind anyway, raises the question of whether to apply a second layer of building paper under the shingles, or to just shingle directly over the plywood. My inclination is to do the former, apply a layer of building paper under the new shingles, or siding. But, would the double layer of building paper tend to trap moisture in the T1-11? What would you do, and why?
Replies
Depends on where you are
Depending on where you are, it might or not not be a problem. Also, building paper isn't really much of a vapor barrier. Particularly, the way most of it was installed in 1959.
Northern California
We're in Marin County which is at the north end of the Golden Gate Bridge. The climate is relatively mild, but we are on the waater and thus have slightly higher humidity, though nothing like a place like Miami, for example.
You are correct on both points about the use of building paper, which is why I'm leaning toward applying another layer over the T1-11 and shingling over that.
Thanks.
Neither the old building felt nor any new housewrap you might use is a "vapor barrier" -- they shed water (to a degree) but still allow water vapor to pass through.
There is a slight danger that if you do a really lousy job of flashing/caulking liquid water could get trapped between the two layers, but even then it would cause relatively little harm.
Whether a new layer of housewrap would do any good depends on where you live and how "tight" the existing structure is. If you live in an especially cold climate (like here in MN) then carefully installed housewrap is pretty much guaranteed to improve comfort in the house (if not reduce heating bills). In, say, Tennessee, however, there would be little advantage unless the existing structure is fairly poor at blocking air infiltration.
Northern California
We're in Marin County which is at the north end of the Golden Gate Bridge. The climate is relatively mild, but we are on the water and thus have slightly higher humidity, though nothing like a place like Miami, for example.
The original is actually building paper and not 15 pound felt which would rarely be used under T1-11, at least at that time in this area. Building paper is definitely lass of a vapro barrier than felt, which is partly what I was counting on to provide some "breathability" if I were to apply another layer over the T1-11 before shingling. Although a Tyvek, or similar housewrap makes sense for most new construction I think I would be inclined to not use it in this case simply because it would form a significant vapor barrier tending to trap moisture in the T1-11.
Of course, the point about doing a bad job of flashing is true, but a bad flashing job will cause problems anytime and anyplace.
Thanks for the thought.
Tyvek a VP?
Don't know where you got the idea that Tyvek is a vapor barrier. It is not, it is a weather barrier, meaning it will stop water and wind infiltration. That is liquid water from the outside, not water vabor .
Several years ago I thought the same thing you are thinking and brought it up on this site. It was suggested that I experiment with water and Tyvek to see exactly what it does. I put about a quart of water in a five gallon bucket and covered the bucket with a single layer of Tyvek, tightly sealed to the bucket with Tyvek flashing tape. I set the bucket in direct sunlight for one week of 85 90 degree weather. We also had a good soaking rain one day durring that week, and at the end of the week I checked the water level in the bucket. To my suprise, although I poored rain water off the top of the bucket, there was only what I would consider condensate water inside the bucket. I would say that 99% of the original quart of water was gone.
Granted that was just a backyard test and not very scientific, but it gave me a pretty good idea that Tyvek does work as advertised. It stops gross liguid water and air infiltration but allows water vapor to pass through it.
I think you would be safe to use it over the T-111. Building paper would also be fine IMO.
I just wanted to clear the air a little on the misconception that Tyvek is a VB.
Just to add my 2cents,
Tyvek is an AIR barrier, it keeps air from infiltrating the wall cavity, and consequently prevents what little moisture is in the air from penetrating the wall cavity, yet it is still supossed to allow air(moisture) to exit from the wall cavity, sort of a one -way membrane.
It is also supposed to offer some bulk water protection from the outside as well.
IMHO.....BS!! I've seen way to many walls where bulk water simply penetrated the Tyvek, rotting the sheathing behind it, I think the stuff is junk, period.
The experiment mentioned by Dave doesn't take into consideration that the Tyvek is laying tight to the sheathing in a real world situation, as opposed to the experiment, where the Tyvek is stretched across the top of the bucket with no solid surface up against it.
To answer the OP's question, the original building paper is intended only as an air barrier, If you were to use 30# felt over the T1-11 I think this would give you a better bulk water protection than the Tyvek and would allow sufficient breathability from inside the wall cavity.
Is there a VB on the inside behind the drywall?
Check the link below,
Geoff
http://www2.dupont.com/Tyvek_Weatherization/en_US/tech_info/faqs_builders.html
Tyvek offers SOME protection against liquid water penetration, but is not intended to replace good flashing and caulking. If water got behind the Tyvek then a) there was a failure of flashing/caulking, and b) very likely poor attention was paid to properly lapping and taping the Tyvek (but we already knew that poor attention was at fault, didn't we). In cases of this sort of failure it's unlikely that felt would have performed any better, and it's a MUCH poorer air infiltration barrier.
And, BTW, there's no such thing as a "one-way membrane", any more than there's such a thing as a "one-way mirror".
Bulk water
As Dan pointed out if bulk water got behind the wrap, there were flashing/siding issues that were wrong to start with.
I had to replaced some siding on a house I built 20 years ago. Much of the house was brick veneer, but the rest was vinyl siding over TyveK and sheating. I replaced 1 square of siding and the torn Tyvek behind, as well as one partial sheet ot sheathing (storm damage).
I saw no sign of bulk water intrusion anywhere.
The devil is in the details.
Sure- just add to the mess.
I've found that there's nothing like a waterlogged, wavy, damaged old layer of material to give your new layer that extra-special "appearance." Then you get to sit back a few years and discover all the new problems you've created.
It's pretty simple: if the old stuff is shot, get rid of it. Take it down. Replace it. The fact is, you've got far more life out of the T111 than you ever had any right to expect. Another advantage to replacement is that your door and window frames will still fit the walls.
If. for some reason, you insist on adding a new layer to the old, space it off. Get some "W" or "hat" channel from a drywall supplier, and create that small air space between the layers. This will allow any moisture that enters to drain, and enough air to flow to reduce mold concerns.
Otherwise, there's a lot to reccomend the hardi-plank stuff. No mold/ rot issues, no fire issues, looks like traditional clapboard siding. Takes paint, and holds it, better than anything else. Best of all, it's fairly inexpensive; it might even be cheaper than shingles.