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Question DetailsAsked on 8/18/2011

I am going to get my house sided. One contractor tells me I should take off the cedar shakes siding and the other says I do not need to.?

I want to get my house vinyl sided. I currently have cedar shakes on most of it. One contractor tells me it would be better to remove the shakes and the other said he can do the insulation and siding over it. They both have good prices. If I do not take the shakes off, my windows will be recessed. They would put a J channel to make them look ok. I am confused. What do you recommend. KB

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5 Answers


If your shakes are all random size and thickness ( hand split) take them down, If they are the machined type ( shingles) where all of them are the same thicknes then you can leave them on. Also it may be better to get them down and check for water or insect damage.

Answered 7 years ago by the new window man


It’s never a good idea to go over existing siding, especially if it is a natural product or made from organic materials. There are many reasons not to and here are a few: Hidden damage that could cause much more severe issues down the road, attaching the new siding correctly is almost impossible, manufacturer’s warranty coverage will be Voided. The only good thing about leaving the existing siding is that it’s cheaper, but it could cost you much more in the end.

Answered 7 years ago by BuildingConcepts LLC


If the siding, cedar or hardboard is in good shape and the substrate is wood it is foolish to remove the old siding.
Going over the existing siding adds to the R value by creating an air space between the insulation and old siding.
It also eliminates the problem of mold forming when insulation is attached directly to the substrate.
NEVER, NEVER, NEVER, let a Contractor remove your old siding and install a Tyvek material with insulation over the top.
Here in the North East we are beginning to see the problems this is causing; mold under the Tyvek.

Find a contractor who is knowledgeable about installing over old siding. There are certain steps that must be taken to insure the windows don't appear to be sunk into the wall.
The advantages of hiring a Contractor with this expertise
1) Less expensive
2) More R value with thinner insulation (Never install more than 1/2 inch over existing)
3) Less chance of water and air infiltratration around openings.

The easiest way is to remove it, but it is not the most effective

Tom Ghysel
President Tom Ghysel Siding Inc.


Answered 7 years ago by tghysel


Also. No siding manufacturer voids a warranty for installing siding over existing siding. and in the 30 plus years installing vinyl I have never had a recall from problems that were missed during the job or were the results of the installation.
Rot and deterioration is something that shows up as you install the siding.
Again, find a Contractor who has the experience to do this.

Tom Ghysel

Answered 7 years ago by tghysel


NOT sure why a 4 year old question like this popped up in today's new question list, but c'est la vie - I will pop in my two bits worth in case it is still pertinent, or if not at least for future readers of this question.

Overlaying an intact flat wood panel siding like T1-11 with a breathable siding like lap siding (with proper airgap and water barrier behind the lap siding) is probably not too bad because that is little different than putting new lap siding over an underlayment (which is standard and commonly recommended practice for lap siding and especially for aluminum and vinyl), but putting new siding over any deteriorated existing siding, or over any uneven siding like shakes or shingles or existing lap siding is almost guaranteed to leave you with an uneven surface, and risks the existing siding acting as a vapor barrier and trapping moisture within the wall. I have seen tons of such houses with waves or bulges from going over shingles or shakes or OSB/particle board/hardboard; and when going over lap siding you very commonly get bulges at the old siding joints as well as out-of-alignment rows because of having to follow the old siding alignment. And of course, if the old siding starts warping or cracking (as shingles and shakes and wood lap siding especially do) you can get additional post-construction bulges in the siding.

Also - the issue of windows ending up recessed, which looks bad, obstructs the field of view, and offer a tremendous opportunity for leakage behind the siding at the bottom of the window frame because you are totally counting on the J or C channel to properly control the runoff, which it rarely does perfectly.

My recommendation - do not overlay old siding, at least not without a full ventilation airgap between the layers (so-called "double wall" or "ventilated rainshield" construction) - it is not worth the risk and issues, and is not very expensive to remove the old, which also gives you an opportunity at that time to see into the walls and identify any leak damage, electrical burning / leaking or badly corroded pipes (though those should not generally be in exterior walls unless for a heating system), wet / moldy / missing / compacted insulation, an opportunityto upgrade your wall insulation at little incremental cost (if planned for in advance), and to make sure there is not already a water barrier under the existing siding, etc.

This latter issue - the existence of a water barrier under the existing siding, is not commonly addressed but if combined with a new water barrier under the new siding the two can add up to a pretty effective vapor barrier for moisture coming through the wall from INSIDE the house, causing that moisture to accumulate in the wall interior or in the old siding (particularly if new insulation is not being put over it, so it can get down to condensation temperature in cold weather), causing in-wall rot. This issue gets real complex very quickly when you look at the mechanics of water vapor transmission, but basically exterior siding is commonly (in the winter when interior moisture release is the biggest issue because of the driving interior heat) "cold", and indeed in cold areas the freezing front is generally 'inside" it, so any condensation that occurs does so in the wall insulation. This is not good, but with fiberglass or foam does not cause a lot of damage either, unless it is excessive. The studs tend not to be damaged unless the mosture amount is pretty high, so it is not at all uncommon to tear into a wall in winter and find frost in the outer part of the insulation and on the inside face of the siding, yet no evidence of damage. However, put another siding layer and water/wind barrier over that old siding, and it is not interior to the wall and can become the surface the moisture accumulates on, resulting in massive decay - like that seen on many 90's homes built with OSB/particle board or hardboard underlayment under foam board insulation under siding.

On the comment about putting thick insulation over existing siding and water barrier without an airgap behind it, basically this is the same issue.Yes, this can cause problems because the insulation itself (especially if closed cell foam board or sprayed-on) acts as a vapor barrier - not fully effective but can be pretty effective if taped or caulked at the joints, can cause condensation on the inside face in cold weather, and certainly limits airflow within the wall which can remove moisture and in the "old days" limited wall moisture issues in the normally drafty and poorly or totally uninsulated walls. And of course, putting an airgap behind board insulation or foam-faced siding largely destroys its thermal effectiveness. I laugh (sadly, actually, for the homeowners being convinced to throw their money away) at installations in cold country using foam-backed lap siding with an airgap behind it, because that insulation is doing very little to insulate because the air gap is little warmer than the outside temperature which is flowing through it. In hot country where the thermal "threat" is from outside heat (high air temp or solar heating of the siding) it is a whole different story - in that case insulated siding can serve as an initial barrier to heat penetration into the wall, and when combined with a substantial and continuous airgap behind it to vent the heat coming through, can keep the walls substantially cooler - similar to the theory behind putting sun-tracking louvers on highrise office building sides and windows. Putting a radiant barrier behind the airgap can reduce the heat transfer even better, though it results in siding heating and can damage vinyul and some wood sidings so has to be used with care.

This situation of putting insulation over existing siding when residing causes the same sort of issue occurring with EIFS (Exterior Insulation as a/and Finish System) where wall rot has become common because the insulation acts as a vapor retarder and is also at the cold surface of the wall, so causes condensation on the inside. Actually, though it requires some radical framing and house construction modifications, the "right" way to foam or board insulate a wall (assuming generally protecting against cold rather than outdoor heat) is to put the interior finish over vapor barrier over insulation board or sprayed foam, possibly a second more breathable insulation layer between the studs if using foam board on the inside of the studs, then a breathable or wind-barrier-water shedding housewrap, then the siding with an airgap behind it to remove any moisture getting through the siding to the water barrier. This sort of "double-wall" or "ventilated rainshield" construction will likely be the coming trend and already is in vogue in some cold areas, along with 2x6 rather than 2x4 construction to allow for a higher wall R value. ITs big advantage, particularly in heavy rain areas liek the Pacific Northwest, is it isolated the water-shedding part of the wall system from the wall, greatly reducing the chance of liquid water penetrating into the wall system.. It does have the disadvantages of separating the siding from the wall thermally so the siding then plays only a very small part in insulating the house and insulsation-backed siding is of little effect, and also because the very environment where it is most useful for shedding very frequent rains also puts moist outside air behind the siding, so does not dry out any penetrating moisture as well as one would hope, or as would happen in nice warm sunny conditions where a good convective flow would develop behind the wall.

Ventilated siding also brings up the issue of support behind the siding, especially with vinyl and aluminum - because just firring out at studs leaves typically 16-24 inches unsupported. The fastening rails are sometimes stiff enough to handle this, but some manufacturers do not recommend that, and it can make for a wavy wall. The alternative, though it takes a lot more firring strips, is to use diagonal firring strips placed at a closer spacing (like 8-12 inches) across the studs, to still make for continuous ventilation passages from bottom to top of the wall, but it takes a lot more firring strips.

One further aside on ventilated siding - you have to be careful where the potentially moist air exiting at the top of the wall is going - you do not want it going into the attic or into soffits if that can possibly be avoided.

Answered 3 years ago by LCD

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