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Question DetailsAsked on 5/29/2014

We are being told aluminum siding expands and contracts creating moisture problems under the siding. True?

Condo association with 63 buildings that are 30 years old is being told to plan to replace aluminum siding with vynal siding by Architect. He says aluminum expands and contracts allowing moisture to penetrate siding and rot wood beneath. He says there is no moisture barrier as it was not requried when built. We are thoroughly confused as there seems to be no mention of this on-line by Angie or anyone else.

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2 Answers


Aluminum, steel, concrete and vinyl (and wood to a lesser extent) all shrink and expand with temperature changes, causing problems with water penetration at the resulting gaps between planks and at the ends, at windows and doors and corners. During expansion the planks can also buckle, bowing away from the wall and letting water in, and of course they can become unlocked and peel away from the wall in extreme wind storms.

The first thing you have to understand is the philosophy of plank siding - which is really not a very good solution when you look at it logically - is that it is not at all "waterproof". It is designed to shed water like a bird's feathers the same as shingles do - water runs downhill off one layer onto the next onto the next and so forth, but in strong winds the water can blow up underneath then unless the planks are the interlocking type, which not all are. The big problem with siding is the joint cracks from shrinkage have to be protected.

The "solution" come up with by the manufacturers is to put a water shield strip under the mid-wall seams or joints, to catch the water and run it back to the top of the next lower plank - like this except there are better truly waterproof materials than roofing felt to use for the strip-

At windows and doors and building corners, the manufacturer solution is basically to let the water get into a metal channel flashing, and run the water down in the channel to the bottom of the door or window or corner, and then run the water out on top of the plank at that point - like these pictures which show what J channel looks like installed, and what the bottom of the channel runout onto the siding looks like

Problem is, the channels are generally not channels for the water at all, just flashing strips, which do not prevent the water from running in laterally under the planks. As you can see in this image, the siding inserts into the channel which holds the ends of the siding, but there is no bend or curve in the channel to prevent the water from flowing laterally off the channel onto the wall under the siding - and the way the channels commonly fit up against the trim, the tendency is for the channel to tilt towards the nailing edge, promoting water flow that way. I reduce this tendency by running a bead of caulk down the side of the chanell short of the nailing slots, but it is hard to install the siding without damaging that small ridge, so water is guaranteed to run off that edge onto the wall sheathing.

Therefore, the only protection you have against this water (and any water getting in under the planking or through breaks or open joints) is a moisture barrier (which you will not the above photo does NOT have) over the sheathing and under the siding, which forces the water to run down the moisture barrier and out at the bottom of the wall - like second photo in this link -

New construction should ALWAYS have this moisture barrier installed under ANY siding, though the specific best barrier varies with some types of siding materials. This barrier is NOT a vapor barrier like commonly goes under your interior drywall - it is designed to run free water down its face without letting free water through, but is permeable to water vapor so moisture thereotically can escape from within the wall to the outside - though if its surface is wet, water vapor also moves into the wall from it, which is not desireable. The synthetics are better at preventing this, as felt (tarpaper or building paper) has a greatly increased water permeability after it gets wet.

I can't say why the architect is prefering vinyl to aluminum siding - both have durability issues, I can only presume he is recommending vinyl because it is now much more common than aluminum or steel, and does not dent as easily as aluminum or have the paint chipping problems of metal siding. However, in my experience, vinyl is fr more prone to popping free and letting water into the wall system.

Here is a pretty good article on the moisture barrier issue FYI - you can find more readable ones (but not unbiased) at the DuPont Tyvek website listed after that -

Answered 6 years ago by LCD


Vinyl siding will expand & contract as well so if the aluminum siding is to be replaced a rain barrier will be installed befor the siding goes up. A potential problem with your existing siding is there should be "weep holes" spaced at the bottom of each lap to allow moisture to escape and due to spiders, bugs, etc the holes can become clogged preventing moisture/rain to escape.

Answered 6 years ago by hosey

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