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Install & Replace

Wallboard or Plywood?

My house is several blocks from the ocean in VA and sits up on stilts with an open carport underneath.  The open carport ceiling is currently painted wallboard, but is beginning to deteriorate (with the help of some squirrels).  I'd like help as to whether it should be replaced with wallboard, or would plywood, or something else, be better - any pros or cons of either, other than cost?  Also, I will likely take the opportunity with the ceiling down, to replace or add to the insulation - any suggestions for the best type of insulation and how much?

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Posted 2012-03-01T16:07:57+0000  by jim88 jim88


Hi Jim88, this is Mike The Home Depot Answer Man. Welcome to the community.


Wallboard is really intended for interior use and being exposed to the weather will shorten its service life. The original contractor used it to save money, as it is an inexpensive and easy product to work with.


Based on your location, near the water, I would use exterior grade pressure treated plywood for the carport ceiling.


This product is rather expensive compared to wallboard, but will last many many years and is rated for exterior use and is insect and rot resistant.


Be sure to prime and paint the plywood after installation to maximize its service life.


As for insulation a spray on expanding foam would be ideal, this usually requires a contractor to apply as it is rather expensive and time consuming to do it yourself.


If that option is not available, I would consider roll fiberglass bats. They are easy to apply and relatively inexpensive to purchase.


The size of the floor joists for the floor above will dictate the amount of insulation you can install.


In your case, R 13 roll insulation will work fine; fill the space completely without compressing the insulation.


I hope this answers your question.






Posted 2012-03-01T18:32:52+0000  by Mike_HD_OC





I personally feel the use of pressure treated plywood would be overkill, especially if you are going to prime and paint it. I will quarantee that the siding on the house is not PP material, although it might be rot resistant cedar, etc. Compared to the siding, the ceiling over the carport receives virtually no sun and no direct rain, except in hurricane type situations. I would use either a T-111 grooved material or a plain sheathing with batten strips over the edges.


I would certainly use the opportunity to check the amount and type of insulation. It is difficult to believe that there would be no insulation in such a location. I would also use the opportunity to close any breeches in the flooring. The average house loses more heat and conditioned air to air infiltration than to conductive heat loss Fill any holes or gaps with either caulk or urethane foam. sprayed insulation would be the absolute best for both conductive heat and air infiltration, however, it is about triple the cost. You should consult with a couple insulation contractors to get their opinions.


This is the type of job where a little elbow grease can save you a lot of money. An insulation contractor is going to work for less if the old drywall has been removed for him. Same for the old insulation, should you decide it has to go. If you are sure you are going to go ahead with this project, it would be a good idea to pull down a section of the drywall and see exactly what is there. Then you can make a more educated estimate of what is involved, so can the contractors who are giving you estimates. There will be no surprises if the area is visible to view.


Posted 2012-03-02T01:30:31+0000  by ordjen

 It has since occured to me that the drywall might have been there because of fire/building codes. In normal construction within a house with attached garage, the living area must be separated by taped drywall. Check your local building department. You might be requred to use some kind of non-combustible material such as cement board.

Posted 2012-03-04T03:06:35+0000  by ordjen

Jim I agree with ordjen pressure treated plywood would be overkill here + in addition PT plywood doesn’t take solid paints as well as the regular exterior grade CDX does. That’s mostly because of the preservatives used.


Also while you're checking if non combustible material is acceptable (plywood) I would also inquire if spray foam can be used as well.


Spray foam is available in open and in closed cell structure.


Closed cell foam incorporates an insulating gas that is retained within the cells. The closed cell nature provides for a highly effective insulator, air and vapor barrier. Vapor permeability ratings for closed cell foam are if I’m not mistaking somewhere around 1.0 perm. According to most building codes, a vapor retarder must have a perm rating of less than 1.0; closed cell foam meets this standard


Depending on your geographical location vapor barrier (closed cell foam) may need to be placed on exterior, interior or it needs to be fully omitted.


Incorrectly placed vapor barrier can create whole another set of issues.


Hope this helps.



Posted 2012-03-05T16:21:31+0000  by George_HD_CHI

 The whole concept of impermeable vapor barriers has been hotly debated in the last few years. Many homes, including mine, have a 6 mil plastic right in back of the interior drywall. However, under certain scenarios, this is a really bad idea. I n hot, very humid climates where the air-conditioning is run constantly, the humid outside air tries to migrate into the cool dry interior air. When it hits the cool backside of the drywall with the plastic on it, it condences. Water then runs down the wall and floods the wall cavity and floor and into the house.


In the situation of the summer home on the warm, humid coast, it would be a bad idea to use a non-permeable barrier under the floor or in the walls if the unit is to be air-conditioned, or might be in the future. You want a moisture retarder, not a moisture barrier! You do not want moisture falling back down on the insulation and/or staining the sheathing or causing peeling.


Here is where foam can really be of benefit. A couple inches of foam sufficiently raises the dew point to where the inwardly migrating exterior moisture will not condence on the surface of the foam. As stated above, it also absolutely seals all air infiltration and insect entry  points. Normally, foam has to be covered with a non-combustible material if it is within the conditioned space. It is also normally covered if it is in a non-conditioned crawl space.The foam does not burn, per se, but emits toxic fumes during a fire. I am not sure how the local building codes would view foam in this overhead carport scenario. In any event, the foam by itself is not aethetically pleasing. I personally would still cover it.

Posted 2012-03-06T06:08:14+0000  by ordjen

Ordjen has a good point on the fire code regulations. However based on the open air nature of the carport and its' proximity to the water, I still feel that drywall or wallboard is not the ideal solution.


The local building code should be observed when the work is completed and your local building department can give you a list of recommended materials.



Posted 2012-03-06T17:30:31+0000  by Mike_HD_OC


The more I think about it, the more I would lean to a cement board product such as HardiPlank which is available as sheet goods. It meets all the requirements: does not burn, insects and critters don't like it and it does it not decomposed under high moisture conditions. It also takes paint well, although shelterred from the sun and water under the home, I am not sure how neccessary that is. Hardi-Plank is also available pre-painted.


It also occurred to me that boxing in the underside of the elevated structure with a material of inherent structural integrity, would  increase the stiffness of the floor structure. This could be of benefit in a hurricane prone area where "racking" or twisting or the structure might be of concern.

Posted 2012-03-06T17:47:29+0000  by ordjen

Ordjen VA falls in zone 4 according to DOE map;


DOE map.jpg


I'm a big fan of Building Science and their publications and this is what their recommendation is for zone 4 (marine);


 Zone 4 (marine) requires a Class III (or lower) vapor retarder on the interior surface of insulation in insulated wall and floor assemblies where the permanence of the exterior sheathing is 0.1 perm or less as tested by Test Method B (the “wet cup” method) of ASTM E-96) and the interior surface of the exterior sheathing shall be maintained above the dew point temperature of the interior air.  Under this design approach assume steady state heat transfer, interior air at a temperature of 70 degrees F (21 degrees C), at a relative humidity specified inTable 1 and exterior air at a temperature that is equal to the average outdoor temperature for the location during the coldest three months of the year (e.g.. December, January and February).

Class III Vapor Retarder

10 perm or less and greater than 1.0 perm



They would  have to go with at least 3 inches of spray foam or thicker . (gives you less than 1 perm at 3").



Posted 2012-03-06T21:53:03+0000  by George_HD_CHI




Let me give you an example with which I had to deal concerning improper vapor barriers:


A few years ago I was removing a Mylar wallpaper from a two story living room in the Chicago area. The upper paper came off fine. However, when we got down to about the 8 foot level, it became obvious that the drywall was soft and spongy. You could merely poke your finger through it.  Wondering what was going on, I walked outside to look at the construction. The upper level had stained verticle siding. The lower level had a porous brick veneer, as is common around Chicago. Apparently, the absorbent brick was passing moisture into the wall cavity where the impervious Mylar (thats the stuff those balloons which stay aloft for weeks is made from) was preventing it from migrating out of the wall.


The wall was saturated. All the old drywall from about 8 feet down had to be replaced along with the wet fiberglass. Fortunately, the studs, though wet, were in good condition. After a couple days of airing out the wall, it was rebuilt. Fortunately, no great amount of mold was involved. My best advice to the homeowner was to never put vapor retardant wallpaper back on that or any  walls. That would be almost all wallpapers, since almost all wallpapers have plastic coatings to make them washable. Unfortunately, it retards vapor flow too. Heavy commercial type vinyl wallcoverings and all foils would be even worse. Further, I suggested that they start a program of regularly sealing the brick with a water repellant sealant.


What innocent homeowner would ever consider that a mere wallcovering could cause so much damage!


I had another home which exhibited extensive past peeling of the exterior wooden  clapboards.. It was built in the 1960's as an exact duplicate of a frame Colonial Williamsburg, Virginia home.  The peeling was largely only on the lower part of the first floor. Again, brick was involved. The house was built with an exposed brick foundation with about 3 feet exposed above ground. Suspecting that the porous brick was involved, I went and got a glass of water and threw  it on the brick. It ALL went into the brick! 


Wood with a moisture content of 10 t0 12 % will probably show some degree of peeling. I went around the house and did not find an area of less tha 12%. Some areas were 20%!


Again, I suspected the brick was getting saturated and passing moisture up into the wall cavity due to an inadequate vapor break under the sill plate. The windows above and the horizontal braces about half way up the wall cavity slowed the vapor from traveling higher and causing as much peeling at higher levels. Again, my best immediate suggestion was to seal the brick well. I am not sure how to improve the sill plate vapor break on an existing home? 


Unfortunately, the poor painting contractor ( me :) ) has to try to deal with the problem. Often the homeowner thinks it is something the paint contractor did wrong. For the most part, the contractor is dealing with symptoms. He can try remedial cures, but poor construction dooms many homes to long term failures unless often expensive repairs are made to correct inherent shortcomings. One only needs to watch an episode or two of Holmes On Homes on TV to appreciate that the cure can really be involved and expensive.


As I was less knowledeable back then about vapor barriers, I never checked to see if that house also had a plastic

vapor barrier in back of the drywall. I would not have been suprised.

Posted 2012-03-07T00:38:17+0000  by ordjen
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