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birch plywood - and MDF interior wall coverings

by David Billinghurst last modified Dec 09, 2006 06:05 PM
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birch plywood - and MDF interior wall coverings

Posted by David Billinghurst at October 18. 2006

We have been building our house for over a year now here is our blog link.

http://members.livemodern.com/Members/purekrista/blog/

We are starting to get ready to install insulation and cover walls.

We are considering using plywood and mdf on some of our walls in addition to drywall. My question is:

We are using unfaced batt insulation, when using drywall the installers of the insulation say that the PVA primer works as the vapor barrier so there is no need for a poly mil covering but is unsure what to do for a vapor barrier if using the plywood/MDF.

We planned on using Benite on the wood to preserve it. Would I need to treat the wood on the front and backside of the wood with some sort of water seal as well as the benite? and/or would I use 4mil poly inbetween the wall and the insulation? Or would you use something like 15lb tar paper as a vapor barrier like you would use on a wood floor?

Re: birch plywood - and MDF interior wall coverings

Posted by David Billinghurst at November 02. 2006

Did I ask the question wrong? Can't believe no one has an answer for me? Saw a neighbors house down the street that did this, Looks like the wood was just installed over unfaced batts to studs with finishing nails. nothing behind it. the wood was preserved with benite and then clear coated with something?

can anyone recommend a clear coat to use?

Re: birch plywood - and MDF interior wall coverings

Posted by mordo at November 02. 2006

daveb, I think any clearcoat would be fine for birch ply - it's really down to what you're comfortable using. There are some low-VOC, waterborne acrylics that work very well.

With drywall installed and joint taped and finished, you will need no vapor barrier or even primer on the drywall. If you fix the plywood at regular intervals at the perimeter, you shouldn't need to worry about the panel warping even with finish on just one side. If you are plannign on leaving reveal gaps, make sure the drywall is finished and painted at leas where your reveals will fall.

If you are looking to retain the very blond color of the unfinished panel, use a UV-blocking clear finish. Sunlight will patinate the birch to some extent.

If you like a richer color, think about Watco oil finish. It is an oil that will polymerize and get quite tough. It will have a matte sheen and will be easy to renew if it gets worn or dull.

Re: birch plywood - and MDF interior wall coverings

Posted by uncleho at November 04. 2006

Be careful where and whether you need the poly. Depending on your climate and application (above or basement), the poly may help trap moisture and cause mold, etc.

There is a cool book out by ... Libeinski? Crap. Too lazy to go upstairs to find the book. He writes about everything smart building in various books dedicated to specific climates. I assume Seattle is more moist than my home in Michigan, but not as cold.

Example: If interior is really moist and depending on pressure differences on either side of wall (drywall or panel), moisture will make its way through damn near anything and if you trap it from leaking out, it will just accumulate and rot. No amount of sealing will hold back the damage of moisture on wood. It seems he speaks of basically controlling the flow of moisture and not necessarily blocking it.

Re: birch plywood - and MDF interior wall coverings

Posted by David Billinghurst at November 05. 2006

Regarding POLY, i have been told that you don't want to use it cause it will trap the moisture like you said. But i was unsure how to handle the use of MDF or Plywood. Sounds like the best thing to do is put nothing between the unfaced batts and the panels/drywall. then just coat the panels with a clearcoat sealer.

I have read something like you mentioned. I think all that info is on this website.
http://buildingscience.com

Re: birch plywood - and MDF interior wall coverings

Posted by Karl Mead at December 09. 2006

Some form of vapor barrier is generally appropriate regardless of where you live. In the north where one is primarily heating, it should be nearest the inhabited space. In the south where one is normally cooling, it should be to the outside nearest the siding. The reasons for this are quite simple: condensation.


In the north during the heating season the highest moisture is inside the living space. As the air inside is heated, it has slightly higher pressure than the outside air which means it is always trying to get out of the house. As the air moves out of the living space into a wall cavity or other unconditioned space it will eventually get to a point within the insulation where it will hit the dew point and will condense in the insulation within the wall cavity. The farther the moisture laden air progresses moisture will eventually freeze within the wall.

Limiting the amount of air movement and in particular moisture out of the living space is the job of a moisture barrier. This can be done using an airtight drywall system and PVA primer or it can be done with a plastic film, tape and special electrical box surrounds. It can also be achieved by using low perm rate sprayed in foams and with SIP construction (to name a few). In any case the quality of the seal will depend on the fastidiousness of the workers installing the system. Problem areas are any and all electrical outlets, lighting cans, points where interior walls meet exterior walls, the joints at the base of walls and at the ceiling, cantilevers past the outside walls, window openings door openings and so on.


You want to ensure the outside walls are not airtight to the outside (ie some air can move through the wall) in the above situation so the moisture can evaporate as it can (which it will do even at very low temperatures as the outside air is very dry.)

In high humidity, high heat and therefor cooling zones you want the opposite to occur. The vapor barrier to the outside keeping the bulk of the moisture out and any that does get in will evaporate into the dehumidified cooler interior space.

In any case controlling interior humidity is paramount to preventing the growth of mold and its associated health issues. Human beings put out a lot of water vapor with each breadth, perspiration, showers, baths, cooking, plants and tracked in moisture (boots, shoes, wet clothing and so on). Keeping humidity levels well below 80% is critical to the structure's longevity and your health.

Re: birch plywood - and MDF interior wall coverings

Posted by Karl Mead at December 09. 2006

Previously David Billinghurst wrote:



Regarding POLY, i have been told that you don't want to use it cause it will trap the moisture like you said. But i was unsure how to handle the use of MDF or Plywood. Sounds like the best thing to do is put nothing between the unfaced batts and the panels/drywall. then just coat the panels with a clearcoat sealer.

I have read something like you mentioned. I think all that info is on this website.
http://buildingscience.com



One should always coat both sides of a semi rigid sheet good especially one that was once alive. MDF, plywood or anything else wood based should always be back primed or at least sealed with a sealer. You will be amazed how strong and persistent a warping sheet can be. Think about it from the standpoint of keeping a balanced construction so that all exterior surfaces of the part will try to behave in the same way.

To prove what I am saying take a thin piece of cardboard (not corrugated) and wet one side. The wet side will quickly cause the sheet to bow because it has effectively become longer than the dry side.
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