Advice needed--vapor barrier over bat insulation?


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Old 02-03-11, 06:48 AM
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Advice needed--vapor barrier over bat insulation?

Well, I blame Mike Holmes. He has us all confused!

We are in the midst of building our third home on the coast of Maine. For our first two, we used foam insulation for the roof and fiberglass insulation for the exterior walls, with no vapor barrier.

This house has a concrete crawlspace (maximum height around 8 feet) that will be insulated with spray foam on the walls. The crawlspace has a concrete floor, and 80% of it is over solid granite to China. The roof (it's a 1 1/2 story Cape) will be spray foam insulated, as well.

We plan to use bat insulation between the floor joists in the crawlspace (the first floor will be gypcrete throughout) and also for all exterior walls.

So here's the question...vapor barrier over the bat insulation or no? Depending on the time of year, we can have 100% humidity here for days if not weeks (gotta love fog). I'm concerned about condensation inside that vapor barrier degrading the performance of the fiberglass, plus the mold potential.

But anything that improves the energy efficiency of our home is worth doing...and that Mike guy seems to be putting plastic over fiberglass everywhere.

Help!??

Thank you so much for your advice.
 
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Old 02-03-11, 07:47 AM
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I forgot to mention that the main floor will have radiant floor heat throughout, and that the exterior is cedar shingles over Tyvek.
 
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Old 02-03-11, 08:45 AM
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Hi Downeast and welcome to the forum. Bangor area, so I am somewhat familiar with the fog you speak of. Seems to be there every time we drive the coast .

First, consider mineral wool in place of fiberglass, that is what you see Mike using most often. Then stick with the VB on the inside if you choose to use one. With today's air sealing standards, and mineral wool as the insulation, condensation should be almost eliminated. The question is, would a VB be bad and is it required by codes? As long as the house can dry in at least one direction, in cold country that is best to the outside, the having one presents no problem. Since codes can vary, check you local office.

Remember, the currently recommended insulation value is based upon the current energy costs. If you meet that standard and energy prices double, you will have half the insulation you need. Not exactly, but that is the general idea, so add as much as you can while it is easy.

Bud
 
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Old 02-03-11, 10:03 AM
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Bud, thanks for getting back to me so quickly! If you're in Bangor, then you know Jonesport. Fog is our major export

Okay, mineral wool sounds superior for what we have in mind, thank you. One website I looked at said it wasn't a DIY project? To me, bats are bats. So to speak. What do you think? The same website also said that mineral wool acts as a vapor barrier all by itself, so that part sounds good to me. Less work = Good.

Thanks, Donna
 
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Old 02-03-11, 11:16 AM
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Here is some reading for you: Vapor Barrier Guidance — Building Science Information

Building Science has much information on many topics. From the link above be sure to read RR-0004.

I just picked up my first bale of mineral wool (Roxul) to get my hands on it, and see no reason it would be different than fg for a DIY project. Fire rating, mold, and other specifications seem impressive as well. If you plan to do a lot of the construction, read heavily on the new energy efficiency methods, especially air sealing and replacement air.

You mention cedar shingles over tyvek, I hope that the tyvek will be over a solid barrier, plywood preferably with that moisture.

Since super insulated homes rarely need an energy audit, Most of what I run into is old construction. But from what I read, you can really have fun building from scratch and going energy efficient.

By the way, I grew up with an old Jonesport family. When in his 80s he returned for his class reunion, but the other two graduates had passed on. Only three in his graduating class, at least that's the way the story goes. I've only been there a couple of times and remember that when you think you should be getting close, you are probably only half way.

enjoy
Bud
 
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Old 02-03-11, 11:34 AM
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Thanks, Bud. I guess the good news is that nobody contradicted his stories, right? Not to mention zero line at the open bar...

Yep, tyvek over plywood. Serious plywood. We have a bit of a wind issue here on Wohoa Bay (yikes).

Great link! Thank you. For this project we are acting as our own general contractor to save money, and therefore the more research I can do, the better.

Much appreciated!

Thanks again, Donna
 
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Old 02-03-11, 01:58 PM
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Use a serrated bread knife to cut the mineral wool.
 
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Old 02-04-11, 08:05 PM
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Lignin

Is a natural occurring substance found in many materials, like cedar. It is a detergent, which means that it reduces surface tension. Tyvek had a problem with this some years back. The Lignin degraded the water shedding properties of the Tyvek allowing water to penetrate the walls. I imagine the company corrected this problem but to be on the safe side, I would strongly recommend you check with the manufacturer. Bach then the manufacturer recommended building felt be applied over the Tyvek before applying cedar or Hardiboard or other products that contained the substance.

Though air sealing is an important factor involving moisture within a structure, vapor barriers primarily deal with "Pressure Induced Moisture Flow". The rule "High to Low" where an area of high pressure, humidity or temperature moves towards an area of low pressure, humidity or temperature has its exceptions. For example where pressure influences moisture flow. In other words normally an object of higher humidity will give humidity to an object of lower humidity. With Pressure Induced Moisture Flow, the high pressure area induces the moisture within this area to flow. In other words the lower humidity inside the house can be influenced to give humidity to the higher humidity inside your walls, ceilings and floors.

When ever you heat air, this heating causes the air molecules to move. Some refer to it as the air is expanding. But since your home cannot get bigger because of this so called expansion, it creates an area of higher pressure inside the house.

It is for this reason I recommend the proper application of vapor barriers to any area that experiences more than 2,200 HDD's (Heating Degree Days). which your area does.
 
 

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