air and vapor barriers for pole barn batts insulation. Also pole barn framing. Pros?

[check]

Questio: Why use finreglass batts when Roxul is so much better?

 

[bob112]

I would really prefer a cellulose batts! But it seems nothing else can compete with the price of glass per R value and I will need it to stay in place. There are even wet applications of cellulose that I considered too but I don't think I can do a roof like that.

For clarification, I have some machines that have flood coolant to the tool which creates some mist. I will be building mist arrestors to reduce this problem. All shops with this equipment don't really have issues but I know it will increase the humidity in the building. Have to plan for it.

 

[Looking4new]

Questio: Why use finreglass batts when Roxul is so much better?

Maybe Roxul is way more expensive there than here. I much prefer Roxul over fiberglass but fiberglass is not an irritant to me.

 

[Cord]

Tyvek is an air barrier, not moisture barrier, as above stated. I have seen the cheaper woven products breakdown as well, but not actual tyvek. I hear that tyvek will be going away soon as it is believed to trap moisture in cases. I find this total BS personally. If there is that much moisture in the wall, there are problems elsewhere with construction, flashing, sealing, etc.

Yes, Tyvek is a air barrier and not a moisture barrier. It's also a product that was invented and then applied to the construction industry without actually having a problem to solve. Gotta have it now though! Actually the Tyvek (or Typar) products are more effective than felt at keeping moisture out of the wall cavity. That said, the increased need for moisture control has come from the extensive use of vinyl siding which has absolutely no capability for controlling water. When wood siding is installed over Tyvek the product does not allow moisture to move through the wall resulting in it being trapped behind the siding. Just normal seasonal changes are enough to drive moisture into the wall where it stops when it hits the Tyvek. With loose fitting siding like vinyl, this isn't a problem as there is enough air flow to evaporate the water. However, with wood siding, the water collects on the back side of the siding resulting in rot. It's well documented that it's not a good construction practice to install wood siding directly over a air barrier like Tyvek. If you want to use Tyvek with a wood siding product, then a rain screen system should be utilized. This will provide the drainage plane needed to allow the moisture to not accumulate in contact with the wood. Although not as air tight, I'm personally a big proponent of felt behind wood siding. It's worked for a couple of hundred years and with plywood sheathing and modern window flashing there isn't all that much air moving through the wall anyways.

For a pole building, what is your source of moisture that is driving the need for a vapor barrier? The occupant load is low in a pole building and there is no cooking or hot water uses to raise the humidity level. The greatest source of condensing moisture will be in the spring when the first warm days come and the surfaces are still cold. That said, if the building is heated (it is heated right?) then spring condensation will not be an issue. If it's cold then the moisture will quickly drive through the insulation and condense on the metal. Although the metal joints do provide a air infiltration source, a simple butyl strip at the joint will result in a adequate air seal. Don't forget, it's not the lap joints that are the major source of air infiltration, but the joint between the roof and wall panels. The use of foam gaskets at the connection will go a long way to sealing the building. Although you don't want to use spray foam, the use of foam to seal the building is exceptionally effective, especially at the panel joints. What are you planning for doors? Sliding an overhead doors are massive air leakers and are exceptionally difficult to seal.

 

[bob112]

Its not so much that I require a moisture barrier there as an air barrier. Hard to get one without the other, thus the selection of Tyvek as it will allow moisture to migrate. Felt is actually a great selection for this but will be nearly impossible to install. I realize felt has special "variable perm rating" specs. It can actually permeate more or less depending on conditions and does it well. However, I cannot ignore the radiant issues with it. It is a massive heat collector.

If I install Tyvek on the outside framing, there will always be an air gap in there due to the ribbing of the metal. That air circulation can be tuned depending on need but convection in the space will always occur so some level of top venting may be needed.

I am certainly not married to Tyvek and considering alternatives. I realize Tyvek was already a product and some bean counter got the job of finding more applications for it. All the generic brands of "house wrap" are nothing more than plastic with perforations. That will certainly leak water....

I do have plan for 1 or 2 OH doors and I realize they can and do leak. I have developed some ideas to minimize that. One in which is installing foam seals to the top of each panel which compressed and seals on closing. Regardless, I am not sure I can accept a plan to totally exclude an air layer on the outside of the batts. We already know air will move between the metal and batts which will cause moisture to get into the batts and possibly hold moisture against the metal. This will ruin the batts. It needs to stay as dry as possible.

Totally open to thoughts and ideas here!

 

[woolyAcres]

You might look into a smart membrane such as this one from CertainTeed.

 

[Billrog]

Bob112 don't know if this helps answer your question but my shop has metal ,1x4 strapping, fiberglass insul, 6 mil poly then drywall last year after 23 years I took the end out of the building to add on and everything was a dry and sound as the day it was built. With R40 in the ceiling during the summer even when over 100 F the temp. inside is 69 to 74 degrees and I assume the 1" space between the insultion and the metal shting has something to do with this.

 

[Billrog]

I've put up a couple pole barns for shops for others using the same principal but don't know how well it worked as I've never been back to see them but have had no complaints either.

 

[bob112]

Bill, I am unclear, you installed a membrane between the metal sheeting and the insulation? Or no? What was it?

The generals as I understand it with batts is it needs to stay clean, dry, and placed properly. People tend to throw it in not realizing air infiltration leads to moisture build up in the batts, thus problems. I read a LOT that batts will "lose R value over time" but the physics do not support that because the glass itself did not degrade. I have opened buildings before to find severe dirt infiltration in the batts. Combine that with moisture and there are issues.


I have done a lot of crunching on the concerns of "thermal bridging" and one of the reasons I am using wood construction. I know for certain that a thin layer of foam board between the sheeting and frame can hugely help with this, there becomes a point of diminished returns for me and that will be hard to install. At every screw there is heat conduction from the sheeting to the frame, but the R value of wood helps. There is not R value in steel so when guys put up a metal building, Every exposed interior I beam is a heat radiator, regardless of the insulation.

I am hopeful that it will perform on the mark as I intend.

 

[Billrog]

Bob112 I have no membrane between the metal and fiberglass just a 3/4" space the strapping provides. When I sheet with board and batt over the strapping I first put tar paper but still maintain the space to the insulation. Over the years I've remodeled or renovated building during winter where the insulation was a solid block of ice. In every case this was a malfunction of the interior vapor barrier having rips or tears or just missing. I think a layer of foam under the metal would be a good plus.

This is what the insulation looked like under the metal after 23 years.