Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Building & Construction

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Search this Thread Display Modes
Old 12-06-2010, 01:21 PM   #1
Newbie
 
Join Date: Dec 2010
Posts: 2
Share |
Default

Insulation and vapor barrier


Let me just say up front thanks for the help.

I am building a new home and have a question on insulation and vapor barriers.

What is the proper way to install a vapor barrier and what do i use.

I am looking at using faced fiberglass insulation and then wraping the inside of the house with plastic. is this my best option or is it to much..

Insulation ---- should i be using faced or unfaced insulation if i install a continuos plastic vapor barrier.

if i should be using faced fiberglass insulation should it be stapeled to the face of the studs or to the sides. seems like if i staple it to the face it is going to make for uneven drywall.

does it matter what i use for a vapor -- 8 mil clear plastic and tape all seams...

newhome2010 is offline   Reply With Quote
Old 12-06-2010, 01:27 PM   #2
Newbie
 
Join Date: Dec 2010
Posts: 2
Default

Insulation and vapor barrier


thanks for the help

newhome2010 is offline   Reply With Quote
Old 12-06-2010, 07:42 PM   #3
KCB
Member
 
Join Date: Jun 2010
Posts: 56
Default

Insulation and vapor barrier


If you use bat with facing you dont need plastic.I usually use bat with facing in the cieling.Then regular bat no facing on the walls then cover with a plastic vapor barrier.Its easier to manage the insulation in the walls when you can see it.Behind wall sockets,wireing etc...You can go the extra mile and fill in any major gaps with can foam insulation or silicone between studs,bottom and top wall plates,around headers etc...To keep air infiltration down before you insulate.

Last edited by KCB; 12-07-2010 at 05:50 AM.
KCB is offline   Reply With Quote
Old 12-06-2010, 07:51 PM   #4
Newbie
 
Join Date: Dec 2010
Posts: 2
Default

Insulation and vapor barrier


If you use faced insulation you don't want to use plastic over the top. You need to staple the vapor barrier to the inside of the stud.
snowroski is offline   Reply With Quote
Old 12-06-2010, 08:15 PM   #5
Member
 
Join Date: Jun 2010
Posts: 45
Default

Insulation and vapor barrier


Ah... vapor barriers.

Depending on where you live, a vapor barrier can either be a blessing or a curse, depending on where you put it and where you live.

I've seen Mike Holmes repeatedly make his point about vapor barriers being on the inside of the house-- but that's for Canada, and even in parts of Canada you wouldn't want a vapor barrier on the inside.

Engineers use the term "heating degree day" to describe the amount heating a house will need during a typical year. For example, Minnesota is mostly a 10,000+ region, while parts of the Southern US are barely 2,000. See here:



If you live somewhere that has very high HDD (over 10K), then you want a vapor barrier and you want it on the inside. This is because the summers are not that hot, and you won't have the A/C cranked and making the interior much colder than the outside-- rather, the greatest differential between indoors and outdoors will occur in winter-- warm, moisture-laden air meeting cold home structure means possible condensation and thus, mold.

Now consider if you live in Louisiana-- hot-humid climate (per ASHRAE). If you put a vapor barrier on the inside in this climate, your cranked up A/C will make the plastic sheet cold, and the heat and moisture from outside will condense on the backside of the vapor barrier. Result? MOLD EVERYWHERE, with the vapor barrier making things far worse than having no barrier at all.

Even in Minnesota, it gets hot and humid enough in the winter that vapor barriers on the inside can lead to mold. Lstiburek says vapor barriers on the inside are a no-no in the entire Lower 48 US, and I agree.

Instead of a vapor Barrier, what you want is a vapor RETARDER. You don't want to stop all vapor from the inside of the house-- you just want to slow it down so that it dissipates at a rate that the house can dry (no condensation). You want to slow it down, not stop it cold. Stopping it cold means you have no drying mechanism, meaning you'll have mold. You don't want air movement-- but you do want the vapor to be able to dissipate.

So, if like me you live in the Central US with both cold winters and hot summers, the the correct solution is no barrier at all, because they will only do harm. Instead, a retarder should be chosen. The definition of what is what depends on permeability (perms). Retarders have perm between 0.1 and 1.0. Lower than 0.1 is a barrier.

It helps my understanding of this to think in terms of dew point. If you want to prevent mold, the surface must be kept above the local dew point (point where moisture in air will condense). That means keep it warm and/or dry. If you can keep it warm, it doesn't have to be dry, and if you can keep it dry, it doesn't have to be warm.

This is why I chose rigid foam for my basement before I even framed it. The foam allows the concrete to breathe enough to dry if water come in through it, but also insulates to where moisture in the house won't condense on the concrete surface. Both point towards warm and dry.

http://www.buildingscience.com/docum...ence/#Photo_05

Last edited by Hohn; 12-07-2010 at 09:00 AM.
Hohn is offline   Reply With Quote
Old 12-06-2010, 08:27 PM   #6
Newbie
 
Join Date: Dec 2010
Posts: 2
Default

Insulation and vapor barrier


Hohn is correct, and I had mistakenly called the vapor retarder on faced insulation a vapor barrier. The vapor retarder needs to be on the warm-in-winter side of the wall. Also, considering moisture, around showers and in bathrooms you may want to consider plastic, with no vapor retarder.
snowroski is offline   Reply With Quote
Old 12-06-2010, 09:56 PM   #7
Member
 
Join Date: Aug 2010
Location: metro
Posts: 1,038
Default

Insulation and vapor barrier


good insight in this thread! may i chime in-what about insulating the roof? i want to insulate under the decking of the roof. should i use a vapor retarder in addition to insulation? i am using spray foam
federer is online now   Reply With Quote
Old 12-06-2010, 10:00 PM   #8
Member
 
Join Date: Jun 2010
Posts: 45
Default

Insulation and vapor barrier


Do you mean spraying foam up in the rafters under your roof sheeting?

Do you already have a lot of insulation in the attic floor area (above ceiling of highest floor)?

If you are using spray foam, you don't need a retarder (most closed cell foams are low perm). Ultimately, it depends on sprayed thickness, as spray foam can be considered a barrier if thick enough.

Help me to understand your situation more, and I can give you better feedback.

JH
Hohn is offline   Reply With Quote
Old 12-06-2010, 11:47 PM   #9
Member
 
Join Date: Aug 2010
Location: metro
Posts: 1,038
Default

Insulation and vapor barrier


thanks for the reply. the proposal is for spraying foaming right underneath the roof decking directly to the plywood. this will be open cell. the idea is to make the attic become part of the conditioned space as well. right now there is fiberglass batts above the ceiling, but its not doing much. so they want to tear down the ceiling and spray foam right against the plywood. i made another thread about it let me link you
federer is online now   Reply With Quote
Old 12-06-2010, 11:48 PM   #10
Member
 
Join Date: Aug 2010
Location: metro
Posts: 1,038
Default

Insulation and vapor barrier


The After-roof roof project
federer is online now   Reply With Quote
Old 12-07-2010, 08:58 AM   #11
Member
 
Join Date: Jun 2010
Posts: 45
Default

Insulation and vapor barrier


The correct answer in any project like this depends *heavily* on context and the details matter. I see so many people make mistakes because they extrapolate from one person's good results in one application to their application where it turns out not to be a good idea. (Not directed at you, just generalizing the importance of context and detail).

If you use open cell foam, the permeance is too high. This means it will not have sufficient vapor retarding to prevent the condensation on the underside of the sheeting. Venting attics can help a bit, but attic venting is also one of those things that depends on your house and where you live (for example, attic venting in the hot, humid South is a no-no if you have air conditioning).

Think again in terms of dew point. Will the sheeting get cold enough to cause condensation if the airspace under it is conditioned and vapor can access the sheeting? Almost certainly in most US locales.

For example, BASF shows their comfort foam closed cell is .46@4" thickness, making it an effective retarder of vapor and an air barrier as well. This should prevent air movement to the sheeting (air carries moisture) and also severely retard the rate of vapor movement through the foam to where condensation will not be an issue at the sheeting surface.

I'd recommend going thick on the spray foam (4") because it's not my money and easy to recommend something expensive for someone else. Seriously, though, this would be the best way (imo) to address part of bringing that attic into the conditioned space.

There's probably a lot more to it than just shooting SF under the sheeting, so take a thorough examination of all that you are seeking to do.

JMO
Hohn is offline   Reply With Quote
Old 12-07-2010, 09:39 AM   #12
Member
 
Join Date: Aug 2010
Location: metro
Posts: 1,038
Default

Insulation and vapor barrier


Quote:
Originally Posted by Hohn View Post
The correct answer in any project like this depends *heavily* on context and the details matter. I see so many people make mistakes because they extrapolate from one person's good results in one application to their application where it turns out not to be a good idea. (Not directed at you, just generalizing the importance of context and detail).

If you use open cell foam, the permeance is too high. This means it will not have sufficient vapor retarding to prevent the condensation on the underside of the sheeting. Venting attics can help a bit, but attic venting is also one of those things that depends on your house and where you live (for example, attic venting in the hot, humid South is a no-no if you have air conditioning).

Think again in terms of dew point. Will the sheeting get cold enough to cause condensation if the airspace under it is conditioned and vapor can access the sheeting? Almost certainly in most US locales.

For example, BASF shows their comfort foam closed cell is .46@4" thickness, making it an effective retarder of vapor and an air barrier as well. This should prevent air movement to the sheeting (air carries moisture) and also severely retard the rate of vapor movement through the foam to where condensation will not be an issue at the sheeting surface.

I'd recommend going thick on the spray foam (4") because it's not my money and easy to recommend something expensive for someone else. Seriously, though, this would be the best way (imo) to address part of bringing that attic into the conditioned space.

There's probably a lot more to it than just shooting SF under the sheeting, so take a thorough examination of all that you are seeking to do.

JMO
thanks for the input. just to be sure by sheeting you mean the plywood decking of the roof right?
so the problem at hand is the vapor traveling up from the room to the underneath of roof, correct? i was under the impression that open cell is more suitable because although the perm rating is lower, i plan on finishing the ceilings so whatever vapor/moisture is in the room will not travel up into the attic space. thus it elimnates the condensation problem.

also, if i used closed cell which essentially is a barrier, if the roof ever leaks i wont find out until its too late. with open cell if the roof leaks it will seep through the decking and the open cell foam and stain the ceiling giving me notice. does this make sense?

the proposal i got is for 5inch of sf.

please let me know what you think!
federer is online now   Reply With Quote
Old 12-07-2010, 10:37 AM   #13
Member
 
Join Date: Jun 2010
Posts: 45
Default

Insulation and vapor barrier


Yes, sheeting=decking (does anything have more vernacular variation than construction? oy..).

Yes, problem is vapor reaching underside of decking. Finished drywall isn't a vapor barrier. It does slow things down a bit

Keep in mind that vapor permeance is really a sliding scale of time-- how long does it take X amount of vapor to move through a given sample of material under a given test condition. So when you're talking about permeance, a low rating is good (a "barrier" is <0.1, meaning it takes a LOONG time for vapor to move through it (though it still moves!!). A higher perm means vapor moves like the brown matter through a goose. MOre here: http://www.buildingscience.com/docum...apor-retarders

Permeance is additive (or subtractive, if you want to think of it that way.) So if you have something that's rated 1 perm/inch, then 4" of that material is .25perm rating.

So the closed cell has less permeance for a given thickness. If you are planning 5" sf, you might not need closed cell.

I don't think the roof leak situation is reason to choose open cell over closed. Closed cell does everything you want SF to do better-- better R value and better barrier to vapor and air movement. I wouldn't hang my hat on closed vs open being the difference between being able to save a roof or not if it's leaking. Differences in vapor and air movement do NOT translate into passes liquid water or doesn't pass liquid water.

If the roof leak scenario is important to you, then maybe SF isn't the best option. You might want to consider other options.
Hohn is offline   Reply With Quote
Old 12-08-2010, 11:30 AM   #14
Newbie
 
Join Date: Oct 2007
Posts: 2
Default

Insulation and vapor barrier


Thanks for the help guys

I am in NE pennsylvania.

Looks like i will just be going with face insulation for the most part but i do have 1 locations that i am still going to need a vapor barrier. I have barrel ceiling that I am going to have to do with a vapor barier.

So if you can not use plastic what is the best thin to use.

Any sugestions on insullating a Vaulted ceiling. Is there anything special that i can do with that or just use the same bat insulation as thick as i can get while still mantaning the air space at the roof shething.
djb282000 is offline   Reply With Quote
Old 12-08-2010, 02:21 PM   #15
Registered User
 
Join Date: Mar 2009
Posts: 9,968
Default

Insulation and vapor barrier


If your zip starts with 185 and gas heat, R-49 in attic, 38 in cathedral, 25 in floor, 15 in walls, 30 at rims, etc.: http://www.ornl.gov/sci/roofs+walls/...on/ins_16.html

No v.b. required, as mentioned, I’d use some foam on the outside with a WRB: http://www.buildingscience.com/docum...ecommendations

Face staple, never side staple batt with paper: 1st picture- http://oikos.com/library/insulating_...lls/index.html

Why use low density f.g. when all other choices are better for attics: http://infrared-energy.com/files/Spe...onProblems.pdf
For walls: http://findarticles.com/p/articles/m...6/ai_n8582994/

Convective loops form in all low density f.g.and cellulose, blown or batt, rated at around 0.5#per cubic foot. The pink stuff is R-13= .93#c.ft. R-19= 0.55#c.ft. R-30= 0.58#c.ft. Remember the testing with convective loops was 0.50#. If you must use f.g., get the high density R-15 for walls, etc.

Gary

__________________
If any ads are present in my answer above, I do not condone/support/use the product or services listed, they are there against my permission.

Last edited by Gary in WA; 12-21-2010 at 08:19 PM.
Gary in WA is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Top of Page | View New Posts

Copyright © 2003-2014 Escalate Media. All Rights Reserved.