While reframing some inside walls we discovered that the insulation in the ceiling of the house had been installed between the roof rafters with no insulation above the actual rooms. Pulling down the insulation revealed the plywood on the roof was already starting to mould.
Our plan now is to insulate the ceiling but the question of a vapor barrier comes up. Can we expect problems with the ceiling joists if we cut & fit vapor barrier to fit between the joists prior to insulating. Will moisture use the wood as a conduit? Thanx for any help
Replies
Geez:
I don't fully understand your description, but it sounds like you now have a conditioned attic (heated and cooled), and want to convert to a unconditioned attic. Is that correct?
First, as I think you know, the mold on the underside of roof sheathing is being caused by moisture, and hopefully, not from a leaky roof. Assuming a good roof, then the moisture must be migrating up from the living space to the attic space. The moisture is generated from normal living activities like breathing, taking showers and cooking to name a few.
You need to do what you can to prevent the moisture from migrating up from the living space to the attic. Moisture is largely carried on air currents. So, if you plug air leaks, especially large air leaks from the house living space to the attic, this will help. If you have drywall ceilings the serves as an effective air barrier, but it must be "detailed" to get rid of air leaks, specifically around light fixtures, plumbing penetrations, HVAC penetrations, ceiling soffits, etc, etc. Also, make sure that you have proper ventilation for the moisture point load areas - kitchen and bathrooms; Exhaust fans that are ducted to the outside - definitely not dumping moist air into the attic. And get in the habit of using these fans. Also make sure moisture in not coming in from the bottom of your house - your basement or crawl space needs to be dry. If it is a crawl space house, there must be a plastic film type vapor barrier on the crawl space "floor". All of the above are your first steps toward resolving your house problems.
Now - I am going to assume you want to abandon the conditioned attic and go to the more traditional unconditioned attic. This is how 95% of the homes are built here in NC (North Carolina): No vapor barrier in the ceiling; just blow loose fill insulation to a thickness to yield R-30 or R-38 (or more) right on the house's top ceiling. This would be roughly 12" - 15" of loose fill insulation, depending on the R value you are trying to achieve, and the type of loose fill insulation being used. Using a layer of batt type insulation is a distant 2nd choice because it will not cover over the tops of the joists (thermal transfer through the wood, and because it is very difficult to get a complete blanket with batt insulation that doesn't have air leaks. Also, you may find that loose fill cellulose insulation is the best performer for stopping air migration.
Of equal or greater importance as compared to insulation, you need to be sure that your unconditioned attic is properly vented with ridge and soffit vents - actually, this would be done before the insulation. The venting is roughly split between the soffits and the ridge, with at least 50% of the venting in the ridge. This balancing of venting sets up air currents that help the attic ventilation work properly. Insulation baffles will be required to prevent the attic floor insulation from blocking the air ways to the soffit vents. There is a specific minimum amount of ventilation required - can't remember it at this early hour, if need be, I'll look it up later.
So, in summary, NO, I would not put a vapor barrier in the ceiling. Of course I have no idea where you live and my recommendations are for our climate here in NC. Also, a better description of the house would be helpful. After providing a bit more info, you can get some more specific recomendations, and probably from someone who lives in your same climate.
Good luck,
Edited 10/31/2004 5:14 am ET by DIRISHINME
The previous poster summed it up properly.
The ratio of vent is a minimum of 1 sq ft of free area vent to every 300 sq ft of attic floor area. .. divided equally 50/50 between soffit and ridge.
As far as the vapour barrier is concerned, applying under the insulation and trying to seal it would be a very difficult task.
Paint the ceiling below instead.
There are v/b type paints on the market that can be applied to the ceilings below this insulation that have very good vb qualities.
I'm thinking you ment to post that to Geezer rather than me?
Matt
You are correct. Excuse me.
Thanx matt.. This is a renovation job to a house that has both cathederal ceilings and some unconditioned attic space. Most homes that I have run into have the proper insulation or the just need more added..when we opened this up I couldn't believe someone would have done this.The roof was two years old and already in a state of deterioration The option I would normally recommend would involve redoing all the ceilings as I am sure that the cathederal portion of the house probably has little or no ventilation...but the owner wants to make do... I think you have it figured out ..but being from the north once I have it vented and insulated my concern was with the moisture getting into the insulation and freezing thus rendering the insulation useless....if there is someone out there from one of the northern states or if you spend a lot of time wearing mitts and you have seen this situation I would appreciate any advice Once again thanx...Geez
What's a mit? ;-)
BTW - around here, a tabogan (sp?) is a kind of winter hat! Matt
A tobaggin here is a flat sled or a sort of a horse drawn skidder...
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming
WOW!!! What a Ride!
Yep, ventillate the attic well, paint the ceiling with a VB paint, and seal the tops of studwalls somehow to prevent air leaks up through cracks between drywall and top header. Also seal any pipe/wire penetrations (though observe clearance rules for flues).
To seal studwall tops I've used pourable roof coating -- the rubberized stuff.
Interesting idea about the ruberized roof coating. I have never used the stuff. Comes in 1 gallon or 5 gal? How thick is it? Aprox price?
Matt
Been awhile since I used it. It's like thick paint or thin tar -- you gotta stuff any visibly open cracks, but it will cover hairline cracks with no trouble. Came in 1 and 5 gallon buckets, if I recall, and probably would run $10-20 a gallon at today's prices.
A number of different brands available, probably with different characteristics/prices.
Instead of or in addition to (under) the roof coating one could probably use regular premixed drywall mud, maybe thinned a fair amount, to smear on top of the stud walls and plug penetrations. Not as good a vapor barrier, but good at filling gaps and stopping air leaks.