How to prevent frost in my walls?
joesepa
| Posted in Energy, Heating & Insulation on
All,
My Pennsylvania house is built with vertical 1.5 inch thick by 8 inch wide T&G cedar planks. They are my exterior walls, 2×4 studs with 2 inches of foam insulation (board type) and T&G cedar 1×6 for interior walls. Yes it’s kind of unusual.
I want to lighten up the place and insulate so I removed the interior T&G and existing insulation which was little or none in most spots. I reinsulated with Roxul and noticed frost between the Roxul and exterior boards. This house has no vapor barrier.
How can I insulate and prevent moisture between the insulation and exterior wall? Should I install a vapor barrier on the living side of the wall?
Any help would be greatly appreciated.
Joe
Replies
Vapor barrier must go on the living-space side of any insulation, and be well-sealed where penetrated at all.
The idea is to prevent water vapor from reaching surfaces behind the insulation that can be at or below the dew point, where if left to itself it'll condense.
Or freeze, in your case.
Problem is: in summer, when living space might be air conditioned, that vapor barrier's on the wrong side of the cavity insulation & you may get condensation against it again, this time on the back of the interior surface.
Key point is to allow whatever water vapor that might get inside the wall a way out. Newer housewrap fabrics permit this, get installed over the exterior sheathing (which your house lacks) but under exterior siding. Vapor barrier still goes between studs and interior finish surface but is less vapor-permiable than housewrap, because the highest humidities will be inside the living space.
Thanks spclack
I did some research and you are correct. I’m using Roxul insulation because of its high R value and other desirable qualities; however Roxul does not have a craft face paper or other vapor barrier. According to the county I live in I’m required to have a class 1 or 2 vapor barrier. I was thinking of getting some 30 lb asphalt paper for my barrier I think it’s rated at class 2. I don’t really want to use plastic (class 1) because I feel the walls wouldn’t be able to permit moister transfer in the summer like you said.
Any thoughts?
vapor barrier...
..".is less permeable on interior of studs because the highest humidities will be inside the living space"...??
please explain!
i assume the interior vapor barrier you are referring to is polyethylene sheeting, correct?
What you see is reasonably expected, given the construction. The Roxul has virtually no vapor resistance while the foamboard had a lot. When you install the Roxul that causes the temperature at the surface of the foam to drop (for a given cold temperature outside) and condensation would be likely below maybe 0F (exact temp would depend on indoor humidity).
Probably your best bet, short of doing it all over again, is to install a very tight (all seams taped,etc) poly vapor barrier over the Roxul. There should be enough vapor leakage around the foam boards to allow the wall to "dry to the outside".
Thanks DanH
Thanks for the info.
Maybe I was misleading. I removed the foam board insulation all together and only have the Roxul insulation now. It’s a 2 x 4 wall so the Ruxol gives me R 15 which is the best I can get, I think for a 3.5 inch space. And yes I have removed all the insulation and let the walls dry for now until I get a suitable vapor barrier.
I’m thinking the reason for all the moisture is because I was using a humidifier in the room because the wood fireplace was drying out the place.
Maybe a class II barrier would be sufficient.
Any advice would be greatly appreciated.
Yes, in that case a simple poly vapor barrier (just behind the inside wall) should work quite well.
Given the labor involved I'd stick with polyethylene for a vapor barrier rather than tar paper, no question. Cost difference doesn't outweigh the difference in effectiveness or ease of handling. Be far easier to tape-seal seams in poly too.
You want as little moisture as possible moving into the insulated wall cavities, as much because that decreases problems inside the wall as because the higher the relative humidity in your living space (within reason, of course!) the more comfortable you'll feel at normal room temps.
Yes the humidifier assuredly was contributing to your frost problem. Even with doors on a fireplace though much of your interior air goes up the chimney when you're enjoying a fire.
(With all that cedar around, need I add a word of caution about risk of fire? Please be careful - there are good reasons to have drywall on the inside of your studded exterior walls, even if you decide to put cedar on top.
Thanks too for turning me on to Roxul! I'd heard of rock wool years ago, seemed to have fallen out of favor over fiberglass or rigid board in many applications. I just moved into a 'new' (for us at least; it's a late '70's pre-fab) house with a full poured concrete basement. The previous owner glued 3/4" white polystyrene board to the inside foundation walls, but it's a nuisance at best so I'm researching alternate methods for more effective insulation down there. Will include a 6 mil poly vapor barrier too, rest assured.
you should have reused...
...the foamboard.
I would have left it in place and air sealed it to the stud cavity framing.
You would have achieved a very effective moisture barrier and retained the high R value of the 2
' foamboard insulation while allowing for air movement within the cavity and thereby helping to reduce or eliminate condensation in the cavity.
Your cost would have been for the caulk and or canned foam that would have been used for the air sealing.
What part of PA are you in?
Indoors high water content, outdoors low water content.
Would just like to clear up a point.
The inside of a home has more water vapor in the air, than the colder drier air outside.
That is why we install exhaust fans in our kitchens and bathrooms, to blow the warm wet air out and replace it with drier colder air, that we then heat up. (at great expense)
Water vapor is programmed by nature to always move towards a cold surface to condense.
In a typical area where the home is heated in winter.
A typical room should have drywall, then a plastic water vapor proof membrane, then insulation followed by the frame, and if you like more insulation inside the frame.
The principals being that wood is hygroscopic, it absorbs water out of the air when cold and is also not a good insulation.
In a typical home the outside of the frame is in contact with the siding, which is in direct contact with the cold outside air, the inner side of the frame is in contact with the drywall and the warm indoor air.
The frame comprises a very large area of wood in a typical home, being cold on the outside it is also colder on the inside attracting and passing your heat to the cold outside by conduction.
You can see that it is important to prevent your expensive heat and water vapor from entering the frame, by fixing insulation over the indoor side of the frame and fixing a plastic water vapor proof sheet over the insulation and under the drywall. Drywall is also hygroscopic being able to hold a large amount of water vapor = this makes it a good half hour firewall.
As far as the outside of your home is concerned, the heat of the summer sun, will initially drive water vapor into the walls, looking for a cold place to condense, but later the heat of the sun will also warm the siding and frame forcing the water vapor out, drying the wood again to the outside. It is important that nothing is done to trap water vapor inside the frame.
How exactly does the heat of the sun (summer or otherwise) "drive water vapor into the walls"?
Thanks Perry525
I appreciate your detailed information. This is going to help me out a ton.
Now that the current insulation is removed I’m going to proceed as followed.
1. Re-install Roxul insulation which has a great R15 value for a 2 x 4 wall. Roxul is also good for sound proofing, fire proofing, and will not grow mold or hold water; however Roxul insulation doesn’t come with a craft faced paper or any other vapor barrier as does the pink insulation.
2. Install half inch ridged foam insulation against living side of studs.
3. Install Certainteed’s MemBrain which is designed to be applied over unfaced fiber glass insulation, loose-fill applications and spray foam. (I’m going to call and see if it’s suitable for my application.) MemBrain looks similar to typical polyethylene sheeting, but is actually a polyamide-based materialthat gives MemBrain its unique ability to adapt its permeability depending on the climatic conditions. With a high resistance to water vapor in winter or low humidity, MemBrain behaves like a moisture vapor retarder such as poly sheeting. When the relative humidity increases, as in the summer season, its water vapor permeability increases dramatically. This allows water vapor to escape the wall cavity easily. This information was obtained from the certainteed.com web site.
4. Install half inch sheet rock.
5. Celebrate the completion of a project!
Since my house has little or no insulation and is 100 percent cedar I feel like this is going to provide safety and warmth for the future. Thanks for all your help.
There's a lot of talk about vapor barriers. More important is an air barrier - Several orders of magnitude more moisture can move into walls on air currents than is likely to diffuse through most building materials. Keeping interior air out of your framing cavities is more than 90% of the battle. My point here is that if you caulk the seams of your foam board, you probably don't need the MemBrain. Foam can function as both a vapor retarder and as an air barrier.
yes to foam, no to membrain
Agree with Andy.
The foam makes the MemBrain superfluous. And the foam will give you better performance; thermal, air, sound, vapor permeance.
The sun and its effect on water vapor.
The way the world works.
The sun shines on the equator region of the world, lifting millions of tons of water into the sky every day.
As the world rotates this mass of water vapor moves towards the poles, driving our weather.
The key thing is the water vapor is programmed to head for the coldest spot = the poles.
The same thing happens in our homes, the sun raises the temperature of the air , the water vapor moves towards the nearest cold , perhaps a rock, some shade under a tree, under our home, inside a wall, wherever.
The wall itself warms and dries, as it heats, the water vapor moves out into the air again looking for the nearest cold space.
Except when the home has air conditioning, then if the homes insulation is not effective enough, the water vapor condenses inside the wall, possibly leading to all sorts of problems.
Actually, water vapor does not move towards the cold. Water vapor moves to areas of lower partial pressure, which is to say lower dewpoint. You don't get condensation because water "moved towards the cold", you get condensation because the air temperature dropped below it's dewpoint.
(If "water vapor is programmed to head for the coldest spot = the poles", how does it ever rain in the tropics?)
Cold heat relative temperature precipitation
You are both right and wrong. The fact that water vapor also moves to areas of low pressure is irrelevant, the difference in air pressure up wind and down wind to a home doesn't effect the way vapor moves, the significant issue being temperature.
Condensation as far as homes is concerned is always the result of warm wet air arriving adjacent to a cold surface and loosing its heat and thereby ability to hold moisture = result condensation. In the sky it is a mixture of pressure and temperature.
The reason it rains, is that the warm air holding the water vapor cools and precipitation occurs.
I try to keep it short and simple to understand.
I didn't say low pressure, I said low PARTIAL pressure. It's a term in physics that relates to how gasses move. Gasses will always diffuse in the direction of lower partial pressure.
And condensation is very often due to "black body radiation" cooling a surface below both air temp and the temp of the building behind the surface. This is why you see dew on the grass in the morning, eg.