eeCurtis

I'm a general contractor in Canada and have noticed quite a few new and older homes installing 1-2" styrofoam board on the outside of their homes for better insulation. This seemed like a good idea until I started thinking about why this type of rigid insulation is desirable. I will briefely describe my concerns and reasoning below. Your thoughts on my reasoning are appreciated:

- "R" value of porous insulation is negatively effected when moisture and air movement are introduced into the equation. Therefore, it is generally accepted that forms of rigid insulation block the elements better than porous insulation.
- Vapor barriers (poly) are only installed on the inside wall of the wall, because while poly controls the elements, it is not perfect. Therefore we know that there is some air and moisture movement in walls. Find a contractor that poly's both sides of a fibreglass insulated wall, and you will find a lawsuit for molding and decaying walls.
- rigid insulation significantly reduces air and moisture movement, but is also not perfect. Most codes will still require poly on the inside of the wall.

So by installing rigid insulation over porous insulation, are we creating a mold and rot situation within wall? This is my question to the forums.

Frankawitz

I live in Michigan in the south east region of the State, just outside Detroit. I have used R-19 and R-13 together in my attic and in my walls I used R-19 & R-13 6 mil visqueen, in the ceilings and walls we also used the visqueen, the problem I have found is when we built the dormer (24'x25') we wrapped the house with tyvek then I installed 1/2" Dura Rock cement board then a 1/2" of concrete then finished off with 1/4" CSC-4 Portland cement. I took down a section of the sheetrock to run some wires on an outside wall, I had found that the OSB had mold on the inside cavity of the wall. I called the company that made the Tyvek House Wrap and explained what was going on they told me it was because the tyvek couldn't breath because of the cement stucco I had applied to the outside of the house. So now the whole dormer is covered in mold, I was also told that because the house is so well sealed that it is trapping the moisture in the house and they said I should have got a dehumidifier installed and I could have stopped it, but now it's in the wood to bad. so we live with it cause the Homeowners Insurance only covers $5,000.00 to take care of Black Mold. and to get all the Black Mold out of the walls we would have to tear down the dormer and more then likely the rest of the house since it's been here since 1926, I have no problems from the Mold I have been dealing with it for 21 years doing repair work. so what are you going to do? Mold has been around for a long time. and with mold it only needs a little moisture to start growing with in 24 to 48 hours, so if your heating a building and the insulation isn't all in place mold will start to grow in the wood from the condensation that forms on the wood from heating the interior space and have colder temptures on the outside of the walls and roof areas. I think we are trying to build these air tight structures to save energy but at the same time creating a perfect place for Black Mold to grow within the ceilings and walls. I feel a dehumidifier needs to run all the time once the heating system is turned on for the Fall and Winter seasons in the Northern regions. I don't know if that was the type of answer you were looking for but that is what I have found.

red_cedar

I believe dryvit had similar issues with moisture not escaping. Perhaps a small air space brteen the stud walls and styrofoam may help take some of the moisture away..
If not, then allowing the moisture laden air to exit thru the ceiling to the attic then, out, might work.
In an opinion for your question, if the foam is applied and sealed up without thought other then sealing the building up, yes, one could be setting them selves up for moisture. problems. The winters would be the worst because moist inside air would be drawn towards the out side dry air.

eeCurtis

Thx guys. This is the type of discussion I was hoping for. The Tyvek info is very interesting because lots of siders up here are moving back to tar paper for the reasons stated. My one sider told me that there is White Tyvek and Blue Tyvek. White is inexpensive, but not very breathable. The Blue is more breathable. I haven't checked into this yet, so don't take my word for it.

BTW, I'm quickly becoming a non-fan of fibreglass insulation because of it's propensity to hold moisture.

manhattan42

First,

Rigid Foam in and of itself poses no problem when placed on the outside of a structure, but it depends on the type of foam (open or closed cell) and whether or not it qualifies as a 'vapor retarder'.

A vapor retarder, by definition, is a material that has a vapor permeable rating of 1 perm or less.

Most rigid foams, even closed cell foams, do not qualify as vapor retarders. Even 1" polystyrene foams only have a perm rating of 1.1. This means water vapor will fairly readily pass through them.

6 mil polyethylene plastic has a perm rating of 1 if I remember correctly.

Only foil faced rigid foam insulation qualifies as a true 'vapor retarder'.

So the first point is, even when using 6 mil polyethylene sheets on the exterior (or interior) of wall assembly, does not mean you are going to stop or trap moisture in the wall cavity.

The second point is, that by insulating the exterior of the thermal wall envelope, you in effect increase the temperature inside the wall cavity and therefore reduce the possibility of condensation.

Remember, water is a gaseous state is harmless. Only water condensed to a liquid causes problems with rot, mold, etc. If the temperature in the wall cavity is always kept above the dew point at which condensation occurs, it doesn't matter how much gaseous water vapor gets trapped in the walls because it will never condense.

One does still need to be careful when using exterior foams and interior vapour retarders however, because in the rare instance that liquid water enters the wall assembly or condensation does occur, the walls must still be able to dry.

With thermal walls insulated on the outside with foam, then, it is normally recommended that the vapor retarder on the warm-in-winter side of the thermal envelope be removed to promote drying to the inside of the structure rather than to the outside.
------------------------

As a side note, Housewraps and tar paper are not designed to do the same things.

Housewraps are typically designed as 'air barriers' and only act secondarily as water resistive membranes. Housewraps generally do not inhibit the flow of gasous water vapor.

Tar paper, on the other hand, is designed specifically as a water reistive membrane which may only secondarily serve (and not at all well) as a air and water vapor barrier.

eeCurtis

Good points manhatten42, thank you. The variable I also wanted to touch on was porous insulation like celluose or fibreglass batt. These materials do tend to 'trap' condensed moisture in a northern climate like Edmonton. My main question is whether or not walls with porous insulation that are then covered with rigid insulation would have moisture problams in the future.

From your post, it sounds like it could pose a problem. I don't want to put words in your mouth though

manhattan42

No, I am saying quite the opposite.

By insulating the exterior of the thermal envelope with rigid insulation, one keeps the wall cavity warmer and hence condensation is less apt to occur. The stud cavity is less likley to reach the dew point temperature.

By insulating the exterior of the thermal envelope, one further reduces the chance of condensation because one eliminates 'thermal bridging', heat loss directly through the studs.

By insulating the exterior of the thermal envelope, the result is a drier stud cavity. This means 'porous' insulations like fiberglass work BETTER when an exterior continuous insulation is used...because condensation is less apt to occur, and the chance of having wet insulation which also increases heat loss also does not occur.

Using an exterior rigid foam ins nearly all cases reduces moisture problems in wall cavities than if one relied upon cavity insulation alone...regardless of type.

red_cedar

I know there has been problems with styrofoam insulated roof panels.
What I understand happens and have seen first hand in a period of less then 10 years, is the exterior wood in the panel system near the ridge starts to disinegrate, due no controlled means of the moisture laden air exiting.

Why was there problems with dryvit sytems? Under the stryfoam the wood was rotted. Is anyone else aware that there was a problem.

I do understand, changing the cold portion of an exterior wall, but moisture still is the same and still needs to go somewhere.

manhattan42

Are you referring to structural insulated panels used as the roof assembly or are you referring to rigid foam insulation placed in rafter cavities?

In either case there should be no moisture issues or damage to the structrual panels if the attic space/rafter space is adequately ventilated, no excess moisture is dumped into the attic, uncontrolled air leaks are sealed as required into the attic, and a vapor retarder is placed on the ceiling below.

The moisture problems you describe do not at all sound related to the use of foam insulation, but please explain...

red_cedar

I supppose I wasnt clear.
SIP's ( Structual Insulated Panels) have had problems. Which I believe now the panels have an air space( venting ) in between.

It may that I am pointing out problems in the industry that foam happens to be involved. And that the foam is not the real culprit.
But I believe the orignal post may have been directed towards the lack of correct use of foam or lack of venting moisture.

eeCurtis

Ah. I misunderstood. Thank you for taking the time to clear that up for me.

eeCurtis

Well I was mainly wondering if sheathing the envelope with rigid insulation would trap moisture. Manhatten says it does not, but rather reduces the chance of condensation.

red_cedar

''In an opinion for your question, if the foam is applied and sealed up without thought other then sealing the building up, yes, one could be setting them selves up for moisture. problems. The winters would be the worst because moist inside air would be drawn towards the out side dry air.''




part of my first reply to the original post

manhattan42

Be careful here.

Although adding foam to the exterior may in fact reduce the chance of condensation, it may not eliminate condensation in walls completely.

Condensation is a function of air tempertaure and relative humidity.

For example, if the air temperature in summer is 90F and the relative humidity is 95%, the dew point temperature is only a 2 degree difference, or 88F. This means that exterior air entering a wall cavity from the outside will condense because the wall cavity will likely be more than 2 degrees cooler.

Or in winter, if the interior temperature is 70F and the relative humidity is 55%, the dew point inside the wall is a relatively high 53 degrees. Even with foam on the exterior, one can still get condensation within wall cavities because of high ambient humidities even though the stud cavities are warm.

This is why in many locations that interior warm-in-winter vapor retarders my need to be removed when foam is used on the exterior of the structure (usually warm/moist southern climates). The reason is to allow any condensed moisture or liquid water that might enter the cavity from the outside to dry to the inside of the structure....

But in other locations, an interior vapor retarder may still be required (as in extremely cold climates).

One needs to know and understand all the dynamics of one's local climate and local energy code to design for proper use.

Generally speaking, however, using foam on the outside moderates temperatures in wall cavities by making them warmer and reducing convection within the walls...and thereby reduces the potential for condensation to occur....

72chevy4x4

how do walls with blown foam (such as a 1lb density open cell) compare to the wall systems being discussed here in terms of its ability to impede the flow of moisture?

red_cedar

As a side note to moisture problems I have seen with foam used, ( the incorrect appliction of it ) is the
fasteners where rusting. The moisture was drawn to the fasteners. I suppose due to the metal being cold when the panels where warm. Could have been a poor coating on the fasteners.
Just thought I would mention it.

Also think Manhattan's answers are very well explained.

mighty anvil

ASHRAE defines any material with a water vapor permeance of 0.1 perms or less as “vapor impermeable”, in other words it is considered a “vapor barrier”.

6 mil polyethylene plastic sheeting has a water vapor permeance rating of 0.06 perms which is below 0.1 and a long way from 1.0. Therefore, it can stop or trap moisture in a wall cavity if there is no alternate path for vapor escape or drying.

Ice won't form inside a wall without condensation but wood with a high moisture content can support mold and fungus growth without condensation.

You won’t find what you need to know about moisture control in the IRC or in this forum. There are too many variables in construction and climate.

The best source for this information is http://www.buildingscience.com/resou...re/default.htm

manhattan42

Uhhh...Not a 'vapor barrier' but a 'vapor diffusion barrier', Mighty Anvil.

You need to brush up on your DOE and IECC terms....

Definitions are EVERYTHING as you keep forgetting...especially when defining a 'masonry veneer' and a 'masonry foundation"....lol

mighty anvil

The DOE/EERE uses the terms “vapor barrier” and “vapor diffusion retarder” interchangeably, but it does not use the term, “vapor diffusion barrier” contrary to your claim, not that it makes any difference to anyone, but for some reason you brought it up as if it did.
http://www.eere.energy.gov/consumer/.../mytopic=11810

What you need to learn is that 4 to 6 mil polyethylene allows almost no water vapor to pass and you should stop claiming otherwise in internet forums; it's irresponsible.

mighty anvil

I have only seen "blue" Tyvek in France. In the US, "white" Tyvek is probably the most vapor permeable ("breathable") housewrap on the market so it certainly doesn't need to be any more permeable. I don't know what is available in Canada but your information doesn't apply to the US market..