Foam board under siding

[Eric Smith]

Have a family member that is building his home all by himself and doing it slowly as he has the money with no loans. It is very rustic as that is what his wife wanted. It s 2000 sq ft ranch with a crawl. The inside is mostly barn wood directly on the studs (lots of gaps and cracks) unfaced insulation was used with no vapor barrier. Now he is seeing signs of condensation in a small room with no wall covering up yet(utility room). He is now worried, as am I, that behind the barn wood is going to have the same issue. Can he put foam insulation board on outside over the house wrap before he sides it? Will this help the lack of vapor barrier inside? I thought it might move the condensation point out more? Advise??

[Scott Patterson]

The condensation if from the cooler air in the stud cavity, adding foam board insulation on the exterior wall might make it worse because you will be stopping virtually all of the vapor transmission through the house wrap from the interior to the exterior. Think of an EIFS home and the problems that arise when you seal water in the walls.

If it has a stud cavity it really needs a vapor barrier, if they want to maintain the rustic look they could put 15lbs builders paper on the stud walls and then nail the barn siding on the studs. This is a common method with pecky cyprus or other rustic looking wall cladding. Heck just using faced batts of insulation would have most likely solved or greatly lessened the issue.

[Lon Henderson]

I think he has some major re-do ahead of him if he has most of the walls done. Shame on the code inspectors if they let him make these mistakes.

There was a recent thread along these lines.

This link will provide great info on getting him back on track. He is not going to be happy.

BSD-106: Understanding Vapor Barriers ? Building Science Information (http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?topic=/doctypes/digest)

[John Kogel]

Barn board with gaps and unfaced fiberglass? That is more than just 'rustic'. :D

In our West Coast climate, we do not use paper on the interior side. We use poly and tape all the seams. This is the only way to effectively block moist interior air from entering the wall cavities.

The only way to see what is going on in those stud cavities is to open up a few spots down low where the outer sheathing is cooler.

[Eric Smith]

So, should I advise him to use the insulation board on outside or no? I guess only part of it has no vapor barrier. The bathroom, kitchen have it and the utility room will since it is only room not covered yet. Will foam board help or hinder? Thanks

[John Kogel]

The storage room is unheated, correct? An unheated room with no vapor barrier is a prime candidate for condensation because of the cold walls.
The living areas might not have a serious issue. I would pull a plank or two and check behind the insulation. If that is ok, go ahead with the foam board and siding. By increasing the R factor it may help more than hurt. But no two houses are identical when it comes to this type of issue.

He should plan to redo the barn board walls. Vapor barrier and wood battens or drywall under the planks.

[Daryl Roche]

The storage room is unheated, correct? An unheated room with no vapor barrier is a prime candidate for condensation because of the cold walls.
The living areas might not have a serious issue. I would pull a plank or two and check behind the insulation. If that is ok, go ahead with the foam board and siding. By increasing the R factor it may help more than hurt. But no two houses are identical when it comes to this type of issue.

He should plan to redo the barn board walls. Vapor barrier and wood battens or drywall under the planks.
IMO he should have vapor barrier on the warm side of all walls. When living in the home moisture is created from humans, cooking and wanting a comfortable home in the winter thus adding an humidifier to the furnace. The vapor barrier keeps this vapor on the warm side and out of the wall cavity. Thus no water problems in the wall. If Foam board is used on the outside and no vapor barrier on the inside you could have moisture problems in the wall. Air filtration will still move the moisture into the wall and the foam board will trap it. Foam board is used in my area on the outside a lot to cover walls except for the corners where you need the stability of plywood or osb to keep the walls from racking, it adds insulation value and air tightness if insatalled properly. I personally have repaired shower stalls and bath tub walls where the tile have come off and when I removed the drywall found no vapor barrier and water droplets sitting on the surface of the foam board that was used on the exterior of the home. Why didn't he install the vapor barrier? Not that big of an expense and by code.:)

[Eric Smith]

thanks guys!!

[Mark Parlee]

Eric, if you install a thick enough foam at the exterior then you can move the theoretical condensation point into the foam where there is no problem. Air moves from warm to cold, wet to dry. Warm dry air is wetter than cold wet air. There is a vapor pressure from indoors to outdoors in the winter. This vapor pressure reverses in the summer as we cool the indoor air. The winter vapor pressure is more extreme outward than the summer vapor pressure inward. That is why in cold climates it is considered best practice to install a vapor barrier to the warm side of the cold season.
If you can get a thick enough foam insulation at the exterior and perfectly air seal the joints across the entire joint then you can minimize this compensation problem. However it is a lot tougher to do, one would expect.
You are right to be concerned about the condensation.

What is the exterior wall sheathing?

[Eric Smith]

Thanks Mark, it is 1/2 inch OSB with good house wrap over it.

[Mark Parlee]

Thanks Mark, it is 1/2 inch OSB with good house wrap over it.

Eric, what is good house wrap?
The problem with adding foam to the exterior is in all of the details.
How thick of foam are you going to add? This thickness needs to be calculated to ensure the dewpoint for the assembly is moved into the foam.
How are you going to handle the details at the Windows and doors when you add this dimension to the exterior?
The house wrap should be a drainable wrap such as Tyvek Stucco so that any moisture that gets behind the foam can drain and not store. Stored moisture will eventually work its way through the WRB and start the process of turning the ODB into vertical mulch.
There are a lot of considerations and details that need to be addressed. What kind of siding are they planning on installing over the exterior foam? At this point a good drainable EIF system with 2 inch foam sounds like a great option.

[Steve Duchene]

Since the outside is done it would be better to put foam board with taped seams on the interior over the studs and insulation. The foam itself is a moisture barrier, taping prevents joint bypass. My insulator recommends this any time we install a wood siding/barn wood interior on new and existing construction whether it is wall or ceiling. It is all about keeping the heat and humidity from reaching any area of the cavity that can possibly have a lowered dew point thus allowing condensation/saturation which results in rot/mold/fungus. Adding foam to the exterior over the house wrap will create more moisture issues than it eliminates. Fiberglass insulation whether faced or un-faced is nothing more than a filter because it allows air and moisture migration unless it is perfectly installed with a continuous sealed moisture barrier.

[Jim Luttrall]

Have a family member that is building his home all by himself and doing it slowly as he has the money with no loans. It is very rustic as that is what his wife wanted. It s 2000 sq ft ranch with a crawl. The inside is mostly barn wood directly on the studs (lots of gaps and cracks) unfaced insulation was used with no vapor barrier. Now he is seeing signs of condensation in a small room with no wall covering up yet(utility room). He is now worried, as am I, that behind the barn wood is going to have the same issue. Can he put foam insulation board on outside over the house wrap before he sides it? Will this help the lack of vapor barrier inside? I thought it might move the condensation point out more? Advise??R

Read the BSC article and plug in the numbers, vapor barrier placement varies by climate. All of the various experience and analogies offered here are based on the climate of the responding person which is likely quite different than yours. There IS a science behind Joe L. and the BSC article that should not be ignored!

BSD-106: Understanding Vapor Barriers ? Building Science Information (http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?topic=/doctypes/digest)

[Mark Parlee]

Steve, with barn board on the interior over the foam and gaps you are missing the required fire barrier over any interior foam.

[Eric Barker]

Can he put foam insulation board on outside over the house wrap before he sides it? Will this help the lack of vapor barrier inside? I thought it might move the condensation point out more? Advise??

If you haven't already, go back and (carefully) read Lon's 12/23 link. It's important to understand the difference between barriers and retarders. I'm about 150 miles north of you and barriers are bad news around here even though builders and the muni guys think they are beneficial.

[Bob Boepple]

The "Cold Climate Housing Research Center" in Alaska has lots of great info on remote wall systems using
foam on exterior walls.
http://www.cchrc.org/

[Robert Hronek]

There are air barriers, vapor retarders and vapor barriers. In this case the interior is lacking an air barrier. Any moisture in the air will pass right through the fiberglass insulation.

My guess is the guy building the house lacks the necessary knowledge to build a house.

You can go to greenbuildingadvisor.com and find out how much rigid insulation. It will be between 3 and 4 inches. It will depend on the thickness of the wall and the R value inside the sheathing.

He will need to define his primary air barrier. The sheathing would be the logical place for the air barrier. Do not count on Tyvek or similar to be an air barrier.

Does he know how to properly flash a window, etc.

He should also read about unvented crawl spaces. No dirt floors. Insulate and air seal.

I should add the fiberglass insulation will not perform well without an air barrier. Fiberglass should be in a 6 sided enclosure. With interior boards that side is effectively open and have a very low effective (as installed) R value. Drywall should have been installed and then the interior boards.

[ROBERT YOUNG]

Have a family member that is building his home all by himself and doing it slowly as he has the money with no loans. It is very rustic as that is what his wife wanted. It s 2000 sq ft ranch with a crawl. The inside is mostly barn wood directly on the studs (lots of gaps and cracks) unfaced insulation was used with no vapor barrier. Now he is seeing signs of condensation in a small room with no wall covering up yet(utility room). He is now worried, as am I, that behind the barn wood is going to have the same issue. Can he put foam insulation board on outside over the house wrap before he sides it? Will this help the lack of vapor barrier inside? I thought it might move the condensation point out more? Advise??

I have reach out to a waitress I have know now for 5 years. Similar story. A Ranch with barn wood.
What's up with barnwood and the ladies. To many Bonanza reruns?

That wood must be treated and the surface removed.
Be Safe When Working With Old Wood with Lead Paint & Rusty Nails! | Greenspiration Home (http://www.greenspirationhome.com/safety-precautions-in-working-with-old-wood/)
This can be life/health threatening.

As for foam board, have you old times forgot organic mulch? TenTest and Celotex.

As for thinking after building.:( Yes they can add rigid extruded insulation on the exterior but must create a vertical drip plane. I would be using organic mulch myself with a perm barrier.