Home>Discussions>INSULATION & HVAC>best approach at insulation and airflow control - old farmhouse
12 posts / 0 new
Last post
best approach at insulation and airflow control - old farmhouse

I have been going crazy doing research on how best to address insulating and "air-proofing" an old 1040's era farmhouse I have purchased in Dallas, which would be categorized as a "mixed-humid" climate.

I will do my best to describe the situation I have but am not sure of the proper terminology in some cases. Sorry it is so long, I hope someone will take the time to read it.

The external siding on the house is what I call old lap siding where the boards overlap each other slightly going down the wall.
Inside there was old paneling nailed up, underneath that was some kind of paper mesh backed membrane. Under that are old 1X6 pine boards nailed up to the studs. Under that is nothing, an empty cavity where you can see daylight through gaps in the lap siding outside. I want to make the house more energy efficient, but do so in a way that I don't have to worry about the wall cavity getting wet and moldy. After much research and speaking to some people, I have come to the conclusion that a vapor barrier is not a good idea (at least in my area). However, it seems to me that a air-barrier (like tyvec wrap or something) that is vapor permeable may be a good idea. I am trying to figure out the best way to incorporate an air barrier and insulation. The problem with the air barrier is that traditionally that is done on the outside perimeter of the house. In this case, I do not plan on removing the siding, so that would mean installing an air barrier from the inside against the interior side of the exterior lap siding and in between the studs. To me this doesn't sound like a good idea. There is an air barrier by Certainteed that goes over the interior wall, stapled across the studs with unwrapped roll insulation underneath. Not sure if this makes sense as I am afraid there would be nothing (no air or vapor barrier) between the exterior siding and the insulation in the wall cavity. I am afraid it will get wet. After I determine how to incorporate an air-barrier (if I should), I want to put insulation in the wall cavity. Not sure what I should use. I do not have the budget to do spray in foam, I will have to stay with the old-style batt insulation. Finally, once I add the air barrier and insulation, should I simply install drywall over the studs on the interior, or should I put the 1X6 boards back up and install the drywall over the top of that? If the suggestion is to place an air barrier directly underneath the exterior siding, do I need to implement some kind of shim to leave a gap between? I am really stuck on how to proceed. Please let me know if any of you have advice. Thank you. Brad Fisher

Re: best approach at insulation and airflow control - old farmhouse

I would worry about the attic insulation first. Do you have enough there? Then, I would move to making sure windows are sealed properly. Last, I have used p2000 on my house and installed it over the similar type of wood siding on my house and sealed it with tyvek tape at the joints. Then installed vinyl siding over top of the p2000. The insulation has a good value and will allow you to then install fiberglass on the inside cavities as well. No air should be able to infiltrate if you seal all of the joints from the outside, allowing you to install fiberglass, then drywall right over the studs.
just my thoughts.

Re: best approach at insulation and airflow control - old farmhouse

You didn’t say if your house is air conditioned but being in Dallas I expect you would have air conditioning. Since your exterior sheathing is so full of gaps the hot, humid air has no trouble getting into your wall cavity. When the hot, humid air hits the back of your cooler interior wall it will condense and get that area wet. With no insulation in the wall cavity it can now dry out. If you put insulation in the wall cavity it will absorb the water and stay well longer which is not a good situation. You must keep the hot, humid air out of your wall cavity. The only way to do that is to take your siding off. If you then put on a good air barrier and meticulously seal any exterior penetration and insulate from the exterior (foam board insulation, two or more layers, stagger the seams) you will keep the humid air out of wall cavity. The hot, humid air will come in contact with the foam board insulation but it will not be cold so it won’t condense. Also, use a rain screen under the siding to allow any moisture that gets behind the siding a chance to dry. You then don’t have to insulate in the wall cavities themselves.

I would not put the vapor barrier on the back of the outside sheathing from the inside. Your wall studs are then in the outside environment between two sheets of plastic. They will get cold from being in contact with your air conditioned wall. They will be a place for moisture to condense. If your wall studs get wet and don’t dry adequately you will have problems.

There’s no easy way around this. If you don’t do this right you will trap moisture in your wall cavity which will cause you problems… probably big problems.

Check out this presentation by Pat Huelman from the University of Minnesota on exterior insulation.

Re: best approach at insulation and airflow control - old farmhouse

Sherry brings up some interesting points that apply to many areas along the gulf coast, but do not apply to the Dallas/Ft. Worth area. Dallas has a subtropical humid climate, but not humid enough for condensation to form inside the wall cavities of an air conditioned space, unless the AC is turned down very low. For the normal recommended interior summer temperature, condensation will not form.

This is a link to historical temperature and humidity data for Dallas.


You can see in the charts that as temperature rises, RH (relative humidity) drops. On a hot day of around 100°F, the RH is around 40%. The temperature record is around 113°F with a heat index of 117°F which would indicate a very low humidity on that day.

This next link is to a chart for temperature, RH, AH (absolute humidity), and dew point. Dew point is the temperature where condensation occurs when the the air temperature and RH are at various values.


The chart is from Europe so the temperatures are in centigrade. For comparison to Fahrenheit, use the following:

15°C = 59°F
20°C = 68°F
25°C = 77°F
30°C = 86°F
35°C = 95°F

So on a typical hot day in Dallas with 95°F and 40% RH, we look on the chart and see that the Dew Point for 35°C and 40% RH is 18°C which is between 59 and 68°F. Unless you set your AC below 68°F, you will not get condensation inside your walls.

The bottom line is, leave the outside alone. If you have removed the interior sheathing of the stud bay (the lath and plaster) then you can insulate this house the same way you would in most of the US, vapor barrier on the room side.

The AH inside your house will be higher that the outside in winter. The greater danger from moisture and condensation will come in winter if the warm moist interior air migrates into the stud bay and condenses on the colder surfaces. Putting a vapor barrier on the inside will block most of this moisture, and what does get out will mix with cold dry air and not be a problem.

I'm pretty sure your house wasn't built in the 1040's and since the 9 key is next to the 0 key, I'm guessing 1940's era. That means the studs are probably on an even spacing, i.e 16" OC. For this, my recommendation is standard faced batts, but do not staple the batt wings to the inside face of the studs as most commonly done. Instead, staple the wings to the ends of the studs and overlap the wings from the batt in the adjacent bays. Any air gap between the batts and the finished surface should be on the outside of the stud bay, next to the pine boards and should be flush to the interior wall (sheetrock). This will keep the insulation dryer and it will work better.

Re: best approach at insulation and airflow control - old farmhouse

i think you should put tyvek on the inside mainly because you aaid you can see light between the clapboards. this will let water and air in wich will bring moisture and get your new insulation wet causing rot mold and lots of other problems

Re: best approach at insulation and airflow control - old farmhouse

I may have misunderstood the wall construction. Are the 1x6's on the interior side of the studs? I was thinking that they were under the clapboards on the exterior side of the studs. If the clapboards are nailed directly to the studs, then you might need to address the "daylight issue. A lot depends on whether water gets on the inside of the clapboards. If the gaps are such that any water is shed down to the claps below, then the gap is not an issue, but if water can blow into the cavity, the it needs to be addressed. Just seeing daylight is not always a problem.

If there are only a few gaps, you may be able to fix the problem with small pieces of Tyvek or similar product. If it is the butt ends of the claps that is the cause of the gap, then some type of flashing can be used. What ever you do, any water that gets behind a clapboard must be directed back to the exterior. Just sticking Tyvek behind the claps will not fix the issue. The water must shed like water off a ducks back.

Re: best approach at insulation and airflow control - old farmhouse

i totally agree that the claps need fixed but covering the back with tyvek is a good temporary fix till the siding issues can be addressed. not sure about weather in texas but here in pa if you can see light through the siding you will get water for sure

Re: best approach at insulation and airflow control - old farmhouse

I have to express a conditional disagreement about the use of Tyvek as a temporary fix. I hope that bfisher posts back to clear things up, but typical construction in the 40's would have had the 1x6's nailed to the studs, with a gap between each one, then a layer of tar paper or other similar paper based membrane over it, then the clapboards. The clapboards would not have been attached directly to the studs and 1x6's would not have been on the interior side of the studs.

If the 1x6's are on the exterior side of the studs as they should be, the membrane over that and then the clapboards, it would be possible to see daylight in some of the spaces and still not get water inside the stud bay. This applies to Texas and Pennsylvania.

But if the claps are attached directly to the studs and there is daylight between the claps at the butt ends, water will get in. Attaching tyvek top to bottom in the stud bay behind the claps is not a good idea. It may keep the insulation dry, but water will still get into the bay, migrate down to the bottom and soak the sill and the ends of the floor joists causing them to rot. It will also aid in holding the moisture in the sill and joists, further aggravating the situation. It could also hold moisture against the backs of the claps causing them to rot too.

What is needed is a flashing that bridges the butt end gap of each clap and directs water back out to the exterior of the clap below. You can make the flashing out of tyvek, but I think a piece of sheet aluminum would be better. Its a tedious task to do from the inside, it should have been done when the claps were first installed, but it is the best solution.

If there is daylight coming through due to broken claps, then the broken claps must be replaced. There really isn't much you can do to fix them. There are epoxy wood repair kits, but they are expensive and time consuming, and they really require a little artistic talent, which I have none of. It would be a solution if there were on a couple of small sections of broken claps and the breaks were small, but if the area is very large, it would be easier and cheaper to just replace the clap. Individual claps can be replaced if you are careful.

One more caution to bfisher, if the 1x6 boards are on the inside, you cannot remove all of them at once, especially if they are mounted on the diagonal as they should be. If you do, you run the risk of racking the walls and the house will fall down around you.

Re: best approach at insulation and airflow control - old farmhouse

Hello, it is me again. I had posted a lengthy (maybe too lengthy) reply back expressing my gratitude to all the fantastic feedback I have seen from all of you and answering the posted questions. I appreciate it so much. I knew this would be the site to get quality feedback. For some reason it said it would need moderator approval. Anyway, the clarification near the bottom is correct. The 1X6's are on the interior, directly under some paneling I pulled off. the cavity inside is empty and what looks to me like the external siding clapboards attached directly to the studs. I am going to try and post another reply with pictures. That would probably work better than my explanations. Stay tuned.

Re: best approach at insulation and airflow control - old farmhouse

Let me post this one to answer questions that were posed:

1940's era - yes, I did mean to say 1940, not 1040. The tax rolls say built in 1951, but the neighbor who has lived there for 60 years and the previous owner (only second owner) said it was built before then as well.

attic - agree with snyder that attic insulation is place to start for best bang for buck. There is absolutely no insulation there either. The attic had been converted for living space, but very poorly. Had to demo that and will rebuild. Also had to demo it due to completely rewiring the house with new service. I wll address the attic insulation when that project and the hvac is complete. I think I have a bit of a clue as to how to go about that.

Sherry's ac question - no, the house is no longer air conditioned. There were 4 window ac units and old Dearborn heaters in EVERY room. I am putting new HVAC as part of the remodel.

Keith - you have captured the climate conditions in Dallas/Fort Worth very accurately. That is what makes it more difficult, we are subject to extreme heat, and occasionally fairly severe cold. I think my pictures will help better than my explanations. The "daylight" I see from the inside looking out is primarily in the horizontal space where the overlap should be. The butt ends look pretty good. I appreciate your warning about the 1X6's adding support to the structure. I really didn't think about that, as I was thinking to sheetrock back to the studs without putting the 1X6's back in place. I just had the house leveled, it is pier and beam. The house is shifting and groaning all over the place.

Re: best approach at insulation and airflow control - old farmhouse

Well.... I cannot seem to get the pictures to compress enough to meet the posting requirements. I have tried straight jpeg, zipping them, putting them in word. They are about 1 to 2 meg apiece. I will find a program to compress them or post them in a link for everyone to see. I was going to post my email address so anyone could communicate with me directly, but apparently I cannot until I reach 10 posts. Again, I really appreciate the help. This house is for my Mother-in-law. My biggest concern is making it a safe and healthy place to live.


Sponsored Stories

TV Listings

Find TV listings for This Old House and Ask This Old House in your area.