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Using insulation board on top of OSB and under siding

stryped1 | Posted in General Questions on

I have a 1500 square foot house with a 500 square foot bonus room upstairs. I have a downstairs window that is broken and I need to replace. It is a vinyle window with a nailing flange.

I live in southern Kentucky (42206). My house has 2×4 walls with fiberglass batts in all walls including the interior walls.

I have osb outside walls with no tyvec or anything on them. Nailed directly to the osb is vinyle siding.

Since I need to replace this window and it is on a small north sided wall, I got to thinking if it would be a good idea to remove the siding, replace the window, then install 1/2 inch insulation board on top of the osb, then put back the vinyl siding? Maybe, as I get time working my way around the whole house with this process.

I remember when the house was built in 1998, plastic vapor barrier was used on the inside of the wall covering the batts. Will putting 1/2 inch insulated foam sheeting cause a moisture problem? I cant seem to find a direct answer to this.

Also, is it possible to put the foam on top of the nailing flange of window instead of removing all the windows and re installing on top of the insulation? Or would that cause water problems?

I appreciate any help!

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  1. GBA Editor
    Martin Holladay | | #1

    Yes, you can put rigid foam on the exterior of your walls -- although 1.5 inch or 2 inches would be much better than 1/2 inch. You probably also want to install a WRB (like housewrap).

    This article is a good place to start for more information: How to Install Rigid Foam Sheathing.

    Here is my standard answer to the question about interior polyethylene:

    Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly weren't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

    That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

    If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

    To summarize, here are four caveats:

    1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing (

    2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

    3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

    4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

  2. stryped1 | | #2

    I guess my concern about going over 1/2 inch in thickness is getting everything to fit in its existings spots. I would not be doing this all at once, but a wall at a time. I am also still unclear as to if all the doors and windows have to come out before installing the insulation board of if the insulation board can go on top of the windows and doors.

  3. GBA Editor
    Martin Holladay | | #3

    It's always better to remove the windows and flash the window openings. But if you have wide roof overhangs and good flashing skills, you can come up with details that will work without removing the windows. It partly depends on whether you feel comfortable living with a certain (low but real) risk of water entry.

  4. Terryssr | | #4

    I have read that OSB doesn't like moisture & expands and/or delaminates when exposed to moisture. If that's true, and one has OSB exterior sheathing under one's house siding, should it be a priority to install a Tyvek-type house wrap between the OSB & the siding material?

  5. GBA Editor
    Martin Holladay | | #5

    Yes, a water-resistive barrier (for example, a housewrap like Tyvek) is required by building codes to prevent wind-driven rain that gets past the siding from wetting the wall sheathing. Your house doesn't have any, and it should.

  6. stryped1 | | #6

    Is it hard to take a window such as these out, then put them back in if I were to go that route? Can anyone think of a way to work this where I only have to take some of the sideing off at a time rather than the whole wall? I will be working as I have time totally by myself. I have a full time job with long hours and it is hard to do everything at once.

    I would like to use thicker foam. However, wont that make the siding when re installed too short on the corners?

    Someone also mentioned OSB need to be covered. Wont the foam insulation on the outside be doing the same thing as the tyvec?

    I appreciate the help. I am sorry for all the questions. I want to make sure I do everything right.

    So If I understand correctly, rainscreen strapping is not required in my situation (although nice to have), right?

    I am also worried that once I remove the vinyl siding it wont go back on exactly the same. (The siding has been on there since 1998.

  7. GBA Editor
    Martin Holladay | | #7

    If you pack out your walls with rigid foam, you will obviously make your walls longer than they used to be. This fact can be somewhat disguised by reducing the length of the vinyl siding overlaps or reducing the depth of the vinyl siding's insertion depth into the corner trim.

    Whether the work is easy or difficult for you will depend on your skill level and experience.

    If you want, you can use rigid foam as a WRB (instead of housewrap), but I don't recommend this approach. For more information, see Using Rigid Foam As a Water-Resistive Barrier.

  8. stryped1 | | #8

    Thanks so much. If I were to install Tyvec, would it go on top of the OSB and under the insulation board? It seems I have even read about a type of house wrap that has corregations in it that creates a slight air space between it and the insulated board or house siding. (I assume this is a positive)

    However, my only store available is Lowes so I am limited in what is available.

    Honestly, is it worth doing all this and using 1/2 inch board? It is R 3 I believe. I just am afraid to get started and use anything much thicker because of the siding gap potential problem. When I measured my corner covers, I have only 21/32 of "play" area.

  9. GBA Editor
    Martin Holladay | | #9

    Q. " If I were to install Tyvek, would it go on top of the OSB and under the insulation board?"

    A. See this article: Where Does the Housewrap Go?

    Q. "It seems I have even read about a type of housewrap that has corregations in it that creates a slight air space between it and the insulated board or house siding."

    A. For more information on housewrap types and rainscreen gaps, see these two articles:

    All About Water-Resistive Barriers

    All About Rainscreens

  10. Expert Member
    Dana Dorsett | | #10

    If you use unfaced Type-II (1.5lbs per cubic foot nominal density) EPS for the exterior half-inch foam it will not appreciably slow the drying rate of the OSB, since it's vapor permeance is 3-5 times that of the OSB itself. The R-value of half inch Type-II EPS is R2.1. The R-value of a 2x4 stud on the 3.5" depth is about R4.2, so you would be reducing the thermal bridging by 33% without reducing the drying rate of the assembly.

    If you can bump that to an inch it'll cut the thermal bridging in half, and still provide a reasonable drying rate. At an inch it has a vapor permeance comparable to that of interior latex paint, and under vinyl siding (which is inherently back ventilated) it's in drying mode MOST of the time.

    An inch of XPS would be on the vapor tight side but still OK, but any foil or plastic facers would be VERY tight at any foam thickness. ( It would take 3-4" of EPS to reach the vapor tightness of 1" XPS, if you have places where you're willing to bulk it out that far.)

    Going with R3 foil-faced polyiso would be the riskiest from a moisture-trap point of view, and with only a modest performance up-side over EPS. An OSB_sheathed vinyl sided 2x4 assembly with R13 batts comes in at about R9.5-R10 "whole-wall" after factoring in the thermal bridging of a ~25% framing fraction (typical for 16" o.c. studwalls.) Adding a half inch of EPS bumps that to about R12, a 16-17% improvement in thermal performance. Adding half inch iso bumps it to about R13, for a 24-25& improvement, but at some increase in risk.

    Also, EPS gains performance at lower temperatures while polyiso loses performance fairly dramatically when the average temp through the foam is below 35F. The de-rating/uprating curves with temperature of half-inch EPS and half-inch polyiso is such that when it's +25F outdoors the performance of either will run about R2.3 in your stackup, labeled R-values notwithstanding. (The testing for labeling purposes occurs at a mid-depth temp of 75F.)

  11. stryped1 | | #11

    Wow, thanks so much for the info. Trying to decide if it is worth it or not. If one inch of xps is borderline from a moisture standpoint, I assume r3 1/2 inch xps would be okay?

    What kind of real world energy bill savings could I expect from adding 1/2-1 inch xps to my exterior walls, considering I am at a zone 4 in location, have central heat and air? (I have propane heat and my ac and heat were installed in 1998)

    One last thing, does anyone know of a simple way to flash/rainproof the installation of a window in this house, which has a nailing flange, if I decided to put the foam on top of the window instead of installing the window on top of the foam? I just worry about installing something structural like a window into something that may degrade over time like the foam. (I guess I would like ot ensure the window is on a solid mount and that I don't have to mess with the interior framing of the window.

  12. Expert Member
    Dana Dorsett | | #12

    Unfaced half inch XPS runs R2.5, with a vapor permeance between 2-2.5 perms, which is fine. But the R0.4 difference between R2.5 vs R2.1 (EPS) isn't worth paying much extra for. Note, XPS is also blown with a much more environmentally damaging blowing agents than EPS & polyiso, but at R2.5 it's still easily a net-benefit on a lifecycle basis when heating with propane.

    The amount of savings you get depends a lot on just how big a fraction of the total heat load is from wall losses, both in conducted loss and infiltration. The wall losses of my tightened-up antique 1.5 story house with cellulose insulated 2x4 walls, the wall losses account for about 1/4 of the heat load. That means if during re-siding I added R2-R3 over the sheathing for a ~16-24% reduction in that 25% of the total would knock about 4-6% of the total heating bill. YMMV. If your walls are really leaky detailing the sheathing as an air-barrier as part of the project the whole project could be be worth 10% or more off the total heating bill.

    Every house has different issues, but any time you are re-siding it's an opportunity moment for improving the thermal performance, an opportunity that typically doesn't come around a gain for several decades. By air sealing, and adding the foam & housewrap you are improving the resilience of the structure too- it's not all strictly an exercise in net-present-value measured against future energy savings.

    Propane isn't likely to get cheaper over time though, and when it's time to replace the HVAC a variable speed high efficiency heat pump may be worth looking into. There are some pretty good ones out there now, with incremental improvements annually.

  13. stryped1 | | #13

    I appreciate the responses. One thing to remember, I am in a southern climate. I run my air conditioner from the end of April until October sometimes. I would venture that most of my expense is from cooling. (But with the price of propane this past year, maybe not).

    I keep hearing everyone talk about heating but a lot of winters where I am are mold compared to most of you. A typical winter may have lowes in the teens to 20's for a short period of time with mostly above freezing weather.

    Will the insulation panel on my walls work for reducing cooling costs as much as it will heating?

    One last thing, in your opinion before starting this, would it be okay to replace the window, put the peel and stick flashing around the nailing flange once the window is in, then put foam on top of the nailing flange and flashing?

  14. GBA Editor
    Martin Holladay | | #14

    Wall insulation is valuable during the summer as well as during the winter, since it prevents heat flow in either direction. But the cost of installing wall insulation is easier to justify in cold climates than it is in mild climates. If you aren't familiar with energy modeling software or payback calculations, you might want to hire an energy rater or energy consultant to help you with these calculations.

    Wall flashing details are a little more complicated than just "replacing the window, putting the peel and stick flashing around the nailing flange once the window is in, then putting foam on top of the nailing flange and flashing."

    If you aren't familiar with window flashing details, I suggest that you study up on these issues here on the GBA website, or hire a contractor who is familiar with window flashing details.

  15. stryped1 | | #15

    Unfortunately, I live in the country with Amish neighbors and hiring a contractor is not an option. (Plus I don't think there are contractors in my area that would remove the siding, and installing insulating sheets like I am wanting to do).

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