Foam Board Insulation Under Vinyl Siding

By Todd Fratzel on Insulation, Siding & Trim

Installing Foam Board Insulation Under Vinyl Siding

Today more than ever before homes are being built with foam board insulation installed on the outside of the sheathing and under the siding. This detail is used with both new construction and also with renovation work. Below are some of the design details along with the benefits of installing exterior rigid foam board insulation.

Rigid Foam Board Insulation Over Tyvek House Wrap

Improved Moisture Control

One of the biggest advantages to installing exterior rigid foam board insulation is improved moisture control. By installing insulation on the outside of the framing it causes the location of the dew point to move to the outside of the wall assembly. This reduces the likelihood of condensation within the wall cavity.

The exterior insulation also helps keep the siding (all vinyl siding leaks!) further away from the exterior sheathing. Basically it creates another drainage plane that helps protect the sheathing and framing from damaging water infiltration.

Improved Thermal Performance

Installing Rigid Foam Board Insulation Over Tyvek House WrapExterior rigid foam board insulation can make a significant difference in overall thermal performance for a building. First of all it adds to the overall R value, which can be quite significant depending on the thickness and type of insulation used. If a 1 inch layer of XPS foam is installed it can result in an immediate increase in R value of 5 (20% increase for a wall with 6″ of R19 fiberglass).

In addition to the increase R value, the bigger advantage in my opinion is the improved air infiltration that’s realized with a properly installed layer of rigid foam insulation. The reduced air infiltration results in a better overall performance of the insulation within the wall cavity. This is especially true if the wall is filled with fiberglass batt insulation.

Rigid Foam OVER Building Wrap

One of the most asked questions on our site is related to this article. Readers are constantly asking if the rigid insulation is supposed to be installed over the building wrap or vice versa. Dupont says you can install it either way, however we typically install the foam over the house wrap. It’s much easier in my opinion to do it in this sequence.

To attach the foam board we use a cool tool from Paslode, their Cap Stapler does a great job holding the foam in place and also attaching the house wrap. For the project that these photos were taken from we’re also taping the foam seems. This is fairly typical however Dupont suggests leaving them “open” to help the wall assembly breath. Seeing that we’re using vinyl siding I think it’s better to tape the seam and keep out any water.

Pay Attention To Details

By installing rigid foam board insulation on the exterior walls you’ll need to pay special attention to the details. You’ll need longer nails for nailing trim and siding. You’ll need to properly flash window and door openings. You’ll also need to pay attention to window and door extension jamb depths as they will increase if the window is mounted flush with the foam board.

Final Thoughts

Energy costs are never going down at least in the near future. So investing in good insulation is always a great investment. Using exterior rigid foam board insulation is a great way to make a building more energy efficient, quieter and more likely to prevent moisture infiltration problems. The method works for both new construction and remodeling and doesn’t affect siding and trim details very much. Bottom line is exterior foam insulation is a good investment.

 

 

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the author of Tool Box Buzz and Today's Green Construction. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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34 Comments

  1. Excellent article Todd! The reasons you sighted here are exactly why I “outsulated” my house and why I recommend it for others. Moves the dew point and the drainage plane. I used poli-iso, fully taped, windows set on top of the foam, and then rain screen details.

  2. Scott Pearson says:

    Todd, would you recommend using 2″ rigid foam? I have a guest at my store who wants to do this. I have yet to find a good answer for him. Any input would be greatly appreciated. Thank you, Scott.

    • Todd Fratzel says:

      2″ is a mixed bag. There are people out there using 2″ but you’ve got two issues to deal with. First is you’d really need to strap over it so that you can nail siding (1″ is about the most I’d do with long nails). Secondly, at 2″ thickness, the foam acts like a vapor barrier, so it’s possible that water vapor that’s inside the wall might get trapped between the foam and interior vapor barrier. If an interior vapor barrier isn’t used, then this is less of an issue.

  3. Dennis says:

    Todd, I’m in the process of replacing old celotex fiber board with 7/16″ Zip-System sheathing. Now before I put up the new vinyl siding, I’m thinking that adding at least a 1/4″ fanfold foam layer would be a good idea, especially after reading your article. Is it a truly necessary step to have the foam on the sheathing before installing windows/doors or will it suffice to bring it right up the edges and just make sure things are taped securely? I ask as I have already installed a few windows directly to the sheathing. Thanks in advance.

    • Todd Fratzel says:

      Dennis – with 1/4″ foam it doesn’t matter at all. Thicker foam requires different details that make it tough to do after the fact. Good luck.

  4. Don says:

    I was thinking of installing 3/4″ rigid insulation on the exterior of a stucco house. I have already installed new windows and firred them out 1 1/2″. I was thinking to install strapping over the insulation and siding over the stapping. Would you recommend I wrap the house before I insulate, after I insulate, or not at all? I live in Alberta Canada and am worried about condensation. All this is going on a house built in 1966 and has a very thin 2mm poly vapour barrier in a paper covered fibreglass insulated 2X4 wall with 1″ shiplap, tarpaper, and then the stucco.

    • Todd Fratzel says:

      Don – Tough question. I’m going to pass on this one! I’ve never done it and I’d hate to lead you down the wrong path. My gut is telling me: house wrap, strapping, foam, then siding, so that there is an air space between foam and stucco….but I’d say speak with an local insulation contractor. We just don’t deal with stucco here much in our area.

  5. Greg Zitzelberger says:

    I’m replacing my vinyl siding on our ranch home and would like to install 3/4″ rigid foam board underneath but I’m concerned that this will change the measurement around the windows. There is no foam board under the siding now. The house was build in the 70’s. Should I go with 1/2″ foam board to be safe? This is my first time replacing siding. I’m trying to do my homework before I start. We live in PA when the temperature can get pretty cold in the winter at times.

    • Todd Fratzel says:

      Greg – The bigger issue is sealing that foam well. If 1/2″ works better around the windows and doors, I’d stick with that, but do a great job sealing the sheets, air infiltration is a far bigger problem than the R value.

      • Greg says:

        OK. I’ll make sure I seal the sheets well. Any opinion about putting Tyvek over or under the foam insulation. There’s a lot of confusing information. Tyvek recommends putting the HomeWrap over the foam board. Correct?

        • Todd Fratzel says:

          Greg – This topic is all over the place. The last two projects we built the architects had the foam on top of the building wrap.I think this is good because it allows the building sheathing to breath and not trap moisture against the sheathing or siding. If you install the foam board against the building directly, it can’t breath in the same way.

  6. Patty says:

    I’ve been searching the web looking for an answer to a siding question and came across this site so perhaps you’ll be able to ease my mind. I have a contractor installing some vinyl shake siding on an existing porch over my garage. a few years ago I enclosed the porch so it’s Windows on top and wooden rail on the bottom. I’m installing the shake for appearance and because I don’t want the wood to rot…inside the porch I have bead board, insulation, then the wood rail and the contractor is simply putting wrap over the wood then the shake. my question is this, is simply putting the wrap then the shake sufficient? He stated no further insulation under the shake since I do have insulation under the bead board and the concern is what I’ve read above about condensation. Just want to make sure the job is being done correct I don’t want to find out in hindsight that a 1/4 or 1/2 inch of rigid foam board should have been put under the vinyl shake. thank you

    • Todd Fratzel says:

      Patty – I’d tend to agree with his approach, I’d be more concerned about the actual detail used over the old railing. Is it plywood, then building wrap, then siding? if so that sounds fine.

  7. Bill says:

    I am thinking about replacing my siding. If I use 1 inch foam and tape the seams will I have moisture problems from the inside? I live in northern Kentucky. House was built in 1960. It has what looks like paper backed batting insulation. I think r13. 2×4 studs. This is only the top half. The bottom has some stone and some brick exterior against block. Thought about getting spray foam in that. Thanks for your help.

  8. Sue B says:

    My house in New Hampshire was built in 1983 and was the original contractor’s home. Almost all the houses this contractor built in my neighborhood were constructed without exterior sheathing–just the foam and siding. A framer who lives in one of those homes said there is metal diagonal bracing. He sheathed all the sides of his home (took him 8 years for all 4 sides) before re-siding. My home has wooden clap boards which I am tired of repainting, with quite a few areas of rotting trims and boards. Would like to have house sheathed –but the cost of doing it is too much too afford (house is 24 squares). Is replacing wood siding with vinyl and structural foam sheathing enough?

    • Todd Fratzel says:

      Sue – without tearing off the old siding and inspecting the frame of the house it’s very difficult to say what is best. Foam plus wood siding is one thing (the wood has some strength, especially for impact loads), but foam and vinyl does give me some heart burn. If it were my home, I’d save some extra money and do the sheathing.

  9. Tom Durand says:

    I bought a older mobile home in las vegas Nevada. to reduce noise from a nearby road I want to side the home with Hardee board. the existing sheathing is aluminum. This is a very dry area but should I use a vapor barrier with the insulation. Will insulation
    help with the noise.

  10. Miles says:

    Thanks for the info, Todd. I’m preparing to replace the 30-year-old vinyl siding on my house in Virginia. Living near a busy highway, noise reduction is also a factor. I was thinking to apply housewrap (there currently is none), then 1/4 fan-fold insulation and non-insulated vinyl siding, or the housewrap then insulated vinyl siding. But different websites have me concerned about creating moisture issues with the sheathing in either case. Which choice would be best for noise? Could I create a new problem in the process?

    • Todd Fratzel says:

      Miles – The 1/4″ won’t cause a moisture problem in my opinion (it will breath). I’d install house wrap then the foam. The foam will help a bit with noise.

  11. pat silvestro says:

    Todd, I am looking for best insulation wrap to go over asbestos shingles, and then siding. contractor said to use 1/2 inch wrap but r value is only 2.0 doesnt sound good to me.. any advise?

    • Todd Fratzel says:

      Pat – The issue is a bit more complex. 1/2 inch doesn’t effect any of the window and door trims typically so it’s used quite often. When you go to thicker insulation, it usually means modifying the trim too accommodate the thicker combination of insulation and siding.

  12. John says:

    Todd,
    I am considering building a new house. I am looking for the best combination. I will be using 2×6 walls with vinyl siding. I am either going with spray foam or wet cellulose. I had plans of 1/2 inch foil faced foam to overlap past the mud sill to have all openings covered. Am I creating moisture issues? What would you suggest? I will be building in central Illinois. Thank for your input.

    • Todd Fratzel says:

      John – The best approach these days is having a continuous layer (usually about 2″) of foam on the exterior of the home. Then inside, the stud bays are usually insulated with additional insulation (fiberglass, cellulose (dry, not wet), or spray foam). This helps move the dew point out of the wall assembly to prevent moisture problems. The next best approach is using spray foam in the wall cavity. If you go with cellulose, my preference is using the BIB system with dry cellulose.
      Good luck.

  13. John says:

    Todd,
    Thanks for taking the time to answer my question. Please take some more time to explain the BIB system. Also, is the exterior of the wall sheathed with OSB or something else? Would metal T bracing and foam over studs be sufficient?
    Thanks again.

    • Todd Fratzel says:

      For BIBS, there is always some sort of exterior sheathing. Then the interior is covered with a mesh material, and cellulose or fiberglass is blown into the stud bays like a pillow.

  14. Jude says:

    Hi Todd,
    Please explain the BIB system. Do you put OSB sheathing on the outside or do you metal T brace and put foam on studs or is that not recommended? Thanks in advance for your advice.

  15. Michael S says:

    I have a cement block wall on my garage that i want to side with vinyl siding…what type of insulation should i put betwwen the siding and the block wall, i am using firing strips 24oc…..i think the insulation will help firm up the siding as well.

  16. Paul H Fopeano says:

    I am in the process of modernizing a 1960 ranch, on the water in Downeast Maine. The original sheathing is fire proof exterior gypsum board. My research tells me that it is structural but it is not intended as a nailing base. I’ll be replacing all the siding, windows and doors and intend to use 1″ or more rigid foam to beef up the insulation. I will most likely replace almost all the interior sheet rock and insulation as I redesign the framing. I’ll use only moisture resistant sheetrock. If I apply foam over house wrap, over gypsum sheathing, or wrap over foam, over the gypsum, I will have to apply vertical firing strips attached to the framing (long screws) in order to have some way to attach vinyl siding. Is this the simplest way to deal with all the issues I have to face in an exposed, moist environment?

    Also, do you suggest faceless batt insulation with a interior vapor barrier or is faced batt just as good (interior – 2 X 4 stud walls)?

    • Todd Fratzel says:

      Paul – I would likely install the building wrap over the old sheathing, then the foam, then firing strips, then siding as you outlined. Faced is fine, just be sure to seal everything really well.

  17. tim ryam says:

    hi todd I have a ranch home in MD built in 1988,I recently took off the brick on the front which only went half way up and vinyl siding and wanted to replace the windows with anderson,well I found out that the house was only sheated on the corners and silver foam on the walls,I removed the foam and found areas missing isulation and a few areas that were not up to my standards.so Icorrected them,sheated the front and put in anderson new construction windows put house wrap on then bricked the front all the way up.now after seeing a mess on the front, I want to take off the vinyl siding and silver foam on rest of the house and correct any problems,then sheat the rest of the house with plywood and fir out around the windows with 3/4″ put new construction anderson windows in ,house wrap it and put on 3/4″ foam because we live in the country around many farm fields and we get much wind. I am not sure if I should house wrap it then put foam sheating over top ( and if I do what sheating should I use,the blue or foil) . thanks in advance tim

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