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I am building a new house in southern Maine. It is a low-cost, affordable-to-build house.

RCRIII | Posted in Energy Efficiency and Durability on

I can add a layer of 3/4″ rigid insulation (R-4.88) to the inside of the exterior walls and flat ceiling for less that $1,500.00. The walls are planned to be 2×6. Keeping the cost to build as low as possible is very important to me.

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  1. GBA Editor
    Martin Holladay | | #1

    Thanks for sharing your plans. Do you have any questions?

  2. RCRIII | | #2

    yes, my question is about whether or not it makes sense to add 3/4" rigid insulation to the inside.

  3. GBA Editor
    Martin Holladay | | #3

    Yes, it makes sense. It makes even more sense to add foam insulation on the exterior, where the foam will do more good (since it would span the rim-joist area and partition intersections). If you can afford it, install 2 inches of polyiso foam on the outside of your walls. In Maine, the R-12 foam will prevent condensation on the interior side of your wall sheathing.

  4. RCRIII | | #4

    So inside is ok. outside would be better. but if i put it on the outside won't I have isues with nailable surface for the siding, and issues with having to build out windows etc.?

  5. GBA Editor
    Martin Holladay | | #5

    Yes and yes.

    When it comes to fastening siding, the best solution for most types of siding is to install vertical strapping (1x2 or 1x3) over the foam. One piece of strapping is located over each stud. This method creates a rainscreen, which will help your siding last longer.

  6. RCRIII | | #6

    Ok. Understood. This will add cost though and for $1,500.00 for the rigid inside it might be the easiest cheapest way to add R value and make the hosue tighter. Would you have any concern that 3/4 inside would make the house too tight?

  7. GBA Editor
    Martin Holladay | | #7

    No. There is no such thing as "too tight."

    Of course, every new home needs a mechanical ventilation system. Read more about mechanical ventilation here:

  8. RCRIII | | #8

    Ok, i guess I'll read about that. I had not planned to install a mechanical system.

  9. RCRIII | | #9

    So, I am planning on a bathroom vent fan in each bath (2), and the heating system will be a direct vent natural gas fired Laars boiler. total house size is about 1400 square feet on two floors, about equal spacing on each floor. Do you think a mechanical ventilation system is a must?

  10. GBA Editor
    Martin Holladay | | #10

    I assume you are posting your questions on this Web site because you want to build a green home.

    A green home MUST be energy-efficient. In most climates, including the climate in Maine, a green home needs to be as airtight as you can possibly make it.

    Once you've built a very tight house, you need a mechanical ventilation system.

  11. Riversong | | #11

    If you build the house as tight as you should for energy efficiency, then a mechanical ventilation system is necessary both to maintain indoor air quality and to control indoor humidity, which is the enemy of modern homes. A minimum 0.25 air changes per hour and a maximum of 40% relative humidity in winter is required to maintain a healthy and happy home.

    But, contrary to Holladay's musings, an exhaust-only system using efficient bathroom fans controlled by both an intermitent switch and a 24-hour programmable timer, coupled with passive make-up air inlets (I prefer American Aldes Airlet 100) located in bedrooms and living rooms works very well in a tight house. This is the simplest of all controlled ventilation systems.

  12. RCRIII | | #12

    I understand where you are coming from. I guess i am shooting for a shade of green. I was thinkng that spending $1,500, was resonable, and would pay back quickly. If i need to add an air to air exchanger etc. then that is probably more than budget will allow.

  13. GBA Editor
    Martin Holladay | | #13

    I'm not sure why you wrote, "contrary to Holladay's musings." I like exhaust-only ventilation systems, which are simple, affordable, and energy-efficient.

    I wrote, "Research shows ... that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well. If the exhaust fan is well chosen — my own favorite is the Panasonic WhisperGreen fan, which uses only 11.3 watts to move 80 cfm — exhaust-only ventilation systems have very low installation and operating costs. If you choose this type of ventilation system, it’s important to remember to undercut the bathroom door."

  14. Riversong | | #14

    I wrote "contrary to Holladay's musings" because your article on mechanical ventilation stated: "many energy experts (including Lstiburek) disparage exhaust-only ventilation systems." and "If you do install an exhaust-only ventilation system, don’t bother installing passive fresh air inlets in the walls."

    While it's true that you stated that an EOV system can work in a limited spectrum of homes, you relied on a higly flawed study to "prove" that a system such as I described above won't work as intended.

    See my last comment "Flawed Study" on your Musings page. I just installed - and tested - such a system and it works as intended.

  15. RCRIII | | #15

    So...I am at a loss, as to what to do. Should I add 3/4" rigid for $1,500.00, and assume my bath fans will move enough air?

  16. homedesign | | #16

    Rocco, Are you the builder or the owner?
    What you need is an airtightness strategy and a measured amount of ventilation.
    Rigid insulation may contribute to airtightness .. but it does not guarantee it.
    Talk to some Energy raters in your area and look for builders who are achieving good blower door test results...and good HERS ratings
    Look for people to network with who understand EEBA.
    There are many more questions than whether to incorporate rigid insualtion or not.

  17. Expert Member
    Michael Maines | | #17

    It's hard to go wrong by installing the 3/4" foam on the inside, provided you have and USE your bathroom and kitchen fans, and you don't have another vapor retarder in your wall assembly. You might get more bang for your buck, though, by building with 2x4 studs, cross-hatching with 2x2's, and using dense-pack cellulose.
    Are you in the Portland area? Send me an email (mmaines at finelinesmaine dot com) and I can suggest some resources for you.

  18. Darin Zurliene | | #18

    Rocco call your local Nu-Wool Certified contractor for a proposal on a great product.

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