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Community and Q&A

Polyiso insulation and siding techiques for wall system in the Northern Rockies?

Alice Oliver | Posted in Green Building Techniques on

I’m going a bit nuts trying to figure out what I must do to properly side my new timber frame backyard office in western Montana. I’ve been reading and reading online, but find that my specifics are important, and haven’t found anything that fits my particular situation. Many thanks in advance for everyone’s input.

The structure is 8’x16′, made of kiln dried Doug fir 6×6″ and 6×8″ beams, in a timber frame design. There is electricity, but no plumbing. It will be heated with an infra-red electric fireplace/space heater (and my body heat!) and the building will sometimes go unheated. I may use an electric tea kettle in there sometimes.

The wall system as it is constructed thus far is as follows, from inside to outside:

1. Interior: timber frame post and beam, raw wood

2. Behind the frame: 2″x6″ tongue and groove pine, new raw wood, completely wraps the frame

3. 30# builder’s felt (aka tarpaper), completely wraps the outside of the T&G

4. 4″ polyiso rigid foam, reclaimed/used, held in place and cracks and holes filled in with spray foam, wraps the entire building, except where 2x4s on edge create a nailable surface for applying the future siding. The 2x4s and the 4″ foam are flush with each other. The 2x4s are screwed into the frame’s girts.

A local timber framer got me this far. Now it’s my job to put on the siding. I want a reverse board-and-batten or a reverse board-and-batten “look.”


1. Do I need any kind of moisture or vapor barrier over the polyiso? Is leaving the foil-covered polyiso exposed under the siding okay? Since it is used there are some minor tears in the foil–should these be covered?

2. The polyiso seams have been filled with spray foam–is tape also needed?

3. Do I need a rainscreen–and if so, would reverse board-and-batten (using 4″ battens and 8″ boards) provide enough of a rainscreen? If not, would going up to 10″ boards help? I’m using locally milled, green rough sawn pine or fir.

The local average annual precip is 13.6″. (and it gets as cold as 25 below zero for a couple of weeks or so.) We’re in gardening zone 4-5 in this valley. Rot is not usually an issue in old buildings here, but wood dries out–we get single-digit humidity in the hottest part of summer.

4. Will using rough-sawn pine for the exterior siding be adequate, or would rough-sawn Doug Fir be better? Must either of those be stained or painted or can I let them just naturally gray out? And if they must be treated, is it necessary, in my climate, to stain/paint on the side facing the polyiso?

4. Would using LP SmartSide and furring strips be a viable alternative to rough-sawn full-dimension wood? Advantages? Disadvantages? It is cheaper, but will it hold up? Is doing it right too complicated for a beginner?

My experience: I have done some small woodworking projects and am a fast learner and very careful, but I have no prior experience with building construction. I’m a writer creating a back yard writing retreat on a shoestring budget.

Many thanks to you all for your kind assistance!!


  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do I need any kind of moisture or vapor barrier over the polyiso?"

    A. Not necessarily. Every wall needs a water-resistive barrier (WRB), but in some cases you can use rigid foam as a WRB. Here are two articles with more information on the topic: Using Rigid Foam As a Water-Resistive Barrier and All About Water-Resistive Barriers.

    Q. "Is leaving the foil-covered polyiso exposed under the siding okay?"

    A. If I were building the wall, I think it would be easier to use housewrap as your WRB. Housewrap is easier to integrate with window flashings.

    Q. "Since it is used there are some minor tears in the foil--should these be covered?"

    A. It wouldn't hurt to cover the tears with foil tape.

    Q. "The polyiso seams have been filled with spray foam--is tape also needed?"

    A. No, unless the polyiso is your WRB. If the polyiso is your WRB, you have to follow the details on the foam manufacturers' documents for use of their product as a WRB.

    Q. "Do I need a rainscreen?"

    A. I think it is an excellent idea.

    Q. "If so, would reverse board-and-batten (using 4" battens and 8" boards) provide enough of a rainscreen?"

    A. Yes, as long as it is properly detailed.

    Q. " If not, would going up to 10" boards help?"

    A. No.

    Q. "Will using rough-sawn pine for the exterior siding be adequate, or would rough-sawn Doug Fir be better?"

    A. Either species will work, as long as your roof has generous roof overhangs (always a good idea).

    Q. " Must either of those be stained or painted or can I let them just naturally gray out?"

    A. Gray is fine.

    A. "If they must be treated, is it necessary, in my climate, to stain/paint on the side facing the polyiso?"

    A. No.

    Q. ". Would using LP SmartSide and furring strips be a viable alternative to rough-sawn full-dimension wood?"

    A. Your choice. I'd go with real wood.

    Q. "Advantages? Disadvantages? It is cheaper, but will it hold up?"

    A. Cheaper, but its longevity is unknown.

    Q. "Is doing it right too complicated for a beginner?"

    A. Maybe. It's easy to screw up flashing details. If all of this scares you, it might be a good idea to work with an experienced contractor.

  2. Alice Oliver | | #2

    thank you, martin!

    a couple of follow up questions:

    1. since i already have 30# builder's felt between the T&G interior wall and the polyiso foam, wouldn't adding a second WRB on the outside of the polyiso foam create a moisture trap or other problem?

    2.can i use builder's felt as a WRB?

    3. can the WRB be attached to the foil-faced polyiso? if so, what method should be used to attach it? since this is a timberframe building, there aren't a lot of places to staple a house wrap. suggestions?

    4. i have no idea what company manufactured my foil-faced polyiso, as it was bought used. i'm sure the builder and the guy he bought it from don't know either. also there are punctures and other compromises in the surface of the foil. should i therefore simply assume this is not suitable for a WRB? i have no way of knowing what manufacturer instructions to follow to use this product properly for a WRB. any links or suggestions? It is foil-faced on both sides.

    5. when you say that reverse board and batten siding can be used to create a rainscreen if it is properly detailed, what are those details?

    btw, my roof overhangs will be 18" out from the siding. is that sufficient for using wood siding?

    I have to get some siding up in the next day or so, enough so that my electrician can install the exterior lights and boxes, so i hope to learn enough here to make a final decision on how to treat the outside of the wall system.

    from what you are saying I'll go with the wood reverse board and batten since this must be a strictly DIY job. (I'm hiring someone to help with the windows, however.) Thanks!!

  3. GBA Editor
    Martin Holladay | | #3

    I'm sorry if my answer was misleading. You're right: the #30 felt is a perfectly acceptable WRB. Just make sure that the asphalt felt is integrated with the window flashings.

    Concerning rainscreen details: you need a screened opening at the bottom and the top of the rainscreen gap to encourage ventilation and drying. You need to be sure that your window flashing is properly detailed. Windows and penetrations are the usual problem areas for water entry into walls.

  4. Alice Oliver | | #4

    Thanks Martin. My question wasn't really about if I can use builder's felt as a WRB.

    What I need to know is can I use it *twice*? I already have 30# felt against the inside wall. I thought that repeating it on the outside wall (on top of the polyiso) was a No-No. Doesn't that trap condensation in the polyiso?

    Please clarify? Is it okay to use the felt *both* against the inner wall (T&G) and as the last layer before applying the outer siding?

    If I can't use it twice, how do you recommend I weatherproof the windows?


  5. Alice Oliver | | #5

    Also is there a flashing product that will niftily screen the bottom edge of the reverse board and batten?

  6. GBA Editor
    Martin Holladay | | #6

    Yes, you can install a second layer of asphalt felt on top of your polyiso if you want. It won't trap moisture, because asphalt felt is a "smart" vapor retarder. As it gets damp, it becomes more vapor-permeable.

    However, it is essential that you know which of these two layers is your WRB. Only one layer can be integrated with your window flashings. Choose which layer to make your WRB, and be consistent.

    Your question, "How do I weatherproof the windows?" is worrisome. If you don't know how to answer this question, it's essential that you hire a contractor who knows the answer. If you get these details wrong, you can ruin your wall.

    If you will do your own flashing work, it's time to go to university. Start reading every article you can find on window flashing, here at GBA and

    Concerning screening the bottom of your rainscreen gap: you can use ordinary insect screening or strips of Cor-a-Vent "plastic wool" used for ridge vents. You can see insect screening used in this GBA video:

  7. Alice Oliver | | #7

    thanks, the video and your answers are very helpful.

    it's too late to use the interior felt as a WRB on the windows, plus i would think that would not work, since from everything i'm reading on the websites of polyiso manufacturers, foil faced polyiso is already a WRB.

    my question was *if* i don't have felt on the *outside* of the polyiso, will i be able to adequately flash the windows?

    from what i can read, the polyiso i have is very heavy duty and is meant to be used as a WRB. i'll fill all holes and cracks, use tape to smooth over imperfections in the surface, and i should be good to go.

    but then i won't have a house wrap or felt to wrap around the windows for weatherproofing. is that a problem? is using the tape and the flashing sufficient?

  8. GBA Editor
    Martin Holladay | | #8

    Check back what I wrote in my first answer. If you want to use the rigid foam as a WRB:

    1. You will be depending on the chemical bond of tapes and peel-and-stick flashing for longterm waterproofing, rather than gravity and overlaps.

    2. Building code official will not accept the installation unless the brand of the foam is known, and unless the builder has followed the manufacturer's instructions, if any exist, for the use of their product as a WRB.

    3. More information can be found in my article, Using Rigid Foam As a Water-Resistive Barrier.

  9. Alice Oliver | | #9

    thanks, actually i did find the answer in your article while you were replying. so now i believe i will be using #15 felt as a wrap around the rigid foam before i install my vertical wood siding.

    which leads me to what i hope is my last question:

    is it okay to staple the felt into the foam panels? if not, what is the best way to attach the felt? i have some 2x4 surfaces to staple felt into (they create frames around the rigid foam panels), but there are also areas where there is no other attachment point except the foam panels.

    any ideas for that?

    many thanks!

    oh, and we are getting a pro's help with the windows--he'll install two of them with us so we know exactly how to do the rest. but in the meantime, we have to get some siding up so that the electrician can finish his job.

  10. GBA Editor
    Martin Holladay | | #10

    I would use long cap nails to attach the asphalt felt. The nails should be long enough to bite into wood. Eventually, once your reverse-board-and-batten siding is up, the battens will secure the felt.

  11. Aj Builder, Upstate NY Zone 6a | | #11

    Alice, the last time we did board and batten siding we hung the felt underlay in vertical overlaps. Works perfectly, very easy to install as you side too. Less ladder moves.... Conservation of E yaa know. Also should solve your attachment issues.

  12. Alice Oliver | | #12

    thank you aj! i was just wondering if vertical application of the felt would work just as well, as that definitely solves the problem!

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