0 Helpful?

Can one place rigid foam over existing panel siding?

I recently purchased a '70s modified A-Frame cabin at Lake Tahoe w/ 2x4 framing.
The exterior walls are constructed with vertical 4x8 redwood panels directly nailed to the studs.
There is no insulation.
The interior walls are sheet rock covered with '70s simulated walnut paneling.
The windows are single pane sliding aluminum framed,
as are the two sliding glass deck doors.
The structure is 24' x36', 2br/2bath, loft...1200sqft.
The county building dept requires permits and retrofitting to replace siding to the studs.
The existing siding is sound but for a few small areas at the sill plate.

My thought......
repair the damaged sill area siding.
Remove all existing exterior window and door frame molding.
Build out the window/door frame area 2.33 inches using 2x4 and plywood.
Cover existing siding with 2 "Rigid Foam Board.
Tape all joints
Cover rigid foam with James Hardie cement 'fiber board Sierra panels.
Replace single pane windows, sliding glass doors, with energy efficient dropins.

Again, this is a recreational cabin, not a full time family home.
Does this sound reasonable?
Input encouraged and welcomed.


DSC_0308-002.JPG757.66 KB
DSC_0311.JPG574.22 KB
DSC_0320.JPG598.59 KB
DSC_0318.JPG581.52 KB
Asked by Whitney Johnson
Posted May 17, 2014 1:41 AM ET
Edited May 18, 2014 2:08 AM ET


5 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

It's certainly possible to install exterior rigid foam over smooth siding panels, as you propose, as long as you pay attention to airtightness, and as long as your wall assembly includes a water-resistive barrier (WRB).

The only problem with your suggested approach is that your rigid foam is too thin. Depending on what type of rigid foam you choose to install, 2 inches will have an R-value ranging from R-8 to R-13. That's not enough to meet minimum code requirements, so make sure that your plan meets the approval of your local code official before proceeding -- and consider doubling the foam thickness.

Answered by Martin Holladay
Posted May 17, 2014 6:54 AM ET


Thank you Martin,
Dang, 4.3 inches out is pretty thick.
2inch x 4'x8' R-Max Thermasheath 3 claims to be R13, is that OK?

If I did as you suggest, how would you recommend handling the existing window and door frames.
I'm currently in the SF Bay area so I can't measure for sure, but I suspect the existing windows/doors are typical off the shelf sizes. Would this revised method require customs?
Might you have a Plan B in mind?

And just to keep full disclosure, the roof is metal attached to T&G Pine boards(maybe sheathing) and huge beam rafters that are left exposed in the interior (no roof insulation).

Answered by Whitney Johnson
Posted May 17, 2014 11:43 PM ET
Edited May 18, 2014 2:31 AM ET.


I'm not an expert in California Title 24 (the state's idiosyncratic energy code), so you should get local advice on code compliance.

It's hard to give a tutorial that explains everything you need to know about installing exterior rigid foam in this column; I suggest you start with this article: How to Install Rigid Foam Sheathing. If you have further questions about the topics covered in that article, I suggest that you click on the many links found in that article.

Most window companies will sell you a window in any size you want, so you should be able to order custom sizes.

For information on insulating your roof assembly, I suggest that you read How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted May 18, 2014 6:00 AM ET


Thanks Martin for the advice and links.
I'll read them.
I'm not going to insulate the ceiling as I like the look, just stating that it has an uninsulated ceiling.
Thanks again!

Answered by Whitney Johnson
Posted May 18, 2014 11:45 AM ET


If you ever change your mind, and decide that you want to insulate your ceiling, the best approach would be to install a thick layer of rigid foam on top of your roof sheathing. (That way, your ceiling's appearance on the interior will be unchanged.) Of course, this method of insulation will require you to install new roofing.

Answered by Martin Holladay
Posted May 19, 2014 7:06 AM ET

Other Questions in Energy efficiency and durability

Insulating Wall Between two Different Slab Elevations

In Green building techniques | Asked by Jonathan Lawrence CZ 4A New Jersey | May 24, 18

Difficulty finding a modulating multi-zone air-source heat pump

In Mechanicals | Asked by Jonathan Lawrence CZ 4A New Jersey | May 21, 18

Vaulted / Cathedral Ceiling -- Unvented

In Green building techniques | Asked by User-7075735 | May 23, 18

Should I insulate and/or heat my unfinished basement?

In General questions | Asked by J Greene | May 24, 18

How do we hire a green building advisor on an hourly basis?

In GBA Pro help | Asked by Joe Martin | May 24, 18
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!