Subscribe or Renew Membership Subscribe Renew

Insulation under Hardie Plank

Emindenver's picture

I'm planning to remove old vinyl siding and replace it with cement fiber lap-siding with a 4" exposure.  After reading several articles in FHB, I wondering how to handle the exterior membranes of the wall assembly.  My plan is to remove the old vinyl, the 1/2" styrofoam sheets behind it, and the original pressed-paper siding.  This will leave a 1/2" layer of that old, black celotex material as well as the 1/2" plywood at the corners.  In order to increase the insulation values I want to first add a layer of the rigid foam board.  Hardie Plank recommends a maximum of 1" of foam-board/celotex-type material under their lap-siding, so the 1/2" foam and 1/2" celotex would be fine.  Would it be better insulation if I remove the celotex and use a 1" foam board instead?  My research seems to indicate that the foam board product is waterproof.  Is this going to be a problem with possible water vapor build-up between the drywall and the foam board?  Also, would I still need to use a house wrap if the foam board is waterproof? One more question I've seen a product called Poly-iso foam boards.  These seem to have better R values than the Owens/Corning pink foam boards.  Does anyone out there have a preference?  Thanks

(post #100355, reply #1 of 18)

Greetings E, as a first time poster Welcome to Breaktime.

This post, in response to your question, will bump the thread through the 'recent discussion' listing again.

Perhaps it will catch someones attention that can help you with advice.


As a side note since I see you are frequenting BT by your profile.

The next time you post here it might be advantageous to the fellow BT readers if when you are forming your post you would break the sentencing down into different paragraghs instead of one large unbroken block of sentences.

That makes for much easier reading.





'Nemo me impune lacesset'
No one will provoke me with impunity



'Nemo me impune lacesset'
No one will provoke me with impunity

(post #100355, reply #2 of 18)

The housewrap is for air infiltration, the siding should keep the water out, besides some condensation. Compare the r-values of replacing all the insulation board or just relaying a new layer.

(post #100355, reply #4 of 18)

If you want to increase Rvalues and have troublefree exterior in mind I would go down to the exterior sheathing (1/2" ply?) strap material horizontal the size of the intended insulation thickness (1 1/2" rigid), install rigid and cover with 2 layers of 60 min tarpaper, install 1x4 treated vertical strapping and then the Hardi.(horizontal application)

Advantage: more insulation, airbarrier behind siding, here they call this a rainscreen after what they learned from the sick building syndrome

Of course your costs will go up.

PS: happy to see you get rid of that vinylsiding. I also hate to wear a plastic shirt

Edited 2/11/2006 4:32 pm ET by semar

(post #100355, reply #3 of 18)

You didn't indicate where you live, but you could use a layer of tyveck paper( to allow moisture thru from the inside to the outside) and then cover this with lapped 15 # asphalt felt to create a seperate drainade plane for any water comming from the rain getting thru the cement siding ( 1 inch rigid insulation could be applied beneath the siding and over the felt.)

All I ever Needed to Know I learned in Kindergarten- Robt. Fulghum

All I ever Needed to Know I learned in Kindergarten- Robt. Fulghum

(post #100355, reply #5 of 18)

Warren, Thanks for the ideas.  I live in Denver, Colorado.  I've read a few threads on the general subject and I picked up several comments about rigid insulation panels and where they should be installed on the wall. 

These other threads recommended foam panels on the outside of the wall in warm climates (air conditioning) and inside the wall, behind the drywall, in colder climates. 

Since I live in a colder climate, and have no intention of tearing out the drywall, would the wall assembly you're suggesting be okay?  It sounds like the Tyvek in your suggested placement will let the water vapor out behind the foam board.  Any chance it would be trapped and do damage?  Would it just evaporate from between the Tyvek and the tar paper?  Thanks

(post #100355, reply #6 of 18)

I have used this in Columbus Ohio area under EFIS ( Exterior Insulating Finish System) ffor a number of years with no failures. The Tyveck lets the vapor through to the tarpaper back and then the water runs down the back of the tar paper onto flashing and out to the outside. The foam helps prevent water intrusion and any water that gets past the foam is conducted down the front of the tar paper to the same flaching and to the outside.

I presently reside in FL and have taken up building here where vapor points are much different than I was used to up North.

For a good advice source check out on the web, and see how they discuss these issues.





All I ever Needed to Know I learned in Kindergarten- Robt. Fulghum
All I ever Needed to Know I learned in Kindergarten- Robt. Fulghum

(post #100355, reply #7 of 18)

I'm in Montana. When i re-sided my old house with Hardie, i felted the sheathing, then layered foam over it held on with 1X2 furring strips every 16", screwed into the sheathing, and 1x6 plumbed corners. Your furring strips would have to hit the studs under the old insulation board to hold properly.

The furring strips allowed me to string the wall and shim it flat before siding since the cement plank tends to show any wobbles in the wall pretty readily. This gave me an unbroken thermal barrier, a rainscreen, and a secure nailing surface, though i have to admit i screwed the Hardie onto the furring strips, too. I was warned that using a nail gun on the plank over an inch of foam could cause the plank to deflect, but i didn't try it to test the theory. In any case, i was entirely pleased with my system, and used the exposure of the sheathing to fill the cavities with cellulose while i was at it.

(post #100355, reply #8 of 18)

I'm wondering what you might think of something slightly different than your assembly.  Instead of the 1 x 2 furring strips how about another layer of OSB on top of the foam? 


Would the OSB nail nice and flat onto the foam, or would it too, deflect somewhat?  The wall studs are 16" on center.  Does the air space created by the furring strips have some other advantage (other than providing a nice flat siding surface)?



(post #100355, reply #9 of 18)

I don't see any advantage to sheet stock over the foam. It's far more expensive than furring strips, harder to handle, provides no rain screen effect ("air space"), and adds little to structural rigidity if separated that far from the studs. OSB would tend to ameliorate any wall problems, but i could shim my individual furring strips to dead flat.

The idea of the rain screen or air space is that any wind-driven water that gets behind the siding has a way to exit easily. Applying siding directly over OSB means you rely solely on the felt over OSB to protect it - something i wouldn't do.

(post #100355, reply #10 of 18)

Thanks for the response.  Those were all very good points and I'll use your original idea.

(post #100355, reply #11 of 18)

Are the furing strips pressure treated? Everything I have read about rainscreens suggest they should be.

I am planning on doing basically what you did this summer. Thanks for the details on what you did.


(post #100355, reply #12 of 18)

Plain white wood. The climate here is very dry most of the year - technically desert - so they wouldn't stay damp for very long. I suppose i would consider treated plywood strips if i were worrried about it.

(post #100355, reply #13 of 18)

Thanks for the response. I am in Oregon, where wet siding is a regular occurance. I was thinking about ripping 3/4 marine plywood, or possibly priming the furring wood.

If you use shiplap-edged (post #100355, reply #18 of 18)

If you use shiplap-edged rigid insulation, you can simply tape the seams and foam or tape gaps around openings. In one unit you have air barrier, vapour barrier and insulation. No building paper required, and the Hardie can be applied directly over top of the rigid. I'm in interior BC which is also semi-arid, rain screen not required (low precip, low humidity). I've seen some very poorly executed paper/tape/flashing work under 20+ year old vinyl siding after removal, and yet very rarely have I found evidence of water intrusion into framing. You live in a forgiving climate.

Buccaneer Contracting

Penticton, BC

(post #100355, reply #14 of 18)

Just finished my house with the same . Remove existing vinyl & foam fanfold. Applied 15 lb, Felt paper over the Homasoate sheathing. Installed 3/4" Dow- Tuff board. Has a R-7 rating, Then Hardie plank w/ Azak trim. Worked great & an easy way to get some extra insulation. We have used this same method on several jobs without problems. Be sure to install felt or a house wrap, & make sure your flashings are installed properly. Good Luck!

(post #100355, reply #15 of 18)

Did you apply the hardie-plank directly to the Tuff Board? Or did you use furring strips?

(post #100355, reply #16 of 18)

Insulation applied to sheathing, hardiboard on top, nailed to studs.

Hardie Plank over drainage mat and insul board (post #100355, reply #17 of 18)

Is anyone familiar with installing Hardiplank siding over a 1" rigid insulating board AND a drainage and ventilation mat such as Driwall Rainscreen 020-1?  After nailing the mat over the insulating board to the studs, can you nail the siding over the two layers to the studs directly with longer nails?  I cannot find any reference in the Hardie literature or Keenebuildingproduct literature that describes this.