I am replacing my conventionally-framed roof with timber frame trusses, covered by 6 1/2" urethane core SIP's. In order to achieve equivalent insulation value in the 2x4 framed walls, I have decided to apply rigid insulation. My questions are: Should I go with 1" or 2" thick sheets? Should they be applied to the inside or outside of the walls? Anything else I should be thinking about? I live in New Jersey, energy zone 5. Thanks for your help.

Jim,
Too many unstated items in your brief description. What kind of roofing material are you going to use ? Would recommend a standing seam metal lookalike (strips are 36" wide, not 12" eventhough they look like 12"). Lightest, cheapest and from the right manufacturer has 100 yr warranty on the metal, 20 yrs on the electrostatic paint (many colors available). More detail if you want.
Now the walls. How are they presently insulated ?
Have you installed air stops around the windows and doors ? Where is the existing moisture barrier ? With this info in hand you may be able to make a wiser choice on how to improve the wall R factor.

Deralte (aka Emmett)

Deralte
The roof is going to be a combination of synthetic slate (Lamarite) on the main roof and standing seam on the dormers, porch roof, and tower.
The walls are presently insulated in fiberglass with the paper backing toward the inside. However, since all of the windows and doors are being replaced, we will be stripping the sheet rock off of the exterior walls and re-insulating. We are also stripping the cedar shake siding off of the exterior, so will have access to both sides of the walls and can start from scratch on insulation material. The only thing we're stuck with is the 2x4 construction of the walls. Our objective is to provide a thermal break to come as close to the performance of a SIP as we can.
That brings up one more issue. Since the SIP's will provide the insulation for the outer shell, should we remove the existing insulation from the attic storage areas? By the way, this is a 1962 raised ranch that we are trying to make look like it was built in the 1920's. Thanks again.

J,
The roof sounds interesting. Are you going to use real standing seam (crimping every 12") or one of the more modern 36" wide strips which you attach with stainless screws where the humps overlap ? In any case recommend you dont forget the ice dams on the lower 3' of every roof section. Also use 3/8" closed cell foam with a mylar skin facing up under the standing seam to both insulate and reflect the sun heat back out. Don't forget to let the main roof breathe to get rid of moisture. A vented ridge cap works well with vented soffits. The insulation in the ceilings should stay in place to give you an enormous thermal envelope on the top.
What sheathing is underneath the exterior wall shakes when you remove them ?
Do you think it is necessary to remove the interior drywall to place the rigid panels on top of the existing studs ? If you plan to remove the existing paper backed fiber glass batts I would recommend you replace them with sprayed in cellulose. Much tighter seal and so much easier to get it airtight. Check the R values of the sprayed in stuff with your vendor and add about R3 for the new sheetrock. If you can get over R20 for the net of the 2x4 walls you will be doing well. All of which is for naught if you don't use double pane windows and spray around those frames with low expansion foam.
Will chat about the exterior sheathing treatment in the next chapter.
Deralte

We plan to install Englert's standing seam metal roof which comes in 16" width with a 1" high seam. It is a snap lock design for easier installation. We will probably apply ice shield over the entire roof surface of the dormers, as they are not that large, and plan to apply it, as you recommend, to the lower 36" of the main roof.
The sheathing is 1/2" plywood. The issue of removing the sheet rock stems from the fact that the new windows are much larger than the old ones and will take up much of the wall space. There won't be much old stuff left on the wall, anyway. That is why we thought we should replace the old insulation while we were in there, but I also wanted to include a thermal break. I like your idea of cellulose, but that gets back to the question of rigid insulation. If the rigid insulation acts as a moisture barrier, should it be applied to the inside, perhaps as the cellulose is being blown in? I know applying it to the outside would be easier, but it seems like we would be trapping moisture.
The windows will be high performance Marvins with simulated divided light.

J,
Aha, new BIGGER windows. Translates to reworking the existing 2x4 framing a lot with appropriate headers, knee walls below the windows etc. So, indeed, the interior sheetrock has to go bye bye.
If you are gonna rework the walls that much for the new windows why not hold the top plate of each wall section in place with a dead man and replace the studs with 2 x 6's. What does your local code require for fire stops in the walls ?
Makes insulating with blown in the best choice and forget your rigid foam panels.
Wrap the house in Tyvek on top of the sheathing before appling the new siding material. Be sure to tape all the seams.
Waiting your reaction.
deralte

Since I have already been asked why we didn't just tear the house down and start over, I think we have to draw the line at putting all new studs in. My first preference for walls would be SIP's. Second would be 2x6's. The fact that we are stuck with the 2x4's prompted my original question. If we can't have the ideal, can we make the best of what we have?
I'm not sure what fire stops are required, but I will check with the architect to make sure we comply.
Thanks again for all the input. I think I speak for a lot of people when I say we appreciate all the time that people like you and Jim Rogers devote to helping with our various challenges.

Jim

I’ve been meaning to reply to your query for days now and just haven’t had the time, but thought I’d make it in that I have some thoughts and techniques which fit your project.

I used to build super-insulated stick framed homes back in the 80’s and most of it comes from that experience, but have also adapted some of these techniques to do built up enclosures for timber frames.

After you lay bare the walls, strip the kraft paper off the glass, fill any voids. If you are rewireing ( more on an additional reason why you might want to later ) and are making a mess of the glass you might consider replacing it with the extra dense 3 ½ ” batts R-13 sometimes labeled as “full wall” Buy this from an insulation installation company, they can install for less than you can buy it from your local supply. Do not expect good work though, the installers are usually paid piece work, have them stuff the full and easy to cut bays and do the fussier things yourself and you all make out.

After that apply 2 X 4’s on the flat horizontally 24” oc, between these lay in EPS panels 20 ½ “ X 1 ½ “ The beadboard stuff – Buy this from a foam plastics maunufacturer not an insulation supply, it will be cheaper and they can custom cut it to 20 ½ “ and save you the trouble of ripping down 24’s and do not be tempted by the additional R’s in extruded Dow board, the difference in price is vast and so is the payback.

Over that layer apply ½ “ rigid foil faced polyiso, shiny side in, logos out, foil tape the joints, this is your vapor barrier. Do not be tempted to increase the thickness of the this layer beyond 1” There is a “creep” issue with cantilevered fasteners and you just don’t need to this system works by eliminating thermal bridges with multiple thermal breaks, And also eliminating the convection current common to stud bays and the resultant heat they lose.

Now apply 2 X 3’s vertically, also on the flat – 16” OC using quality screws.Don’t forget to add a block betwixt these at the bottom for nailbase for you mopboards Run your wiring here in this reflectant dead air space, with no penetration through your rigid vapor barrier, this requires 1 ½ “ deep junction boxes with reducer plates.

Drywall over these.

Done right, adding some gaskets and beds of caulking at the subfloor and your air exchanges will be down in the SIP range. A range where you need add a heat exchanger to your mechanicals to remove pollutants and moisture.

Out of interest what are you adding to the top plates to distribute the point loads imparted by those timber trusses ?

Keep looking for answers and, Be your best

Will,
Sorry for the delayed response. I wanted to check with the architect and engineer before answering your question about point loads. In all cases the point loads will be brought down to the foundation via interior posts (ganged 2x members). This is a method used successfully in the past by the timber frame contractor, Mitch Rowland (Architectural Timberworks), as shown in an article he wrote a couple of years ago for Fine Homebuilding.
Thanks again to you and Emmett for all your advice. I'll try to remember to post pictures of the final product.