Structural Insulated Panel Home - US FPL 1935 Stress-Skin Structural Insulated Panel Repairs

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How to repair rotted or damaged stress-skin insulated panels:

This article offers detailed suggestions for diagnosing and repairing rot, leaks, delamination, or other damage to stress skin insulated panels (SIPs) used in timberframe and other building construction.

This article series discusses the use of stress skin insulated structural panels for timber frame and other construction. Illustration at page top U.S. Forest Products Laboratory Structural Insulated Panel based prefab houses developed in 1937, built for study and evaluation.

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Stress Skin Insulated Panel Repairs

Reader Question: how best to repair damaged stress skin panels

Own a 22 year old home with the stress panels that have T1-11. Our home is three stories and the horizontal decorative board to cover the panel joints was not caulked properly by the painters. Naturally water got behind the board and released water down the T1-11 an we have rot.

Only on one end of the home that gets the brunt of the elements, especially winter in Western NC. How do we fix this rot. Do we cut out the T1-11 if it isn't a large area, do we just cover with all new T1-11 and how to do that.

Winter is fast approaching and we just discovered the damage. It is a post and beam type construction with the SIP panels with T1-11 as the outside covering.

We need help and look forward to hearing from anyone that has had this problem. Is rot common with T1-11 as it ages? The rest of the house is fine. - Richard 9/22/12


Richard, I've asked our expert Steve Bliss for comment on the subject of damaged SIPs.

From my own experience, which is more with T-111 type plywood exterior siding than its use on SIPs, if the siding is not kept weatherproofed by a coating (I prefer pigmented stains to paints) and if trim is not properly sealed to keep out water, we can expect to find spots of delamination of the plywood as well as rot behind and around poorly-sealed horizontal trim. Delamination and buckling of the outer skin of the plywood siding seems to occur first where there was already a small flaw, crack, or opening in the outermost ply. Those problems are not unique to plywood siding however.

Steps in exploring for hidden damage in an SIP building

Without having seen the extent of damage on your building, a detailed repair prescription amounts to pretty much wild arm-waving. I think the first order of business is a thorough damage assessment. While there is less of a water path and space within a SIP than a cavity type wood framed wall, water behind the outer ply of T111 invites rot and also insect activity. I'd want to

Repair Ideas for limited-damage SIP stress skin panels on a building wall

If you find very extensive water, insect damage, etc. in the wall you may have to remove and rebuild it. I'm guessing you are going to find this extreme approach is not necessary.

If you do not find evidence of any significant water entry or deeper damage to the structure then you have two plausible approaches, depending on the degree of adhesion of the remaining plywood outer skin to the foam core:

  1. If it is relatively easy to separate the plywood skin from the foam core of the SIP then with a circular saw set to the depth of just the plywood outer skin,
    • Cut a horizontal line across the damaged siding at a point above the highest water or rot damage to the wall, and cut a second line lower on the wall, below the extent of damaged plywood there.
    • Peel away the damaged plywood;
    • insert Z-flashing appropriately positioned up under the bottom lip of the upper remaining plywood as well as over the upper lip of the lower remaining plywood and then piece in replacement plywood in that horizontal band you've cut and peeled away. If you match the color of z-flashing to the stain of the siding it won't be too ugly and this approach is enormously less troublesome than tearing off and rebuilding the wall.
    • Before replacing the horizontal trim, be sure the new trim boards are back primed, sealed on all sides, and sealed well especially along the top edge.

      Note: I've found that it is difficult to seal horizontal trim along grooved exterior siding like T-111 unless you work meticulously to seal every single vertical groove along the top of the tim board; Even the smallest opening in the sealant (caulk) eventually leads to rot or insect damage.
  2. If you find that you cannot remove enough damaged plywood skin from the foam inside the panel, or if the foam itself has become damaged, then set your circular saw deep enough to cut through the panel to just inside the interior skin (building interior side) and
    • remove a horizontal band wide enough to excise all of the damaged material;
    • replace the removed band with solid foam or foamed-in-place foam and a new plywood skin and
    • continue to flash, seal, and trim as I outlined above.

Send us some photos of the conditions you currently face as well as what you find, and we may be able to offer more detailed suggestions.

Reader Question: how do I remove and patch damaged OSB on a stress skin panel?

2015/11/05 Charlie said:

How do you attach the new osb if the repair is not on part of the timber frame? It seems like all the Windows and out side corners of my repair project need attantion. How do you check the extent of ant infiltration without tearing the whole thing down? No joke this is a problem. 

Reply: basic steps in repairing a damaged stress-skin panel


OPINION: Certainly I don't advocate unnecessary demolition - or as I put it to my clients: "I'm not about to run amok with an axe and chain saw" - but generally we pull trim and probe until we've found the edge of damage.

If an area of OSB that is rotted or insect damaged does not abut intact structural framing members - studs, rafters, or in your case the timber frame - I'd do the following:

  1. Decide if we're going to repair from outside (usually the case) or inside - as that determines what we have to remove.
  2. Avoid cuts into wiring or plumbing: Iinspect and if necessary remove interior wall coverings to assure that we are not at risk of cutting into plumbing or wiring
  3. Cut through the OSB or plywood to the foam if you are working with a laminated, insulated stress skin panel (plywood or OSB glued to foam insulation): you should be able to cut and pick out the damaged OSB or plywood,
  4. Repair damaged foam insulation: re-fill any damaged solid foam insulation in the panel up to flush with the insulation's original mating surface, then glue-in a replacement panel, skipping the backer strips or section I discussed above. You may find it easy going to spray with an open-celled foam that can be cut with a carpenter's saw; small areas of lost foam insulation less than an inch deep you might choose to ignore.
  5. For ordinary repairing uninsulated structural panels, cut a rectangular opening that removes all of the damaged OSB (or plywood as that'd be the same), leaving edges of sheathing in good condition at all sides of the opening

    For ordinary sheathed panels or uninsulated structural panels, cut to fit 1x lumber supports (1x4 may be enough) or if necessary for structural support use 2x lumber (2x4s) to length to fit the four sides of the cut-out rectangle - each piece will need to be long enough so that when you reach through the cut-out opening you can place the supports so that half of the support edge is behind the remaining OSB/plywood wall or roof sheathing and the other half of its width is exposed in the opening. This exposed half serves as a support for step 5.

    For ordinary sheathed panels or uninsulated structural panels, against the supports added at the opening in step 4, cut out and mount a replacement rectangle of OSB or plywood that fits snugly in the opening.

    Alternatively for small areas of damage - a square foot or two - I might simply cut two pieces of OSB or plywood, an inside support that is 3-4" larger in all dimensions than the opening, and an outside repair piece. Drill two 1" holes near the center of the inside or backer support so that you can use those to hold the support in position, then insert the backer through the opening, hld it in place, and use construction screws through the external sheathing into the backer to fasten it securely; to be meticulous you could also have run a bead of construction adhesive around the mating face of the backer support so that it is also glued to the interior face of the sheathing.

    With the backer strips or solid segment now in place fit and screw in place the repair OSB/Plywood that fits snugly in the opening.
  6. Repair the building wrap or roofing felt: when replacing housewrap on walls or roofing felt on roofs, be sure to insert the upper edge of the inserted repair piece under the lower edge of housewrap or felt that you cut above the repair opening.
  7. Replace siding, trim, etc. if those were removed, for example working on a wall or near the lower edge of a roof or at a gable end.

Question: Best repair for rotted T-111 on Stress Skin Panel Walls on a Timberframe House in Ohio

Lee Test said:

I have a 2 story timberframe house with 6 inch foam SIP panels. The panels have problems similar to another consumer has that I read about at this site. The panels have T1-11 bonded to the foam and primarily my west wall is really in need of being fixed and ultimately covered over. To date smal spots I've removed the rotted material and have used Minwax exterior stainable wood filler that has seemed to work fairly well. I end up sanding the area, prime with oil based primer and then stain with Sherwin Williams solid stain. A few large areas, 1 ft sq to 3 ft sq I have cut out the areas and inserted pieces of T1-11 with drip edging and some caulking.

Now a I am finding more spots. I'm 69 years old and can't keep pasting this west wall much longer. Would it be appropriate to fix the new rotted spots as I've done in the past and then either apply cedar lap siding to the west wall area or maybe do the whole house in vinyl siding. I'm worried about underlying moisture causing green mold coming out on the outside of the vinyl.
I do have a few decorative areas above and below windows that are about 4 ft wide x 4 ft high that have cedar lapped siding that do not have rotting problems.

These areas have T1-11 as the underlying surface, then tyvek, then the cedar. The cedar has been prime coated with oil based primer than covered with 2 coats of solid stain. With cedar I question the ability of the SIP panels to carry the weight. Your thoughts? - Lee Test Bellbrook, Ohio 937-528-9251

Reply: Suggestions from InspectApedia & Steven Bliss

Lee, as you'll see in the other stress skin SIP repair discussions above, we have not found any easy path to repair rotted SIP panels;

The patch approach you describe using a filler may not have a long life, especially if you live in a freezing climate; The cut and patch approach is feasible when the damage area is limited; I'd use "ZEE" or F-flashing rather than drip-edge along the upper edges of pieced-in T-111 repair sections and I'd take care to seal, or preferably back-flash and seal the butting vertical edges as well.

Applying a new layer of siding is very tempting as it leaves the foam panels in place; you'd need to install housewrap, carefully placed to drain to the wall bottom, with flashing tape around windows and doors before installing any new layer of siding;

A timber-frame home adds the complication for a side-over job that there are wider un-supported expanses than 16" or 24" stud-spaced homes.

If the contractor wants to add furring strips to support siding between the timber-frame posts I’d worry that the installer is headed for a cluge, or as we say here in Mexico, a "Lata" - a can of worms. The furring would have to be heavy enough to span the spaces between your timber-frame posts, and horizontal furring is generally a terrible way to mount any siding system as it tends to trap water.

Unless she or he can run adequately strong furring from wall top to wall bottom to carry the siding; I'm afraid the sider will want to add horizontal strapping between posts to carry the vertical siding on spacing suitable for adding siding.

Two layers of 2x furring + a half inch or more of siding thickness approaches a 4-inch wall build-out. In turn that means that we have to build out all of the wall trim in order to avoid the "sunken eyes" look of homes with recessed trim. (Recessed trim also becomes a leak point). This seems a terrible approach.

In that case, for a wall over which there’s not enough nail base in the old SIP to carry new siding (over house wrap and flashing tape as we agree), you might either

As our original article on stress skin structural insulated panels for timber frame homes was writen by my friend Steve Bliss, over at our sister site “Building Advisor” (see link at page top), I’ll pass a note on to Steve to see if he has other suggestions.

As with lots of new-fangled building ideas that seem terrific at the time of invention, the passage of time shows what is durable and what’s not.

Finally, the “green mold” problem you describe, if it’s mold, is a cosmetic concern and an indicator of dampness or shade regardless of the mold color. The green goop you most often see on vinyl siding is not mold but algae. Details are at

Stress Skin Panel Damage Diagnosis & Repair Suggestions from Steve Bliss

Steve Bliss - has some additional comments that we'll post here:

I have more questions than answers and am always reluctant to give specific advice without understanding the problem. A few issues that occur to me:

Bottom line: If the T-111 is intact enough to hold nails and the lamination to the foam is good, the panels should make a fine nail-base for siding. If the T-111 is decayed and delaminating from the foam, then I am skeptical about trying to attach siding to the underlying timberframe.

Excerpt from my article on Sheathing Wrap: -

Draining Housewraps. With growing interest in rain screens, manufacturers have responded to the need for an air space and drainage plane with a variety of plastic housewrap products that are either wrinkled or corrugated to provide an integrated air space. These include products intended primarily for stucco, such as DuPont StuccoWrap, and others developed for use under siding. These include Pactiv Raindrop 3D, Barricade Drainage Wrap and Barricade WeatherTrek, Benjamin Obdyke’s Hydrogap, and Valeron Vortec.

The small air space created by these products generally range from 1-2 mm (one mm = about 4/100 inch). Although these materials may allow for a capillary break and some drainage (assuming they are not crushed by the siding installation) it is unlikely that they will provide any measurable airflow to promote drying. Where better drainage and ventilation are needed, consider using a plastic drainage map, such as Benjamin Obdyke’s Home Slicker HP, which creates a 6-10 mm gap.

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