ICF Window Preparation
This method uses a full-length cedar backing piece and several 3″-wide beveled cedar strips (installed perpendicular to the backing) to form a sloped seat for the window. The cedar sits on top of the waterproofed sill area (Blue Seal membrane in your photo), with beads of acoustical sealant for redundancy. Finish with a peel-and-stick membrane over the cedar and follow the window manufacturer’s instructions for window installation and support spacing. Be sure to check out our list of services that we offer for your upcoming project.
Tools & Materials
Materials Beveled cedar siding (main backing piece + at least three 3″ wide strips) — from Windsor Plywood (Edmonton) Blue Seal liquid-applied waterproofing (or equivalent) — already applied under the cedar in your photo Acoustical (butyl or polyurethane) sealant — for bedding the cedar Peel-and-stick membrane (sill flashing) — to protect cedar top surface Corrosion-resistant fasteners: 1½″ corrosion-resistant roofing nails or 1½″ hot-dip galvanized or stainless ring-shank nails for the first piece 1″–1¼″ galvanized or stainless finish nails for the 3″ leveling strips (or 1¼″ galvanized brad nails) Optional: construction adhesive compatible with cedar (exterior grade) Optional: metal drip edge or sill flashing Masking tape, rags, isopropyl alcohol (for clean up) Timber buck or wood backing (if required by window manufacturer) Tools Tape measure, pencil / marking knife Circular saw (with finish blade) or compound miter saw Utility knife Cordless drill and appropriate bits (for pilot holes) Hammer or pneumatic nailer (suitable for finish/roofing nails) Caulking gun Straight edge / level / chalk line Safety gear: gloves, eye protection, dust mask
Preparation & layout
1. Verify window rough opening and bucks.Ensure the window buck (wood/foam nailing block) is installed, plumb, square and secure as per the window manufacturer. The cedar sill sits on top of the ICF buck/threshold area—do not rely on the cedar alone to carry structural loads unless the manufacturer approves. 2. Measure and cut the sloped cedar sill piece.Measure the full width of the ICF opening. Cut one full-length sloped cedar piece to fit the bottom threshold. Dry-fit it into place. 3. Cut 3″ beveled cedar strips (minimum 3 pieces).Cut at least three 3″-wide shim strips of beveled cedar siding. These will be placed on top of the sloped cedar piece to form a level threshold for the window to sit on. One at each end plus one in the center is typical; add more shim strips if the window is wide or heavy—follow the window manufacturer’s support spacing. 4. Mark the window seating line.Use a chalk line or pencil to mark exactly where the bottom of the window frame will bear on the sill. Transfer that line all around the opening so window placement is consistent. A line laser makes quick work of this step.
Step-by-step installation
1. Confirm substrate & membrane condition.Make sure the Blue Seal (or other liquid membrane) is applied and in good condition. Follow the manufacturer’s cure/flash time—don’t place cedar over uncured membrane if the membrane manufacturer advises against it. 2. Apply two beads of acoustical sealant.On top of the cured Blue Seal (where the sloped cedar will sit), apply two continuous beads of acoustical sealant parallel to the window seating line. The sealant provides a air tight bedding and helps hold the backing piece. 3. Place and fasten the full-length sloped cedar sill piece. Seat the sloped cedar sill into the beads of sealant and confirm alignment with your seating line. Use 1½″ corrosion-resistant roofing nails or ring-shank nails to fasten the backing into the underlying buck. Space fasteners roughly every 8–12 inches along the backing—adjust spacing per local code and window manufacturer. Tip: Pre-drill pilot holes or countersink to reduce split risk in cedar. 4. Dry-fit the beveled 3″ shim strips and confirm level.Place the bevel shim strips on top of the sloped cedar sill, in the opposite direction of the full length sill. Confirm the level with a level. 5. Fasten leveling shim strip pieces. Position two shim strips under the window ends location and one or more across the span as required. Secure each strip with 1″–1¼″ corrosion-resistant finish nails (or brads if recommended). Pre-drill small pilot holes if the cedar is prone to splitting. If you want extra adhesion, apply small dabs of exterior construction adhesive (compatible with cedar) between backing and strips before nailing. 6. Protect cedar with peel-and-stick membrane.Install a piece of peel-and-stick waterproof membrane over the top of the beveled cedar sill. Start the membrane behind the inner face (lapping onto the face of the ICF or buck) and extend it over the cedar, creating a continuous, shingled lap with the wall WRB / flashing system. Smooth out wrinkles and ensure full adhesion. 7. Install outer drip edge or metal cap (optional but recommended).For added durability, fit a small metal drip edge or cap at the outer nose of the cedar sill. This reduces direct water contact on the cedar end grain and prevents staining. 8. Final window install & seal.Follow the window manufacturer’s fastening and sealing instructions: set the window on the prepared sill, shim as required (using non-compressible shims under jambs), fasten per spec, and apply exterior sealant at the perimeter where the window frame meets the sill. Integrate window flashing tape and head flashing to the peel-and-stick membrane and WRB system.
Common mistakes to avoid
Relying on cedar alone to support window weight. Always follow window manufacturer support spacing and use structural bucks where required. Insufficient slope. Too shallow a slope allows pooling; too steep can affect window seating. Target 1/8″–1/4″ per foot. Using non-corrosion-resistant fasteners. Cedar and exterior exposure need stainless or hot-dip galvanized hardware to prevent staining and corrosion. Skipping peel-and-stick membrane. Cedar is somewhat water-resistant, but the membrane protects the sill and the structure beneath. Not integrating flashing correctly. Flashing must be continuous and lapped to shed water—check window and membrane manufacturer details.
Tips, checks & troubleshooting
Pilot holes: Always pilot-drill for long screws or larger nails in cedar to avoid splitting. Support spacing: If you’re unsure how many 3″ strips are needed, consult the window manufacturer’s instructions for bottom support spacing. For wide units, add additional strips. Weather & curing: Do not install peel-and-stick in temperatures below the product’s recommended range; adhesives may not stick properly. Maintenance: Inspect the sill and membrane annually; re-apply sealant at the window perimeter if cracking appears. Re-stain cedar every few years depending on exposure. If in doubt, follow the window manufacturer: They may require a sill pan or specific support/blocking technique—follow those specs first.
Estimated time
A single standard opening prep (measure, cut, fit strips, fasten, membrane) typically takes 1–3 hours depending on access, window size, cure times for sealants/membranes, and whether any additional flashing or detailing is required.
Questions or want help on-site?
If you’d like a review of project photos, advice for a specific window unit, or a quote for installation, get in touch: YEG ICF📧 yegicf@gmail.com📞 587-778-5308🌐 https://www.yegicf.com