Building envelope estimating is mostly a coordination problem. The materials are not especially complicated: membranes, insulation, sealant and flashing. The trouble is that the envelope is built by many trades, and the small items at each junction do not clearly belong to any of them. Those orphan items are where bids lose money and where buildings leak. This guide shows how to estimate the envelope by control layer and how to find the gaps before you price them.
If you need the thermal and moisture scope extracted from drawings, our thermal and moisture protection estimation service covers insulation, waterproofing, sealants and related items.
The envelope is four control layers
Think of the envelope as four jobs the assembly has to do: control liquid water, control air movement, control vapor, and control heat. Some products do more than one job. A self-adhered membrane, for example, can be both the air barrier and the water-resistive barrier. Others do only one. The first step in a takeoff is to identify which material does each job in each assembly, and then who installs it.
- Water control: roofing, flashing, water-resistive barriers, sealants and drainage planes.
- Air control: air barrier membranes, sheathing joints, tapes and transition materials.
- Vapor control: vapor retarders, where the design calls for them.
- Thermal control: insulation, thermal breaks, and glazing with thermal performance requirements.
Energy codes generally require a continuous air barrier on commercial buildings, and some specifications require installers accredited under the quality assurance program of the Air Barrier Association of America. That affects who you can use, what you must submit and what testing to expect.
Where the scope gaps hide
Most air and water problems occur at transitions rather than in the field of a wall. Walk through each of these on the details:
- Window and door openings to the wall barrier.
- Wall to roof at parapets and eaves.
- Wall to foundation and slab edge.
- Penetrations for pipes, ducts, conduit, brick ties and anchors.
- Expansion joints, control joints and changes of material.
- Curtain wall and storefront perimeters, covered in our post on storefront vs curtain wall.
For each transition, write down who supplies the membrane, flashing, tape, primer and sealant, and who installs them. If the answer is nobody, you have found a gap. If the answer is two trades, you may have found a double count.
How to take off each layer
Air and water-resistive barriers
Measure wall area in square feet from the elevations, deduct large openings only if your convention does so, and add allowances for laps, overlaps and waste. Count transition and flashing membranes separately in linear feet, since they cost more per foot than field membrane, and count penetrations by number, because each one needs its own detail work.
Insulation
Measure area by thickness and type: rigid board, mineral wool, batts or spray foam. Spray foam is usually priced per board-foot, which is one square foot at one inch thick. Add fasteners, clips and furring for continuous insulation, and thermal breaks where the detail shows them.
Sealants and backer rod
Measure joint length in linear feet by joint size. Volume decides the quantity of sealant, so size matters as much as length. A joint 1/2 inch wide and 1/2 inch deep holds 0.25 cubic inches per linear inch, or 3 cubic inches per foot. A US gallon is 231 cubic inches, so a gallon fills about 77 feet of that joint, and a 20-fluid-ounce sausage fills about 12 feet. For 1,800 feet of joint:
- 1,800 ft × 3 in³ = 5,400 in³.
- 5,400 ÷ 36.1 in³ per sausage = about 150 sausages.
- With 10 percent waste, about 165 sausages.
Joint depth and width vary, so use the sizes on the details. Add backer rod and primer where the sealant manufacturer requires them.
Flashing and accessories
Count flashing by linear foot and type: sill pans, head flashing, through-wall flashing, counterflashing, drip edges and terminations. The masonry and roofing scopes often include flashing too, so check the overlap against our masonry estimation and roofing estimation pages.
Submittals, mock-ups and testing
Many envelope specifications require product data, shop drawings of details, a mock-up of a typical wall with window and flashing, field adhesion tests, and air or water testing of the assembly. These take time and have to be sequenced, so they affect both cost and schedule. If the spec requires them, price them. If it does not, state that as an exclusion.
A scope-gap checklist for bid day
- Which trade owns the air barrier at window and door perimeters?
- Who provides and installs flashing at openings and wall bases?
- Are firestopping and smoke seals at the slab edge in the curtain wall scope or another?
- Who seals penetrations through the barrier: the barrier installer or the penetrating trade?
- Is continuous insulation attached by the barrier installer, the framer or the cladding trade?
- Do sealants at control joints and the facade belong to the envelope, masonry, glazing or painting scope?
- Are mock-ups, testing and accreditation included?
Write the answers into your bid assumptions. Where you cannot get an answer, price the item and state who is assumed to install it, or list it as an exclusion. The same habit helps with contingency and risk allowances.
Common envelope estimating mistakes
- Pricing only the field membrane and not the transitions, tapes and primers.
- Assuming that insulation in one trade’s scope also includes its fasteners and clips.
- Counting a window perimeter once when two trades each need sealant.
- Leaving out sealant backing and primers, or using a joint size that does not match the detail.
- Not checking whether the specification requires accredited installers, mock-ups or testing.
- Giving no allowance for protection and repair of the barrier after installation.
Frequently asked questions
What is included in the building envelope?
The envelope is everything that separates the conditioned inside from the outside: roof, walls, foundation, windows, doors, and the barriers, insulation and sealants that connect them.
What is the difference between an air barrier and a vapor retarder?
An air barrier stops air movement through the assembly. A vapor retarder limits water vapor diffusion. Some products do both, but the roles are different and are specified separately.
Where do envelope estimates usually go wrong?
At transitions and penetrations, where several trades meet and the materials needed are small and easy to overlook.
How do I price sealant joints?
Measure the linear feet of each joint size, convert to volume using the joint width and depth, divide by the volume of the product package, and add waste.
Get the envelope scope extracted and organized
We extract insulation, barriers, sealants, flashing and accessories from your drawings by assembly and transition. See our thermal and moisture protection estimation, quantity and material takeoffs, or cost estimation pages.