How we work out costs
Ranges, not prices
A single number looks precise but is usually wrong for your job. We show a low–high range from published sources and round results so they do not imply accuracy the inputs do not have.
Where numbers come from
We use government agencies, building-science references, industry research and published cost guides. Each is listed on the sources page with its type, and the date we last checked it. Cost guides built from contractor or customer data are labelled as such, because their methods are often not published.
When sources disagree
We do not average them into a false middle. We show the typical range from one source and the wider spread across sources, and say they differ.
Location
Costs vary a lot by place. The site is built to apply state and local adjustments, but only where a sourced, transparent dataset exists. Right now it does not, so national ranges are shown and the state spread is mentioned instead of guessed.
Value and ROI
“Resale value recouped” figures come from the Zonda Cost vs. Value report. They are national averages estimated from surveys of real-estate professionals and cost modelling, not measured sale prices. Dollar figures in that report are for particular specifications, so use its percentages as a guide to proportion, not as a promise.
Where each number comes from
We try to put the evidence next to the number, not only on a sources page. Cost tables show the source, its geographic scope and its date beside each row. In the calculators, every line of the result carries one of these labels:
- Sourced: a figure taken from a published source, with its publisher and date shown.
- Calculated: worked out by the tool from sourced figures and your inputs, with the arithmetic shown.
- Rule of thumb: a widely repeated guide with no published study that we could find.
- Your input: a number you entered, which the tool cannot check.
Each result also lists its assumptions, where it applies, what it does not include, and what could change it. An estimate is not a contractor quote: a quote is a price for your job after someone has inspected it.
How to read a cost table
Each cost row shows the item, a low–high range, the unit it applies to, the scope and date of the source, and a short note on what is covered. Under each table we list what usually pushes the price higher or lower and what is not included. A range is a guide to scale, not a quote.
What “value” and “ROI” mean here
Resale price is only one part of return. Where relevant we also describe energy savings, damage avoided, durability, risk reduction, payback time and comfort. Each is labelled with how much evidence sits behind it:
- Sourced estimate: a published figure exists, with its source.
- Qualitative: a real effect, but we have no reliable number.
- No reliable data: we looked and did not find a trustworthy figure, so we do not invent one.
- Depends on you: the answer turns on your bills, your house or your plans.
Rules of thumb
Some home tests and decision rules are widely repeated but not backed by a published study. When we include one, we label it as a rule of thumb.
What we do not do
We do not give quotes, diagnose your particular property, or give legal, financial or engineering advice.
Corrections
Pages show a last-reviewed date. If you find a mistake, please tell us (see About).