Honestly? Before the winter you are currently complaining about.
Since that one has already happened, the useful question is whether to book now or wait until spring. This year the answer is unusually clear, and it has less to do with weather than most people assume.
Why Most People Get It Backwards
The problem announces itself in January. That is when the upstairs will not hold heat, when the gas bill arrives looking wrong, and when the ice at your eaves makes it obvious something is escaping through the roof.
Then March arrives, the house stops being uncomfortable, and it drops down the list. By the time anyone remembers, it is September, and they have paid for a full winter of heat leaving through the ceiling.
That gap is the real cost of waiting. Not the price of the job, which barely moves, but the season you spend heating the outdoors while the decision sits unmade.
If you are reading this in autumn because last winter was miserable, you are already doing better than most people manage.
Autumn: the busiest season, and this year the deadline
September through November is when the phone rings, for obvious reasons. Everyone notices their attic in the first cold week.
That means lead times stretch. A job that takes a day to do may take three weeks to schedule, and by mid-November most contractors in the GTA are booking into the new year.
This year there is a harder reason to move. Ontario's Home Renovation Savings programme, which pays 50% of the attic insulation cost up to $1,250, ends on 30 November 2026. Enbridge can also close it earlier if the funding runs out, which has happened to predecessor programmes.
A quote in late November is a quote for work that probably lands outside the window. If the rebate matters to you, and on a job in the low thousands it is worth about half the cost, the practical deadline is earlier than the official one. Detail on the deadline and the tiers.
Worth separating two things people collapse into one. The assessment and the installation have different lead times. Getting someone out to look is usually days. Getting the work scheduled is usually weeks, and it is the second one that stretches in autumn. If you are unsure whether you even need the job, book the assessment now regardless. It costs nothing, it is not a commitment, and it puts you in the queue with a known scope rather than starting from scratch in November.
Winter: possible, and less mad than it sounds
Blown-in insulation goes in perfectly well in January. The material is not temperature sensitive, cellulose does not lose R-value in cold, and the work happens inside your attic rather than on the roof.
Two genuine drawbacks. Contractors are at their busiest right through the early part of it, so scheduling is the constraint rather than conditions. And you are already partway through the season you were trying to fix, so you capture a fraction of the saving you would have had in October.
One advantage nobody mentions: in winter the problem is visible. An assessment in January can pick up frost on the sheathing, warm spots melting the roof snow, and condensation that simply is not there in July. If you want the most accurate picture of what your attic is doing, cold weather is when it shows you.
Cost does not move much by season, whatever you might assume. What a job actually runs is driven by attic size and scope, not by the month.
Spring: the quietest window
March to May is the easiest time to get booked quickly. Demand drops the moment the heating goes off, and you are not competing with everyone who just had a bad January.
It is also the sensible time to deal with anything winter exposed. Condensation, frost damage and mould show up during cold weather and get discovered when someone finally goes up in March.
The obvious downside is that you have just finished paying for the winter you meant to fix, and the benefit sits several months away. Against that, you are ahead of the rush and ahead of next season.
Summer: available, but unpleasant
Availability is good. Conditions are not.
An unventilated attic on a July afternoon runs far hotter than the air outside, and working in one is genuinely taxing. Crews move slower, take more breaks, and there is a real limit to how long anyone should be up there. None of that stops the work, but it is worth knowing why summer is nobody's first choice.
Insulation does help in summer, incidentally. It slows heat moving in as well as out, and the upstairs bedroom that is unbearable in August is usually the same room that is freezing in February. The mechanism is identical and so is the fix.
It Depends What Is Going In
Here is the part that actually changes the answer, and almost no article mentions it.
Blown-in fibreglass and cellulose have no meaningful temperature constraint. They are dry materials blown into place. Cold does not affect how they install or how they perform.
Spray foam does. Standard formulations need the surface they are applied to be above roughly 15°C to expand and adhere properly. Below that the chemistry changes, adhesion suffers, and you can end up with foam that pulls away from the substrate. Winter blends exist, but only for closed cell, and they are a different product rather than the same one applied in the cold.
So if your attic is a straightforward blown-in top-up, the season barely matters. If the work itself involves spray foam anywhere, timing matters considerably more, and that is worth establishing at the assessment rather than at the install.
What Actually Decides Your Timing
Strip out the seasons and four things determine when you should book.
Whether there is a deadline. This year there is. That overrides everything else.
Whether anything has to happen first. Live knob and tube, vermiculite, a roof leak or a bathroom fan venting into the attic all need sorting before insulation goes anywhere near them. Each of those brings in another trade, and each adds weeks rather than days. This is the single most common reason a job that looked like a fortnight turns into two months, and it is exactly what an early assessment surfaces. Finding out in October that you need an electrician first is recoverable. Finding out in late November is not.
How bad it is now. An attic at R-10 is costing you meaningfully every month. An attic at R-40 is not, and waiting for a quieter season is perfectly reasonable. Settling whether you need it at all takes about two minutes from the hatch and tells you which of those you are.
Whether you are doing other work. If the roof is being replaced or the bathroom is being redone, the attic is easier and cheaper to reach while that is happening.