Do your windows have screens? It is my understanding that it is better to keep them open. How big is your house? Three vents in a larger attic would not seem like enough to me. We were told by our inspector to just put insulation down on the floor of the attic and keep the windows open year round. He said that even if we could close them up (we currently have no glass) that they should be kept open for proper air flow. We had mentioned that we wanted to finish the space at some point and that was when he suggested putting vents in the roof. So in short, I would keep them open. Maybe you should get a professional opinion?

Thanks for the post!
I did staple in screens to keep out any insects.
The inspector told me to do exactly what yours did - put in insulation on the floor and leave windows open. However, there was poor air flow this Summer which led me to put in the three vents in the roof. That helped a lot. Only question now is, with those vents, should I still leave the windows open.
E

The primary purpose of the ventilation openings is to allow moisture that enters the attic (from above or below) to escape to the outside. Your local building code will set a minimum clear opening for the floor area of the attic and the type of vents used (wall openings must be much larger than eave & soffit vents).

There is no reason to not have screens in a window but the opening should also be protected by louvers because otherwise rain might enter the attic defeating the purpose of the vents. Excessively humid air can also be a problem. The louver will have a clear opening ratio so you can size it large enough to meet the code clear opening requirement.

You should check from time to time for roof leaks, seal the ceiling of bathrooms with vapor retarding paint, and make sure no bathroom exhaust ducts terminate in the attic.

Thanks macv
Louvers are a good idea for the rain/snow
The original owners had the bathroom venting directly into the attic. The inspector noted that the saving grace was the lack of any insulation which likely kept the humidity down a bit.
We rectified the venting situation
Appreciate your time and I'll keep those windows open. Now to find some louvres...
E

Here is an article about louvers fabricated for an historic building.

A screen is needed to keep wasps,etc. out and usually a heavier "hardware cloth" is also used to keep squirrels and birds out. I am skeptical of the capability of a contractor who recommends leaving a window open in your attic.

Here is a link that might be useful: window louvers

I too have a garrison colonial. We only one vent on each gable end. The are louvered but we have had to repeatedly replace the screens because of birds and bats (and a giant whole house fan that leaves the upstairs of the house vulnerable to such creatures when we use it). We added more insulation and replaced the roof a few years ago. The roofer never recommended more vents.

Every summer I think/hope that would be the solution to keeping the upstairs cooler. My question is, did the three vents make such a difference that you'd recommend adding them?

The vents are intended to allow excess moisture to escape from the attic. If you want the house to be cooler you need to add more insulation or a radiant barrier.

Guess I have another question
Our attic has a louvered opening (approx 6" x 6") immediately above each window (which are approx 18" x 12". I felt that the air flow through those louvered openings would be minimal and the inspector had apparently agreed when recommending opening the windows year-round. Please correct me if such a small opening is typically adequate to ventilate the attic.
E

The usual building code requirement for an attic ventilation clear opening (that is not located at the eaves and ridge) is 1/150th of the attic floor area.

You don't say what the clear opening of the louvers is but if it is 70% and there are 2 vents they could serve an attic footprint of 52 s.f. That is not adequate to allow moisture to escape from your attic but it would be better than leaving the windows open and allowing rainwater and pests to enter the attic.

Find someone to look at your attic who understands moisture control in buildings and owns a copy of the building code.

A vented attic makes building science guru Joe Lstiburek's list of theTop ten dumb things to do in the South.

And the same applies to summer venting in those "cold" climes that also experience hot humid summers.

For vented attics, according to Lstiburek, the 1:300 ratio is the most common Code requirement.

Here is a link that might be useful: Builder's Guide to Cold Climates (PDF)

The 1/300 code rule is when there are soffit/cornice vents and some kind of vents in the upper part of the attic. The 1/150 code rule is for all other vents like window louvers.

If the roof vents are large enough you could add drip edge vents from AIR VENT Inc. however they don't work as well over gutters because of potential ice dams.

Here is a link that might be useful: drip edge vent

The code requirement in MA is found in section 5806 ROOF VENTILATION.

You can access it online at the link below. Just click on Chapter 58 and scroll to section 5806.

Here is a link that might be useful: MA 1 & 2 family building code

Correct for Massachusetts, where the OP is.

But the requirement differs in other jurisdictions. For example, OBC (Ontario Building Code):

9.19.1.2. Vent Requirements
(1) Except as provided in Sentence (2), the unobstructed vent area shall be not less than 1/300 of the insulated ceiling area.
(2) Where the roof slope is less than 1 in 6 or in roofs that are constructed with roof joists, the unobstructed vent area shall be not less than 1/150 of the insulated ceiling area.
(3) Required vents are permitted to be roof type, eave type, gable-end type or any combination of them, and shall be distributed,
(a) uniformly on opposite sides of the building,
(b) with not less than 25% of the required openings located at the top of the space, and
(c) with not less than 25% of the required openings located at the bottom of the space.
(4) Except where each roof joist space referred to in Sentence (2) is separately vented, roof joist spaces shall be interconnected by installing purlins not less than 38 mm by 38 mm on the top of the roof joists.
(5) Vents shall comply with CAN3-A93-M, "Natural Airflow Ventilators for Buildings".