I have an older house built in 1965. It has three gable vents and no soffit vents. Would you recommend that I put in soffit vents? If so, how far apart from each other should they be? Or are the three gable vents efficient enough?
Homes built in the 1960s are notorious for having inadequate ventilation. In my 20 years as a professional home inspector, I frequently found evidence of delamination and mold in the plywood sheeting of those roofs.
You're smart to add additional ventilation. Here's the best way to do it. The most effective attic ventilation is continuous ridge and soffit ventilation. I would replace all solid soffits on the house with ventilated soffit material.
But that's only half the improvement. The other part is to add a continuous ridge vent along the peak of the roof. Together, the ridge and soffit vents will properly ventilate your attic 24/7, 365. As a last step, I would seal off all the gable vents. If you have continuous ridge and soffit vents in place, gable vents will only cause turbulence and negatively impact the efficiency of your ridge and soffit system, which will carry out heat in the summer and moisture in the winter.