Learn Spanish (Latin America): El aislamiento nuevo fue más caro de lo que queríamos pagar. - The new insulation was more expensive than we wanted to pay.

When you compare an adjective (like caro) to an entire clause with a conjugated verb (like queríamos pagar), you must use de lo que instead of just que. If you were simply comparing it to another noun, like 'more expensive than the roof', you would use que.

Why use de lo que instead of del que? Doesn't it refer back to the masculine noun aislamiento?

We use del que or de la que when the second part of the sentence refers directly to the quantity of the noun itself (for example, 'we bought more insulation than we needed'). But here, the comparison is about the abstract degree of expensiveness, not the physical insulation itself. Because it refers to an abstract idea, we use the neuter de lo que.

Why use the preterite fue for the first verb, but the imperfect queríamos for the second?

Fue is in the preterite because the final cost was a single, completed reality at the time of purchase. However, queríamos is in the imperfect because their budget or 'wanting to pay' a certain amount was an ongoing state of mind leading up to that moment.

Does aislamiento only mean 'insulation' for a building, or does it also mean 'isolation'?