Старая теплоизоляция требует ремонта.

Because it agrees with теплоизоляция, which is feminine singular nominative.

• Dictionary form: старый = old
• Feminine singular nominative form: старая

In Russian, adjectives must match the noun they describe in:

• gender
• number
• case

So:

• старая теплоизоляция = old thermal insulation

The noun теплоизоляция ends in -ия, which is a very common ending for feminine nouns in Russian.

So its nominative singular form is feminine, and that is why the adjective is старая, not старый or старое.

Compare:

• старый дом = old house (masculine)
• старое здание = old building (neuter)
• старая теплоизоляция = old thermal insulation (feminine)

Because the subject, теплоизоляция, is singular.

The verb требовать means to require / to need, and here it is conjugated in the 3rd person singular:

• я требую = I require
• ты требуешь = you require
• он / она требует = he / she / it requires
• они требуют = they require

Since теплоизоляция is one thing, Russian uses требует.

Because the verb требовать normally takes the genitive case.

So:

• dictionary form: ремонт = repair
• genitive singular: ремонта

That is why Russian says:

• требует ремонта = requires repair

This is a very important pattern to learn:

• требовать + genitive

Other examples:

• требовать помощи = to require help
• требовать ответа = to demand an answer

It is genitive singular.

Here is the case breakdown of the sentence:

• Старая теплоизоляция — nominative singular
  
  • this is the subject
• требует — verb
• ремонта — genitive singular
  
  • this is the noun governed by требовать

So the structure is:

• [subject in nominative] + [verb] + [object/complement in genitive]

They are:

• старая → старый = old
• теплоизоляция → теплоизоляция = thermal insulation
• требует → требовать = to require, to need
• ремонта → ремонт = repair

This is useful because Russian sentences often contain inflected forms rather than dictionary forms.

Yes, it is one word: теплоизоляция.

It is built from parts meaning roughly:

• тепло- = heat
• изоляция = insulation / isolation

So literally it is something like heat insulation, which in natural English is often thermal insulation.

Russian often forms technical nouns this way.

The usual stress is:

• ста́рая теплоизоля́ция тре́бует ремо́нта

Word by word:

• ста́рая
• теплоизоля́ция
• тре́бует
• ремо́нта

Stress matters in Russian, so it is good to learn words together with their stress.

Yes, Russian word order is more flexible than English, because case endings show the grammatical roles.

The most neutral order here is:

• Старая теплоизоляция требует ремонта.

But other orders are possible for emphasis, for example:

• Ремонта требует старая теплоизоляция.

This might sound more like:

• It is the old thermal insulation that needs repair.

Even though word order can change, the original sentence is the most straightforward and neutral.

Yes. A very common alternative is:

• Старая теплоизоляция нуждается в ремонте.

This also means The old thermal insulation needs repair.

The difference is grammatical:

• требует ремонта → требовать + genitive
• нуждается в ремонте → нуждаться в + prepositional

Both are correct, but требует ремонта is compact and very common in formal or technical contexts.

Russian has no articles.

So старая теплоизоляция can mean:

• the old thermal insulation
• old thermal insulation

The exact meaning depends on context.

English must choose the or a, but Russian leaves that unspecified unless something in the context makes it clear.

Not always. Требовать can mean:

• to demand
• to require
• to need

In this sentence, it clearly means requires / needs, not that the insulation is somehow demanding repair in a dramatic sense.

Technical or formal Russian often uses требовать in this milder meaning:

• Машина требует ремонта. = The car needs repair.
• Документ требует проверки. = The document needs checking.