Vi holder huset varmt ved at isolere loftet.

Holder is the present tense form of the verb at holde.

• at holde = to keep / to hold
• holder = keep / are keeping

So Vi holder huset varmt means We keep the house warm.

In Danish, after the subject in a normal main clause, the verb is usually finite, so you use holder, not the infinitive holde.

Here holde means keep, not literally hold in your hands.

So:

• holde noget varmt = keep something warm
• holde noget rent = keep something clean
• holde noget åbent = keep something open

In this sentence, Vi holder huset varmt means We keep the house warm.

Because huset is the definite form: the house.

In Danish, the definite article is often added to the end of the noun:

• et hus = a house
• huset = the house

So:

• Vi holder hus varmt would sound wrong here
• Vi holder huset varmt = We keep the house warm

Because hus is a neuter noun: et hus.

When an adjective agrees with a singular neuter noun, it usually gets -t:

• et varmt hus = a warm house
• huset er varmt = the house is warm

The same agreement happens here, even though varmt is linked to the object huset:

• Vi holder huset varmt = We keep the house warm

So varmt agrees with huset.

This is a pattern where the verb is followed by:

1. an object: huset
2. a word describing the result or state of that object: varmt

So:

• Vi holder huset varmt
• object = huset
• object complement / result description = varmt

English does the same thing:

• We keep the house warm
• They painted the door red
• She left the window open

So Danish is working very similarly here.

Ved at means by when you explain the method used to do something.

So:

• ved at isolere loftet = by insulating the attic / loft

This is a very common Danish structure:

• Jeg lærer dansk ved at læse hver dag = I learn Danish by reading every day
• Han tabte sig ved at spise mindre = He lost weight by eating less

So in your sentence, ved at introduces how the house is kept warm.

Because after ved at, Danish uses the infinitive.

• isolere = to insulate
• at isolere = the infinitive marker + infinitive, like to insulate

So:

• ved at isolere loftet = by insulating the attic

You do not use the present tense isolerer after ved at.

Loftet can mean either the ceiling or the attic/loft, depending on context.

In this sentence, because it talks about keeping a house warm by insulating it, loftet most naturally means the attic / roof space / loft area.

So:

• loft = ceiling / loft / attic
• loftet = the ceiling / the loft / the attic

Context tells you which meaning is intended.

Because it gives extra information about how the action is done, and Danish often puts that kind of information later in the sentence.

The core sentence is:

• Vi holder huset varmt = We keep the house warm

Then Danish adds the method:

• ved at isolere loftet = by insulating the attic

So the whole sentence is:

• Vi holder huset varmt ved at isolere loftet

This word order is very natural.

Yes. You can say:

• Ved at isolere loftet holder vi huset varmt.

That means the same thing: By insulating the attic, we keep the house warm.

Notice that when the sentence starts with Ved at isolere loftet, the verb still comes second in the main clause:

• Ved at isolere loftet
  • holder
    • vi ...

That is standard Danish word order.

Yes, a few things:

1. Definite nouns often take an ending

   • hus → huset

2. Adjectives agree with gender/number

   • hus is neuter, so varmt

3. Method is often expressed with ved at + infinitive

   • ved at isolere = by insulating

4. The structure is very close to English

   • We keep the house warm by insulating the attic
   • Vi holder huset varmt ved at isolere loftet

So although there are some Danish grammar details, the overall sentence pattern is quite familiar to an English speaker.