If you are researching wool insulation, chances are you have already seen some strong claims. Healthier. More natural. Better for moisture. Better for the planet.
Some of those claims are true. Some are oversimplified. And some depend heavily on where and how the insulation is used.
This article takes a clear and balanced look at sheep wool insulation, how it really performs in homes, and when it is genuinely worth paying the premium. No hype and no scare tactics. Just the trade offs homeowners should understand before making a decision.
What Is Wool Insulation, Really?
Wool insulation is made primarily from sheep wool fibres, but an important detail is often missed.
Most modern wool insulation products are not one hundred percent wool.
To meet building code durability requirements, wool is typically blended with polyester fibres. A common ratio is around sixty percent wool and forty percent polyester. The polyester acts as a structural backbone, helping the insulation hold its shape over decades and resist slumping inside wall cavities.
So while wool insulation is based on a natural fibre, it is better thought of as a hybrid material rather than a purely organic one.
That blend is what allows wool insulation to perform reliably over the long term.
The Real Strength of Wool Insulation: Moisture Behaviour
The biggest reason homeowners consider wool insulation is moisture, and this is where it genuinely stands apart from many alternatives.
Wool Is Hygroscopic and That Matters
Wool fibres can absorb and release water vapour from the air without feeling wet or collapsing. Wool can hold up to thirty percent of its own weight in moisture vapour while still maintaining loft and thermal performance.
This has several practical effects in a home.
- It helps buffer indoor humidity and reduces sharp swings
- It can lower condensation risk on framing and linings
- It performs more consistently in climates with high overnight humidity
Unlike fiberglass, which loses a large portion of its insulating ability when damp, wool stores moisture inside the fibre rather than in the air pockets that provide insulation.
The Heat of Sorption Effect
There is also a lesser known phenomenon at play.
When wool absorbs moisture vapour, it releases a small amount of heat. When it dries out again, it absorbs heat. This process slightly moderates temperature swings inside the building envelope.
It is not a replacement for heating, but it does contribute to a more stable internal environment, especially in older or less airtight homes.
Wool Insulation Pros and Cons
Pros of Wool Insulation
- Excellent moisture regulation
Wool handles humidity better than most bulk insulation materials and helps protect framing from condensation related issues. - Stable performance in real world conditions
It maintains insulation value even when humidity is high, unlike materials that rely on perfectly dry air pockets. - Naturally fire resistant
Wool has a high ignition temperature and tends to char rather than burn. The char layer helps slow fire spread. - Safe to handle
There is no itch, no glass fibres, and no airborne particulates during installation. - Acoustic benefits
Dense wool blends perform well for sound absorption, particularly for lower frequency noise.
Cons of Wool Insulation
- Cost
Wool insulation is typically significantly more expensive than fiberglass and often more than polyester for the same R value. - Not purely chemical free
Despite being marketed as natural, wool insulation must be treated to be viable in buildings. - Pest risk without treatment
Untreated wool is attractive to moths and insects, which leads directly to the need for chemical protection. - Blended materials complicate sustainability claims
The polyester component improves durability but reduces full end of life compostability.
Fire Retardants and Pest Treatments: What Is Actually Used?
This is where marketing and reality often diverge.
Why Wool Needs Treatment
Wool is a protein known as keratin. That makes it a food source for insects like moth larvae. Without treatment, wool insulation would degrade over time.
Borate Treatment
Most wool insulation is treated with borate salts, commonly borax or boric acid.
These treatments do the following.
- Act as a pest deterrent by killing insects that ingest the fibre
- Improve fire resistance
- Bond to the fibre during processing and do not off gas
Borates are widely used in building materials, but they are still a chemical additive and not a natural by product of wool itself.
Some manufacturers use alternative treatments, but borates remain the industry standard.
The key takeaway is that wool insulation is lower toxicity, but not untreated or purely raw.
Fire Safety: How Wool Behaves in a Fire
Wool performs well in fire scenarios, but context still matters.
- Wool has a high ignition temperature compared to many materials
- It self extinguishes once the flame source is removed
- It chars instead of dripping or spreading flame
This makes wool insulation a strong performer from a fire safety perspective, particularly compared to materials that melt when exposed to heat.
Durability and Longevity
A common concern is whether wool insulation lasts as long as synthetic alternatives.
Does Wool Slump Over Time?
Pure wool would slump. That is one of the reasons it is blended with polyester.
In modern wool insulation products.
- The polyester fibres provide structure
- The batt holds its shape in walls and ceilings
- Products typically meet fifty year durability requirements
When installed correctly, slumping is not a major issue.
Environmental Impact: Not as Simple as Natural Equals Better
Wool insulation is often positioned as the greenest option, but the reality is nuanced.
Positives
- Renewable fibre
- Biodegradable components
- Carbon stored in the wool itself
Trade offs
- Sheep farming produces methane, a potent greenhouse gas
- Wool scouring is energy and water intensive
- Blended polyester reduces full biodegradability
In some life cycle assessments, wool insulation can have a higher overall carbon footprint than recycled polyester insulation, depending on assumptions.
It is environmentally conscious, but not automatically the lowest impact option.
When Wool Insulation Is Actually Worth the Cost
Wool insulation makes the most sense when the following are true.
- Moisture management is a priority
- The home has condensation issues or high indoor humidity
- Occupant health and comfort matter more than lowest upfront cost
- You are renovating an older, less airtight building
- Acoustic performance matters alongside thermal insulation
It is less compelling when the following apply.
- Budget is the primary constraint
- The building is well sealed and dry year round
- You are insulating large areas purely for R value per dollar
Final Verdict
Wool insulation is not magic and it is not just a feel good natural upgrade. It is a technically interesting material with real strengths around moisture regulation, fire safety, and comfort, balanced by higher cost and necessary chemical treatments.
For the right home, in the right conditions, it can be a smart long term investment. For others, especially where cost efficiency is key, alternative insulation types may offer better value.
The key is choosing insulation based on how your home actually behaves, not just how a product is marketed.
