Insulation is a key factor when it comes to lowering energy usage, whether your property is in a cold, or a hot climate zone, or somewhere in between. As well as saving energy and lowering bills it is the environmentally conscious thing to do.
So, when an insulation product arrives that appears to have excellent eco-friendly credentials, like Hemp it’s got to be good news, right? To an extent, Hemp is a worthy competitor for your construction or renovation dollar, but it comes with some downsides too.
We’ve taken a cool look at hemp insulation, and the claims made for it, and made our own, realistic appraisal.
What Is Hemp Batt Insulation?
Hemp batt insulation is a natural wool fiber insulation made from the outer part of the stalks of the hemp plant, a fast-growing, non-toxic crop that has enjoyed a comeback in recent years. Mixed with a polymer binder, the hemp fibers are spun and then compressed into batts.
A cousin of the cannabis plant, it has no active psychoactive properties and is grown predominantly for medical and other uses. It can be cropped from planting within a few months on otherwise unproductive land and needs little irrigation and no pesticides to grow.
So What Are The Disadvantages to Hemp Batt Insulation?
1. Hemp Cannot be Bought in Every Big Box Store
This is a big one. Hemp wool is quite a new product, so distribution hasn’t quite caught up in all states as yet. Rumor has it hemp is becoming popular as a crop in Texas and Montana, but the only current production of hemp insulation batts in the US is in a factory in Idaho.
A previous setup in Kentucky filed for bankruptcy in 2020, so if you can get your hands on hemp insulation batts, make sure you order all that you need to complete the job!
When looking at online retailers for hemp batts, at the time of writing, all outlets were currently marked as ‘out of stock.’
2. When You Can Buy It Hemp is Expensive
At R-20 or 5” in a six-by-two wall construction expect to pay approximately $0.65 per square foot for fiberglass and $1.95 for hemp. It’s true that hemp has some advantages over fiberglass, it is more structurally stable, but then again, so is mineral wool, and that comes in at a respectable $1.20.
It is supposed to be a cheap product to produce, depending upon a plant that is fast to grow, needs no pesticides, and few nutrients, but until the supply chain catches up prices are likely to remain high.
3. Hemp Has Little to No Flexibility
Measure twice, cut once, is the old carpenter’s saying that merits repeating here. Hemp batts are a friction fit, so the best tip is to over-measure by a half-inch to ensure the batts will have enough compression to sit snugly between the wall studs.
However, don’t overdo the compression as this will lead to reduced resistance to thermal conductivity, and lower the R-value, and insulating properties of the Hemp batts.
Hemp won’t wrap around pipework or conduits, so you will either have to leave a gap for services or carve out space for them to fit. The same goes for electric boxes and other penetrations, they will have to be cut around to fit to avoid compression.
4. Hemp Might Not Be the Most Eco-Friendly Insulation
Hemp batts are, at most, 92% natural and plant-based, with the remainder made up of polymer-based materials used as a binder. So what does this mean? What it means is that the insulation has a content level that is 8% plastic.
If you were looking for an environmentally friendly, all-natural insulator, Hemp does not quite measure up to that specification. Sheep wool insulation might be a better all-natural alternative, but as most of the supply originates in New Zealand, it doesn’t help with carbon offsetting.
5. Hemp May Not Perform Well With Fire
Early iterations of the product claimed hemp batts were Class A non-combustible without the addition of fire retarders or other chemicals to inhibit combustion. Some of the more recent products are marked Class E, which means that they are absolutely not fire retardant.
Samples that have been burned in tests also give off a heavy black smoke, that smells terrible, and is likely not helped by the plastic content within the batts. The advice is, that if you are determined to use Hemp, then make sure that electrical boxes are properly fire-sealed, and that the insulation is isolated from any other possible sources of ignition.
Hemp does have good moisture control properties and is resistant to mold and pests, attributes that would make it a great contender for continuous external insulation underneath your siding. However, the combustibility of hemp batts means that you would be better off using a foam board or mineral wool instead.
If you are determined to use Hemp, then make sure that your supplier has used borate, or some other certified fire retardant chemical in the production of the insulation.
6. Hemp Insulation Is Not Compostable
The polymer fibers used as a binder for the hemp batts mean that claims that the insulation is compostable at the end of its life are, well, not quite true. The polymers are not biodegradable and contribute to the longevity of the stability of the batts.
It is estimated that hemp batts should easily last the lifetime of a building, say a century or two, so there is little chance of using hemp batt offcuts in the vegetable garden anytime soon.
7. Hemp Batt Fibers Can Be an Irritant
Despite what you might read elsewhere, handling and cutting Hemp batts will give rise to airborne fibers. Some have claimed that you can fit Hemp without putting on workwear or any protective gear whatsoever.
Whenever a material is sawn, the act will produce airborne particles and, unless you are wearing a Type 3 respiratory mask, it is likely you will breathe these in. While they are not toxic, the dust produced will be uncomfortable, may make you cough, or vomit, and will not be good for asthma or other respiratory conditions.
But cutting with a knife causes less dust, right?
8. Hemp Cannot Be Cut With a Knife
Well, hemp can be cut with a box cutter, but the installation will take days. The fibers in Hemp batts are tough, which is what gives it the stability and rigidity that makes it a good choice for wall stud insulation.
It is best cut with a power saw, either hand-held or table-top. You could use a regular handsaw too. Why is this an issue? It can be a problem due to the fact that the fibers can clog the saw teeth very quickly, and you will need to make sure the blades stay clean so the tools do not overheat.
As you might imagine, this can also contribute to the level of dust and airborne fibers.
Hemp Batts Can Contain Ammonium Phosphate
Ammonium phosphate is a commonly used agricultural compound that is present in many plants and foodstuffs and is used to fix nitrogen. If inhaled, this can also lead to irritation of the airways. It is found in hemp insulation batts, and should be a consideration when weighing up the possible disadvantages of using it for your project.