Sheep's wool, especially the comercially sold hiking variants like Icebreaker*, is the least warm options out of all standard wools. If it has to be sheep's wool, take homeknit, from feltable wool. A cable design will be warmer, since it has more wool in the same size. But it is probably better to look into other animals.
If you are on a budget, I would suggest alpaca, which is much warmer at the same thickness. The downside is that it is also much heavier. Llama performs basically the same as alpaca, but is rarer to find, so not worth chasing. Camel is good. A better, very light choice would be cashmere, which fluffs up to considerable warmth even at thin weights. But the one that beats them all is the yak. Get a yak sweater if you want to stay really warm.
Other wools will not give you anything special for hiking. If you can even get mohair in thick gauges, it won't be warmer than sheep. Angora is OK, but not better in warmth, weight, price or user friendliness. Some absolute exotics like vicuna are expensive because of their rarity, I haven't tried them, but I can't imagine their functionality being proportionate to their price tag.
Silk is not really warm. It can create a good underlayer, but it's closer to standard polyester in its qualities, more likely to make you sweat than to keep you tempered.
Stay away from the usual plant based fabrics - cotton, kapok, linnen, ramie and juta. None of them provides good warmth or wet performance. Also don't use synthetics, including all kinds of acetate, viscose, milk and soy, even if the label says "natural"**. Without going into philosophical discussions about natural vs. nonnatural, they are not as warm as the other materials I discuss here.
Leathers won't warm you per se, but they will keep wind out, which is quite valuable. They can make a decent outer layer, but most people nowadays would choose other options.
The outermost layer I would suggest is not leather, but something filled with down. It is not strictly fabric, but you cannot beat down for insulation, not with artificial fabrics nor with other natural materials.
A last paragraph: you say "wool is heavier". If the weight of sheep's wool is already a concern for you, I assume that you either have not tried it (and so are not familiar with natural fabrics) or are looking for something that is just like artificial or synthetic fabric, just warmer. To make it perfectly clear: such a beast doesn't exist. Natural fabrics are very different in weight, body climate, washing, drying, coloring, and skin feel. Before you invest too much money, make sure you know what you get and that you can live with its downsides.
* This is not just a general "handmade is always better" snob attitude. First, they tend to use rather thin fabric as opposed to rustically knitted clothes. Second, they almost always sell machine washable nonitchy wool, which is made by removing the microscopic hairs sticking out of raw wool. Sadly, these samie hairs are great at trapping additional air, making traditionally processed wool much warmer at the same yarn weight.
** Nowadays marketing loves to call them "natural", as in "natural bamboo socks" but the standard terminology is to use "artificial" for anything made from oil, "synthetic" for fabrics which have been chemically synthesized from natural materials, and "natural" for fabrics created with non-chemical processing of natural materials.
The best fabric the use for outdoor furniture is polypropylene fabric. Polypropylene was one of the first marine fabrics due to the fact it had superior UV resistance compared to other fibers. Polypropylene is the best choice for outdoor cushions because it dries quickly when wet and has no dye sites. These non-active dye sites prevent stains and can easily be cleaned with bleach and water solution.
Landscape architects, designers, realtors, and building contractors all agree that regardless of the price point on a home, outdoor living spaces have become a standard feature in new home construction. The trend is to create an outdoor living area that mimics indoor living.
Outdoor kitchens vary from a built-in grill to fully functioning kitchen spaces that may include refrigeration and brick ovens. What outdoor space would be complete without a television and a sound system as well as a fireplace or at least a cozy fire pit?
With all these outdoor living areas, comfortable seating is essential. Think of the seating as an expansion of what you would have inside your home... comfortable dining chairs, a sofa or a sectional. The difference is that the seating is made of outdoor cushion material that can withstand the elements. Harsh sunlight, temperature extremes, water and humidity that contribute to mold require special fabrics engineered for outdoor living.
Revolution Outdoor Fabrics are the perfect choice for the best outdoor fabric for living and entertaining!
New to the outdoor fabric category, Revolution outdoor fabrics are woven in King Mountain, North Carolina with olefin and polypropylene yarns.
Olefin is a byproduct of refining petroleum and is extremely durable and stain-resistant. Olefin along with Acrylic is extremely resistant to fading from the sun. Most types of outdoor fabrics on the market will be acrylic, olefin, or polyester. Polyester fabrics will perform the worst because it has the lowest light fastness rating and they require a PFC chemical finish for stain resistance. The two best fibers when making outdoor cushions are solution-dyed olefin and solution-dyed acrylic. The best thing about Revolution outdoor fabrics is that you get superior performance at a great value per yard. Where most outdoor fabrics range from $20 - $50 a yard, Revolution outdoor fabrics cost between $15 and $30 a yard!
Extremely stain resistant without the use for a PFC chemical treatment.
(MUSTARD, KETCHUP, WINE, SHARPIE... IT ALL COMES OUT )
Currently, produce a small line of outdoor fabric. More fabrics will be available in the future.
100% woven in the United States with raw materials form within 300 miles of the factory.
Heavy texture ( great for cushions, maybe not drapes )
Exceeds 30,000 Double rubs and us very abrasion resistant
Wax-based water repellent finish for rain and dew.
Outdoor fabrics are over 2000 hours of light fastness or 3-year fade warranty.
Mildew resistant properties.
Acrylic fabrics are synthetic fabrics made from acrylic fibers. This type of outdoor fabric has a number of advantages including lower specific gravity, mildew-resistant properties and an ability to manage moisture. Made famous by outdoor fabric brands like Sunbrella, most outdoor fabrics are solution-dyed acrylic or a cheap polyester only good for a season. Acrylic is great for lightfastness but is extremely easy to stain when not treated with a stain treatment bath. These fabrics tend to look great at first but become stained as the treatment wears off. Do not let the "solution dyed" marketing efforts fool you. Not all solution-dyed fibers are created equal. Solution-dyed Acrylic can be atmospherically dyed ... This means that the fiber has dye sites. The more dye sites a fiber has, the more wine, ketchup, and stains can penetrate the fiber. Solution Dyed Olefin on the other does not have dye site. This is why Olefin is stain resistant without the use of stain treatment baths.
Pros ConsHigh UV and lightfastness scores ( 2000 hr of lightfastness)
Contain harmful PFC Chemical
mildew resistant properties
Known to pill or fuzz
Multiply use application (drapes, cushions, curtains)
Very expensive ( will had $400 average to your upholstery project)
Some liquid resistant properties ( with stain finish applied )
Flat texture ( not great for upholstery / limited design )
Extremely hard to get oily stains out ( Ketchup, Mustard, ETC )
Mostly used in one season outdoor goods, polyester outdoor fabrics and prints will be your lowest performing outdoor fabric. This type of outdoor fabric on average will have a lightfastness score no higher than 1000 hours of lightfastness. Unlike solution dyed Acrylic and Olefin, Polyester is batch-dyed. Most polyester is woven white and then dyed in drums to its desired color. This process is great for obtaining mass amounts of colored fabrics but these fabrics are extremely receptacle to staining because of the mass about of dye sites in polyester fiber. These fabrics will be cheap and only good for a seasonal throw pillow or two.
Pros ConsLargest selection of product
Low Lightfasntness / Fade rating ( 1 year or 500 hour average )
Cheap fabrics by the yard
Hard to clean oily spills like ketchup and mustard
Can have some liquid repellency after a PFC chemical bath
Abrasion results are across the board
Known to pill or fuzz.
Contains Harmful PFC chemicals if stain resistants properties are claimed.