You have just received your delivery of some lovely engineered doors. Your first instinct is to rip open the packaging and start hanging them. Before you do, have a little read of the fitting instructions attached to most engineered timber products. These instructions will usually state that you must lay the door(s) horizontally on a level surface, raised off the ground within the packaging for one to two weeks, whilst steadily increasing the ambient temperature daily.
A lot of users will simply ignore this clear and quite precise instruction. I am going to explain why you shouldn’t and what might happen if you don’t.
Engineered Timber Doors
An engineered door is made of several timber components. The core is normal a high density particle board, the veneer is normally real timber and the lippings (edges of the doors) and inlays (normally routed decorative parts) are all solid timber (Fig 1). The main reason for allowing a door to acclimatise is because of the solid timber lippings and inlays.
In order to understand why, we must first understand the timber’s journey from cutting to its final use in assembly.
From Tree to Door
Freshly cut timber (also known as green timber) is full of water. When a tree is first cut down the weight can be up to two and a half times the final dried weight, depending on the species and the part of the tree from which the timber comes.
There are two types of water in the tree at this point. The first is free water and the second is bound water. Free water is very fast to remove but bound water is deep in the wood’s cells and has to be forced out.
I like to compare timber to a rag when explaining moisture. Imagine a rag in a bucket full of water. As soon as you remove the rag it begins to lose water, when you ring it out it loses even more but there will still be moisture within the rag. This is equal to bound water. It can only be removed through drying, either mechanical (a tumble dryer) or naturally (a washing line). This is similar to timber. You can remove bound water through two main methods: Kiln drying and air drying.
Once the process of removing the bound water commences the timber starts to move (contracting, cupping, bowing or twisting.) This is also true if the timber absorbs moisture after drying beyond the free water stage.
Bound water is removed in most engineered products by means of a kiln.
Let’s Talk Numbers
When a tree is cut the green timber has a moisture content of 100%. This is calculated as the difference between the weight of a piece of green timber and the weight of the same piece after oven drying , divided by the oven weight and multiplied by 100.
For example, a piece of green timber weighing 800g containing 400g of water has a moisture content of 100%
Once the timber has undergone kiln drying it then has a moisture content of between 6% and 18%. It then heads to a factory to be made into an engineered door. When the timber arrives at the factory it will be allowed to acclimatise to the factory’s conditions for between 10 and 14 days. The conditions can vary from factory to factory. Some factories will be heated while others will simply be a covered location. The timber will generally absorb or expel moisture in the region of 15% to 18%. The solid timber is then used in production. Once the doors have completed production they will generally be packaged and shipped and stored in similar conditions to that of the factory.
When a door arrives at your property, what happens to the moisture content ? Well this is where acclimatisation is very important. In a continuously heated building timber will generally drop in moisture content from between 6% and 13%. But why does that matter? Timber moves in three ways: Longitudinal movement, tangential movement and radial movement (Fig 2). They all affect the timber in different ways but a simple calculation gives a rough guide to movement. For every 1% change in moisture, timber moves 0.25% to 0.38%. So although this is an approximate measurement, in the most extreme situation the solid timber in your door can be dropping from an 18% to 6% moisture content, resulting in an approximate movement of 3% to 4.56%.
So what does this mean for your door? Once a door is hung it is locked into place by the hinges. If it moves it is locked into place at that point. Let’s look at solid timber inlay. It forms a part of the core of the door and if it moves while the door is hung it can split the veneer (Fig 3).
The same applies to the lippings on a door which are also veneered, if they contract, cup, bow or twist while the door is hung they will split the veneer and in extreme circumstances can even cause the lipping to come away from the door (Fig 4). Now this is unlikely to affect the structural integrity of the door but doesn’t make them aesthetically pleasing.
So how does acclimatisation stop this? If a door is allowed to acclimatise inside your house then the moisture content will slowly drop to suit the conditions in your house. Because it isn’t locked into place by hinges, it can slowly expand and contract without causing any cosmetic issues. Once the door is hung it then needs to be sealed on all the edges and behind hinge cut outs and handle cut outs using a treatment specified by the door manufacturer. Once this has been done the moisture content within the timber will still increase and decrease but to a much lower level that the door can cope with.
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