Humidity levels and hardwood flooring

5mcdonag

I wonder does anybody have experience installing solid hardwood flooring over underfloor heating? I have researched thoroughly and the myth that you can't do this is not true. Rather than warping because of heat under the floors, it is the moisture content in the air that causes the warping. So, my question now has become: can I get the moisture levels down to an adequate level inside so as not to affect my flooring? I know the humidity levels in ireland are super high but does that influence the indoor levels too? We are in the middle of a new build and I really want solid floors rather than engineered for many reasons but can't seem to find an answer to this. TIA.

oraiste

TRADA offer advice on installation of solid hardwood floors:

http://www.trada.co.uk/faq/list/flooring/Flooring

The moisture content of your hardwood flooring should be maximum 6-8%.

TRADA also advise the following:

"However, in TRADA's experience, the moisture content that timber flooring will settle to in these service conditions can vary from the values given in BS 8201. In particular, specifying a moisture content for timber flooring, special consideration needs to be given to the heating/air conditioning regime of the building, and to the location in the building where the flooring is to be installed. For example, a conservatory in a house or a glazed atrium in a commercial building may be subject to significant solar gain throughout the year"

Other points you may wish to consider:

- TRADA suggests that a ratio of 4 : 1 width to thickness (eg 80mm x 20mm)
would incur minimal risk assuming good practice (i.e. to prevent cupping of the product). If the boards have a higher width to depth ratio, you should check with product manufactuer that their product is suitable over UFH.

- The latent moisture content in your screed is a very important consideration. If it is still drying out below an installed timber floor, this will impact the flooring above it, cause cupping etc.

- 'Engineered' hardwood flooring (i.e. plywood substrate with 5-8mm solid hardwood finish) is considered to be a more stable product than a solid hardwood product. It may be better to specify an engineered product if installing underfloor heating underneath.

- The three main methods of installation for wood floor systems are as follows:
• Fixed: Where the flooring components are nailed to underlying battens or joists. The resulting floor surface is ‘captive’ with each individualbcomponent of the floor being fixed firmly to the sub-floor.
• Floating: Where adjacent components of the floor system are linked together either by means of clips, edge gluing or interlocking click system to form a rigid floor covering which is held in place by its own weight. The covering is not fixed to the sub-floor and consequently ‘floats’ above it allowing the floor to react to changes in humidity as if it were a single unit.
• Glued: Also known as ‘stick-down’, where the floor components are glued directly onto the sub-floor using a continuous layer of adhesive or adhesive beads.

- TRADA: It is vital to understand that the top surface temperature of the wood floor covering should not exceed 27°C for engineered boards, otherwise excessive gapping between boards may occur. This temperature equates to an average power output of approximately 100W/m2. This is usually sufficient to provide a comfortable environment in newly built properties with adequate insulation and is generally brought about by a water circulation temperature (in the case of water pipes) of 35–45°C although this will vary according to the thermal conductivity and resistance of the floor covering used (species, thickness etc). Older properties with less efficient insulation may need to be set at a higher output. For wider solid hardwood boards, which are more susceptible to changes in moisture, a maximum top surface temperature of between 22 24°C would be appropriate.

- TRADA : Installing UFH on ground sub-floors
When UFH systems are installed on ground floors, the heating elements are covered beneath a screed. The recommended depth of the screed may vary between 30mm and 75mm. Deeper screeds will aid in heat dissipation and will help prevent local hotspots from occurring.
Allow the screed sufficient time to dry fully and ensure that the equilibrium relative humidity is certainly no more than 75%RH but ideally no greater than 65%RH.
Note: It is essential that the equilibrium relative humidity of the screed is no greater than 65%RH for glue-down systems.
The heating system should be run at 1/2 to 2/3 its maximum power for two weeks followed by maximum power for 2–7 days prior to installation of the flooring. The room should be ventilated briefly every day during this period. Install the vapour membrane and underlay directly over the dry screed. Where electric mating or cables are used beneath a 6mm layer of cementitious material, ensure that all moisture has escaped before the vapour membrane and underlay are installed and ensure that no cracks have appeared.
Prior to laying the floor, ensure that the boards are dry and that the moisture content is less than 8–9% and that the conditions in the room are between 35% and 65%RH at 18–20°C.
The UFH is generally switched off when laying the floor in order to prevent moisture being driven to the underside of the boards. Whilst this is not strictly necessary in the case of nailed or floating systems, it is especially true when stick-down systems are installed. After the floor has been installed, the heating should only very gradually be turned back on over a period of several days. Sudden changes in temperature such as testing heating systems at the highest temperature settings and for prolonged periods should be avoided as this can ‘shock’ the timber and cause rapid irreversible ‘setting’ of the wood.

Lots to consider - I would recommend that you use a good quality, stable product, with the support of a professional supplier/manufacturer who will advise on installation over UFH

5mcdonag

Thanks oraiste. I have that info alright but every flooring person in Ireland is telling me only engineered flooring can be used. I guess I was looking for practical experience from people. I know theoretically it can be done but give that Ireland has such a massive humidity level, I'm wondering does it make a difference. Everyone I have spoken to has had engineered floors installed because they were told solid hardwood could not be used.

oraiste

I know theoretically it can be done but give that Ireland has such a massive humidity level, I'm wondering does it make a difference.

From my understanding, the external humidity level could be considered irrelevant.

The humidity levels or moisture levels that are relevant:

- moisture level of the timber flooring product
- moisture level of the screed underneath (if applicable)
- moisture / relative humidity level (& temperature) of the room during installation

Everyone I have spoken to has had engineered floors installed because they were told solid hardwood could not be used.

There are suppliers that offer solid hardwood flooring products designed for use with UFH. Check out www.junckers.ie - however this particular product is expensive!

5mcdonag

Thanks Oraiste. I'll definitely check them out!