Substrate under the warm water floor


Heat­ing sys­tems used in every­day life have dif­fer­ent tech­no­log­i­cal capa­bil­i­ties and are dis­tin­guished by their effi­cien­cy. Accord­ing to many indi­ca­tors, today the first place in pop­u­lar­i­ty is occu­pied by heat­ing sys­tems “warm floor”. In many ways, this atti­tude to this method of heat­ing is caused by the increased pace of small hous­ing con­struc­tion. Coun­try hous­es, cot­tages and coun­try hous­es are increas­ing­ly becom­ing objects of per­ma­nent res­i­dence. Against this back­ground, the inhab­i­tants and own­ers of such hous­ing strive to achieve the most com­fort­able con­di­tions in their apart­ments. Lay­ing a water-heat­ed floor is just ful­ly capa­ble of sat­is­fy­ing such aspi­ra­tions.

Heat­ing built into the floor is con­ve­nient in every way. In addi­tion to the high effi­cien­cy of the method of space heat­ing itself, such a sys­tem opens up wide pos­si­bil­i­ties for the design of the inte­ri­or space of res­i­den­tial build­ings. How­ev­er, in order to achieve an accept­able tem­per­a­ture inside a res­i­den­tial build­ing and make the heat­ing equip­ment work opti­mal­ly, it is nec­es­sary to fol­low the instal­la­tion tech­nol­o­gy and cor­rect­ly set the con­trol mode.

Lay­ing a water-heat­ed floor is a sequen­tial process in which each stage has its own mean­ing and sequence. The deci­sive stage of the whole work will be a prop­er­ly made con­crete screed for a warm water floor. What is the pecu­liar­i­ty of installing under­floor heat­ing at home? On which ele­ment of the floor struc­ture should you focus your atten­tion?

Substrate for underfloor heating. What it is? What is the meaning of this element

The main con­di­tion for the suc­cess­ful oper­a­tion of a home under­floor heat­ing sys­tem is not only prop­er­ly con­nect­ed and con­fig­ured equip­ment. In many ways, the heat­ing effi­cien­cy depends on how com­pe­tent­ly the under­floor heat­ing sub­strate is made, how the water heat­ing cir­cuits are laid, and final­ly, what kind of fin­ish­ing floor cov­er­ing is cho­sen.

Let you have the most pow­er­ful heat­ing boil­er, a pump­ing and mix­ing sta­tion with a mass of aux­il­iary devices and devices, a pipeline made of the most expen­sive pipes, just one detail can neu­tral­ize such advan­tages. Incor­rect pipe lay­ing, incor­rect­ly select­ed water floor insu­la­tion will make even the most mod­ern equip­ment inef­fec­tive. Pre­cious heat will go to waste, giv­ing only a small part of the floor sur­face.

On a note: the heat car­ri­er cir­cu­lat­ing through the pipes of the water cir­cuit must give up to 80% of its ther­mal ener­gy to the floor sur­face, even­ly heat­ing the entire floor area in the heat­ed room.

Such indi­ca­tors are achieved by lay­ing in the cor­rect sequence a puff cake, in which there is a water­proof­ing lay­er and a sub­strate. This ele­ment of the puff cake is made of heat-insu­lat­ing mate­ri­als with low ther­mal con­duc­tiv­i­ty. Due to this qual­i­ty of heat-insu­lat­ing mate­ri­als, the nec­es­sary heat-reflect­ing effect is achieved. The heat, reflect­ed from the sub­strate, rush­es upwards with the main mass, trans­fer­ring the opti­mal amount of calo­ries to the floor sur­face. Under­floor heat­ing with a water cir­cuit hid­den in a con­crete screed or locat­ed in the mid­dle of the floor­ing sys­tem becomes a key fac­tor in the effec­tive­ness of under­floor heat­ing.

With this, we actu­al­ly answered the ques­tion of why a sub­strate is need­ed. Ther­mal insu­la­tion is laid on a rough pre­pared sur­face, increas­ing the heat trans­fer of the water cir­cuit towards the floor sur­face.

For ref­er­ence: the sub­strate con­sists of a heat-insu­lat­ing mate­r­i­al, thus cre­at­ing a “ther­mos effect”, and a water­proof­ing lay­er, due to which mois­ture pen­e­tra­tion into the sub­floor is pre­vent­ed.

It is appro­pri­ate to recall here that water floors are usu­al­ly mount­ed on con­crete and wood­en floors. A leak result­ing from dam­age to the integri­ty of the heat­ing pipe, the for­ma­tion and accu­mu­la­tion of con­den­sate can cause flood­ing of the base­ment and rooms of the low­er floors.

For under­floor heat­ing, the thick­ness of the sub­strate is impor­tant. The qual­i­ty of a puff cake depends on the mate­ri­als used in the con­struc­tion. It will be opti­mal to use an insu­lat­ing mate­r­i­al for the sub­strate with the required rigid­i­ty and with the high­est coef­fi­cient of ther­mal con­duc­tiv­i­ty. It is impor­tant that the mate­r­i­al be tech­no­log­i­cal­ly advanced, i.e. con­ve­nient and prac­ti­cal to work with it. A good under­lay will not be cheap, but it will serve for a long time and sig­nif­i­cant­ly increase the func­tion­al­i­ty of water floors.

The main qual­i­ty char­ac­ter­is­tics in this case include the fol­low­ing aspects:

  • opti­mal para­me­ters of hydro and ther­mal insu­la­tion;
  • the pres­ence of high reflec­tiv­i­ty;
  • envi­ron­men­tal safe­ty of the mate­ri­als used;
  • high resis­tance to sud­den tem­per­a­ture changes;
  • sim­plic­i­ty and prac­ti­cal­i­ty in instal­la­tion;
  • high resis­tance to defor­ma­tion;
  • fire resis­tance.

The list­ed para­me­ters cor­re­spond to the ide­al vari­ant of the sub­strate equip­ment, how­ev­er, in some cas­es, it is nec­es­sary to use mate­ri­als with sim­i­lar char­ac­ter­is­tics. The most com­mon mate­r­i­al that is used as a heater for a warm water floor is expand­ed poly­styrene. This syn­thet­ic mate­r­i­al is best suit­ed in terms of its para­me­ters for installing warm water floors in a coun­try house, in a house or in a city apart­ment.

Basic materials for underfloor heating


And again we return to poly­styrene foam. Why is this mate­r­i­al most often used? What are its main fea­tures in the appli­ca­tion process?

Tra­di­tion­al­ly, a warm water floor in most cas­es looks like this. A foam poly­styrene sub­strate is mount­ed on the draft base, which is cov­ered on top with a lavsan vapor bar­ri­er film and met­al plates. Often, instead of plates, foil film is used. The main role played by met­al plates or foil is the max­i­mum and uni­form trans­fer of ther­mal ener­gy to the floor sur­face from the heat­ing pipeline.

One of the low­est ther­mal con­duc­tiv­i­ty coef­fi­cients should be attrib­uted to the dis­tinc­tive fea­tures of extrud­ed (foamed) poly­styrene foam. Accord­ing to this para­me­ter, judg­ing by the data in the table, expand­ed poly­styrene is sec­ond only to min­er­al wool.

Such mate­r­i­al does not allow heat to escape into the sub­floor, and due to inter­ac­tion with the met­al lay­er, ide­al con­di­tions are cre­at­ed for uni­form heat dis­tri­b­u­tion over the entire floor sur­face with max­i­mum effi­cien­cy. The floor screed, laid on top of the entire struc­ture, becomes a sol­id heat­ing ele­ment with a large heat­ing sur­face area.

Impor­tant! Expand­ed poly­styrene is able to with­stand heat­ing tem­per­a­tures up to 900C, so pipes can be laid direct­ly on the sub­strate with­out fear of dam­ag­ing the insu­lat­ing mate­r­i­al.


Usu­al­ly, in a prac­ti­cal plane, they work with expand­ed poly­styrene, the den­si­ty of which is 25–35 kg / m3. Thick foam is used in the under­lay­ment for floors on the first floors, which are locat­ed direct­ly above the foun­da­tion or above the base­ment. Here a sheet or plate with a thick­ness of 100 mm is tak­en. In the absence of plates of the required thick­ness, the foam is laid in two lay­ers (50mm + 50mm).

For inter­pan­el floors, when installing warm water floors in a city apart­ment, poly­styrene foam 20–30 mm thick is used.

Oth­er mate­ri­als used in the equip­ment of the sub­strate are:

  • foil mate­ri­als;
  • cork coat­ing;
  • min­er­al wool;
  • min­er­al mats;
  • fiber­board;
  • peno­fol.

The choice of one or anoth­er mate­r­i­al is deter­mined by the state of the base, on which the instal­la­tion of the water cir­cuit loop will be car­ried out in the future and the type of floor­ing. A water heat­ed floor that pro­vides heat­ing of the inter­nal space in a sin­gle room must have a sub­strate capa­ble of per­form­ing two func­tions:

  • high reflec­tiv­i­ty;
  • pre­vent the pen­e­tra­tion of mois­ture into the sub­floor in emer­gency sit­u­a­tions.

It is appro­pri­ate to say that recent­ly pro­filed mats, which are uni­ver­sal mate­ri­als, are being used more and more often. Prod­ucts have a heat-insu­lat­ing lay­er and boss­es, with the help of which it is con­ve­nient to quick­ly and accu­rate­ly lay the loops of water cir­cuits.

How­ev­er, in this case, there are also dis­ad­van­tages. The water­proof­ing prop­er­ties of the sub­strate made of pro­filed mats leave much to be desired. There­fore, in such a sit­u­a­tion, it is nec­es­sary to lay an addi­tion­al water­proof­ing lay­er.

Impor­tant! Dur­ing the lay­ing process, it is worth remem­ber­ing that the foil lay­er must go onto the walls in order to com­pen­sate for the ther­mal expan­sion of the screed.


Mark­ing is applied to the sub­strate, in accor­dance with which the water pipe is laid. Here it is nec­es­sary to take into account the pipe lay­ing step, which for under­floor heat­ing should not exceed 30 cm.

Characteristics of substrate equipment materials

In some cas­es, the inhab­i­tants of hous­es try to use for ther­mal insu­la­tion any mate­ri­als that are at hand or have a low cost.

Con­sid­er the char­ac­ter­is­tics of the sub­strate made of var­i­ous mate­ri­als:

  • Poly­eth­yl­ene foil, self-adhe­sive — thick­ness 8 mm. It fea­tures high reflec­tiv­i­ty, hydro and sound insu­la­tion.
  • Poly­eth­yl­ene one-sided, lam­i­nat­ed — thick­ness 8 mm. It has good ther­mal insu­la­tion and water resis­tance.
  • Poly­eth­yl­ene foam (Tepo­fol) — only 2 mm thick. Aver­age ther­mal insu­la­tion para­me­ters;
  • Foil poly­styrene of var­i­ous thick­ness­es, with print­ed mark­ings. High heat-insu­lat­ing, hydro and sound­proof­ing char­ac­ter­is­tics;
  • Tuplex back­ing, pro­pri­etary 3 mm thick. It has the max­i­mum para­me­ters of heat, hydro and sound insu­la­tion;
  • Under­lay made of poly­styrene and lavsan coat­ed, 3 mm thick.

The small­er the thick­ness of the ther­mal insu­la­tion, the low­er the thick­ness of the entire lay­er cake becomes. Accord­ing­ly, the height of the flows in the room is slight­ly reduced. The thick­ness of the screed over a water-heat­ed floor can vary with­in 50–60 mm.

The list­ed mate­ri­als are the set that is pre­sent­ed in the trad­ing net­work today. The cost of mate­ri­als may vary. Here, the amount of mate­r­i­al used in the work and the inten­si­ty with which floor heat­ing should be car­ried out are tak­en into account.

Less com­mon­ly used is cork, which is made from crushed oak bark. There is one impor­tant aspect here — such mate­r­i­al is envi­ron­men­tal­ly friend­ly. How­ev­er, if mois­ture gets in, it can become a place for the devel­op­ment of the fun­gus. This mate­r­i­al is sup­plied in rolls.

The cheap­est option is foil poly­eth­yl­ene. Of all the mate­ri­als list­ed, foil poly­eth­yl­ene is the most com­mon today. Despite the fact that the mate­r­i­al is char­ac­ter­ized by a high degree of ther­mal insu­la­tion and fair­ly mod­er­ate water­proof­ing, foil poly­eth­yl­ene does not have suf­fi­cient rigid­i­ty and strength.

In order to achieve qual­i­ty when installing under­floor heat­ing, try to use foil poly­styrene. Do not pay atten­tion to the cost of the mate­r­i­al, it is high, but the obvi­ous ben­e­fits more than off­set your costs, mak­ing the water floor durable and effi­cient.


The sub­strate is a struc­tur­al ele­ment, with­out which it is impos­si­ble to do when lay­ing warm water floors. Pos­sess­ing the nec­es­sary lay­er of ther­mal insu­la­tion and water­proof­ing, the sub­strate pro­vides a high rate of heat­ing of the floor sur­face, reduces ener­gy costs for heat­ing a res­i­den­tial facil­i­ty.


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