ᐅ Fireplace Kit - New Construction - Load-Bearing Insulation

Hello everyone,

In our new build, we would like to address the topic of a fireplace, and we would appreciate your opinions and experiences.

The following situation applies:

• LAS chimney system from the company Erlus, TYPE SL 18 (11 m total length, 18 cm (7 inches) diameter).
• Location: living room, open kitchen, approximately 50 sqm (540 sq ft) in total, open staircase, extending up to the 2nd floor.
• Plan: airtight fireplace insert, for example Camina S9, weighing approximately 500 kg (1,100 lbs).
• KfW-40 standard, triple glazing, mechanical ventilation without heat recovery, operated by a fan in the attic.

We would appreciate your feedback on the following points:

• At the spot where the fireplace will be installed, a recess in the screed will be made. However, unlike usual practice, it will not be filled with bonded screed. According to the architect, this is because it would create a thermal bridge and therefore bonded screed cannot be used. Instead, Styrodur (extruded polystyrene) will be used as a load-bearing insulation. The architect’s statement is “up to about 1000 kg (2,200 lbs) this is not a problem.” What are your experiences? What do you think? What should we pay attention to?
• Regarding the “air pressure monitor”: presumably only the local chimney sweep or authorized inspector can tell us if this is required, correct?
• Is it possible to tile over the expansion joints between the "normal" screed and the recessed screed? Could we choose a large screed recess and, if necessary, tile over it in case the fireplace footprint ends up being significantly smaller?
• What else should we consider when reading about this plan?

Thank you for your support.

Best regards,
Sebastian

Hello,

- Styrodur (extruded polystyrene foam) with a sufficiently strong screed on top for load distribution should be enough. Entire houses are built on Styrodur after all.

- It’s important to include the mechanical ventilation with heat recovery (MVHR) system from the start, especially with self-build kits. Not only controlled ventilation but also the kitchen exhaust hood with external venting must be taken into account. The keyword here is window contact switch.

- Do not tile over the expansion joint; rather, make sure the joint is visible in the tile pattern. In front of the fireplace, you will need a fireproof area made of tiles, glass, or similar material anyway.

Also important:
The fireplace should minimize direct heat release into the air. A KfW40 (German energy standard) house is so well insulated that even a large surface like this quickly overheats the room. The Camina S9 has a nominal output of 7 kW and a panoramic glass panel. That sends a lot of heat directly into the living space. I would choose a fireplace with a standard glass panel and more thermal mass instead. Besides, 7 kW... well. I have my 14 kW (11 kW water, 4 kW air/stone) fireplace running at 16 kW just into the water circuit. So much for 7 kW. Your entire house will probably use less than 7 kW of primary energy demand!

Hello,

Thank you for the response; it has already been very helpful. I am curious to see if there will be any further useful advice here.

However, the reply raises some additional questions:

a) Regarding the 7 kW: We really like the look of the Camina S9. Apart from it possibly being oversized, which might cause the room to heat up too quickly and prevent us from operating it at full capacity, are there any other drawbacks?

b) Regarding the screed recess: The Camina S9 has a base dimension of 730 x 730 mm (28.7 x 28.7 inches). The required recess in the screed is specified as 701 x 672 mm (27.6 x 26.5 inches). I interpret this as the load-bearing parts measuring 701 x 672 mm (27.6 x 26.5 inches), with the visible parts extending to 730 x 730 mm (28.7 x 28.7 inches). This would allow for tiling underneath the visible area. Would you recommend having the screed recess cut exactly to 701 x 672 mm (27.6 x 26.5 inches), or leaving some tolerance (e.g., 10 cm (4 inches)) to allow for potential inaccuracies? Even with a 20 cm (8 inches) buffer, the expansion joint should still be covered by the visible parts. And if not, appropriate tiling, as mentioned above, could cover that as well. Why do I ask? - If the specified recess is followed exactly and there is even a 1 mm difference on one side for any reason, the fireplace insert might no longer fit because the recess would be too small. What speaks against making the screed recess 720 x 720 mm (28.3 x 28.3 inches)?

Best regards,
Sebastian

General: Our chimney sweep asked about everything that actively directs air outside on the chimney’s floor level: range hood, controlled ventilation system, exhaust fan in the bathroom.

Pay attention to the required clearances to combustible materials. For example, 80 cm (31.5 inches) between glass panes and wooden walls, wooden door frames, etc. (You might be able to measure from the center of the firebox, but this should be confirmed with the chimney sweep beforehand.)

We also debated for a long time whether to simply leave a gap, but ultimately decided to install the chimney right away due to the screed issue...

Ok, so it won’t be installed in the traditional way...
We have a traditionally installed chimney and laid a lean concrete layer on the slab before the screed. 1mm (0.04 inches) difference is nothing; the tolerance in the screed area is larger.
If this is going to be the chimney, I would build the opening exactly to the maximum size. Around it, you will have about a 1cm (0.4 inches) edge insulation strip anyway.

Regarding stove performance: You generally shouldn’t operate a wood stove at only 40% or 50% of its capacity. The fire needs to burn with a long flame; otherwise, you get soot glaze and the stove will clinker up.

We assembled a kit weighing 1.1 tons. It was placed directly on the continuous floor covering or screed, so there is no special insulation beneath this area and no expansion joint either. So far, it is working fine.