Thermal loss through wall ties - is there a benefit to low conductivity ties?

[Joey]

Dear all,

 

My girlfriend and I are hoping to build ourselves a home after finally managing to secure a plot near Manchester, all very exciting! We'll be building primarily with brick and block and want to aim for passive house levels of performance. Current thinking is to have a 250 mm cavity fully filled with DriTherm 37, and a 100 mm thermal block inner leaf giving a U value of approximately 0.13. Based on an approximate wall area of 200 m2 excluding windows and doors, I think this gives 520 W of heat loss at a 20C temperature differential.

 

I've been struggling to find many options in terms of walls ties to accommodate the wide cavity, but have obviously come across Teplo ties whilst taking inspiration from Green Building Store projects and other users on this site, and was initially taken with the ideal of using a low thermal transmittance product. Neutras ties also seem to be quite similar. The problem is, the costs I've currently seen for these ties at 375 mm are £4-6/tie! Assuming a tie density of 2.5/m2, that's £2,500!

 

Using formulae given in ISO 6946 (see picture), I've calculated the following based on using stainless steel ties, not thermally efficient ones:

 

Thermal conductivity of fastener = 17 W/m/K

Cross sectional area of fastener = 16 mm2 or 0.000016 m2

Number of fasteners/m2 = 2.5

Thickness of bridged insulation layer = 0.25 m

Thermal resistance of bridged insulation layer = 6.76 m2/K/W

Thermal resistance of whole component = 7.69 m2/K/W

 

0.8 * 0.0027 * 0.77 = 0.0016

0.13 + 0.0016 = 0.1316 (corrected U value for the wall)

 

Using the corrected U value and same conditions as above, I calculate the heat loss to now be... 526 W. Have I made an error I haven't spotted, or am I missing something? I'd appreciate your thoughts on this, because I've seen arguments both for and against using thermally efficient ties on here, but based on this I can't understand why someone would go to such additional expense for such a small saving? Also, if there really is limited benefit to using thermally efficient ties, can anyone recommend any stainless steel ties for a 250 mm cavity? I've seen Vartry Engineering recommend on here, but worry the lack of BBA certification might cause issues, or I might not be able to get them in the UK.

 

Thanks for your time!

 

Joe

[ADLIan]

Might be better spending your money on lower conductivity insulation, 0.032 W/mK. Look for ‘32’ in the product name/reference. Also check that your wall tie with its small cross-section is suitable for 250mm cavity, you may have to look at a heavy duty tie with larger area. Teplo tie may offer little benefit bearing in mind the cost.

[Joey]

10 minutes ago, ADLIan said:

Might be better spending your money on lower conductivity insulation, 0.032 W/mK. Look for ‘32’ in the product name/reference. Also check that your wall tie with its small cross-section is suitable for 250mm cavity, you may have to look at a heavy duty tie with larger area. Teplo tie may offer little benefit bearing in mind the cost.

Thanks for the reply!

 

I did actually look at DriTherm 32, and ubakus calculated this would take the U value from 0.13 to 0.11, and therefore heat loss from 520 W to 440 W under the above conditions. However, the extra cost for 32 vs 37 from the supplier I've mainly used for prices so far was about £3,000, so again I find it hard to justify the expense for a saving of about 23p/day under the above conditions. I appreciate your suggestion though as I do need to have a look at the different areas of the property and see where any extra investment would be best spent! 

 

Good point about the tie area. I just took the 16 mm2 cross section from the Vartry Engineering website as I'm having trouble finding other stainless steel ties with the required length. Likewise with the tie density, I assume this may also need to be increased due to the larger cavity. Hopefully I can get get answers on both of those points soon.

 

Joe