Heat Loss Savings from the Treatment of Masonry with Water Repellents - Feasibility Study

Transcription

1 Heat Loss Savings from the Treatment of Masonry with Water Repellents - Feasibility Study The thermal conductivity of brick is known to increase with moisture content (see for example reference 1) and hence there may be benefits in reducing heat loss by the treatment of external brickwork to reduce moisture content. A possible secondary benefit of making brickwork nonabsorbent is the reduction of cooling from evaporation of water which results from the latent heat of vaporisation. The purpose of this Note is to bring together information to facilitate an energy loss calculation. The focus of the project is older houses say c.1900 Past work Some previous work is reviewed to provide background 1. In 1978 Roedder (2) reported energy savings could be made by using water repellents. Two different brick types were tested along with a range of other building materials. The results found are shown in the table. Table 1 : Data from (2) with a calculation of heat savings from reduced drying energy After 4 hours of heavy rain Drying energy Fuel cost Water absorbtion water content of wall * % m3 KWhr DM Brick I Brick II * based on 120 m2 of wall at a thickness of 25cm What Roedder has done here is measured the water uptake of two types of brick after 4 hours of heavy rain. He has then calculated that, on the basis of 120m² of wall at 0.25m thickness, what the volume of water in the wall is. (e.g, taking Brick II 18% of 120 x 0.25 = 5m³ ). Using the latent heat of vaporisation of water as 2.3 KJ/g, he then calculates that the drying energy to remove this water is 3290 KWhr. (by 5 x 1000 x 1000 x 2.3 = 11.5 MJ = 3200 KWhr). Based on a cost of 31 KWhr/DM for heating, then the fuel cost to remove this water from 4 hours of pouring rain is 106 DM or 25 (1980 exchange rate). That is for the higher absorbing brick. He then goes on to look at the conductivity contribution and data is presented that shows the thermal resistance to halve from 0.6 to 0.3 m2/h/c/kcal by increasing brick moisture from 0 to 4 wt%. Based on this halving in thermal resistance, the heat loss through the walls can be reduced from 8,100 to 4,050 KWhr/a which gives an annual saving of 44 / a based on 1979 data. So in conclusion, the work estimates that a single period of heavy rain for 4 hours could increase energy costs through vaporisation by 25 and the conductivity increase through wet brickwork might mean an annual cost of 44. There are some obvious inaccuracies in this approximation in that (a) heat loss through walls is not a pro rata calculation on conductivity and (b) it is not reasonable to say that the vaporisation is calculated to the extreme that it leaves the walls dry and then also add a conductivity factor in which assumes the walls are partly wet. 1 P a g e

2 2. In 1995 the BRE (3) reported results on the moisture content of bricks treated with water repellents. The water repellents used were early types and mostly silicone or stearate based. A particularly interesting aspect about the study was that measurements were made of moisture content of bricks exposed to weathering oriented WSW at the Scottish site. Looking at the best silicone performance in 1995 the results were as follows; Table 2 : Data from BRE (3) showing the average on-site moisture content in on year of weathering Brick type Water uptake 5h boil Average on-site moisture content Untreated Siloxane wt % wt% wt% calcium silicate clay clay clay With clay bricks the average moisture content drops from to wt %. In the most porous brick the on-site moisture drops from 15.7 to 1.0 wt%. 3. A measurement of U-value of a brick panel was made by BRE in 2001(4). This found that there was no change in U-value after a 5 hour water spray of a panel treated with silicone whereas an untreated panel showed an increase of U-value of 2.2 (dry) to 2.9 (wet) W/m2/C representing a 24% reduction in heat loss by silicone treating. 4. Kunzl from the Fraunhofer Institute reported how the moisture content of a test wall reduced after treatment with water repellent (5). After treatment the average moisture content of a westfacing brick masonry wall dropped from 10% vol (5.3% wt) to 0% vol over a period of 2 years. The brick masonry had a density of 1900 kg/m3 and a pore volume of 19% vol (= 10% wt) so is relatively non-porous compared with reference 3. The moisture related increase in conductivity was 15% per 1% increase in water amount. Current Work Some measurements of brick conductivity were made by Dr. Zhang at the University of Portsmouth. Brick slips were polished flat and conditioned to various degrees of moisture by changing ambient conditions. The thermal conductivity was then measured. Two different water repellents were applied and the conditioning and conductivity measurements repeated. Tabulated results are given in the Appendix with a Graph shown over. The data from three brick slips are plotted on the same graph. Similar behaviour is seen in each case. The conductivity increases with the amount of water in the brick. When the water repellent is applied there is very little water uptake after the conditioning treatments and the conductivity remains at low and increases marginally from 0.7 to 0.8 W/mK. These results were obtained from off-the-shelf water repellents currently available from Seamless Coatings after allowing only a short time (21 days) for the repellent to become active. It is anticipated after allowing for longer times and with some development work of the repellent recipe zero water uptake can be achieved. 2 P a g e

3 conductivity (W/mK) LABORATORY REPORT Figure 1 : Relationship between conductivity and brick moisture (Portsmouth University) Brick slip No.1 Brick slip No.2 Brick slip No moisture (%) So, previous work and the data here show an increase in conductivity with water content and that water content can be reduced by repellents. The question is how does this then relate to water content in the field. Moisture in walls As part of this project, two small test units of single skin brick wall will be constructed (2m length, 1m width, 2m high). One will be treated with water repellent and the other left as a control. The water content of the bricks will be periodically measured. But for now, using the data from reference 3, and taking the more porous brick of the three, then the moisture content of the treated wall reduces from 15.7% to 1.0%. This wall was WSW facing and therefore relatively extreme. If we take a guess of the moisture content of the untreated building we might have; 16% West facing, 4% East facing, and 10% North and South facing aspects. So the average might be 10%. Assumptions to calculation of U-value To begin with assume; 1. Average moisture content of the untreated brick wall is 10% wt 2. We can reduce this value to 0% by correct formulation of a façade water repellent 3. The data from Portsmouth on conductivity is representative of an old (c1900) wall. (The conductivity v s water graph gradient is actually less inclined than some other sources (1)) Using then the data from Portsmouth gives; Conductivity wet (10% moisture) = 1.2 W/mK Conductivity dry = 0.6 W/mK The U-value from a single skin 4 brick wall can be calculated as follows; 3 P a g e

4 Table: Reduction in U-value single skin wall Dry wall Wet wall conductivity resistance (R) conductivity resistance (R) R se external surface R brick brick 4" R si internal surface Total R U=1/R= Table: Reduction in U-value unfilled cavity wall Dry wall Wet wall conductivity resistance (R) conductivity resistance (R) R se external surface R brick outer brick 4" R air cavity R brick inner brick 4" R si internal surface Total R U=1/R= Reduction 27% Reduction 13% So, this simple calculation shows that the potential reduction in U-value and therefore heat loss through walls is 13-27% (cavity single skin) through conductivity reduction. The website provides an energy calculator to estimate energy costs and potential savings. The accuracy of this is not known. Using the calculator gives the following. See Appendix 2 for the details. For a semi-detached house, in the north of England with an internal temperature of 20C and an average external temperature of 9C with a solid brick wall, gives Annual heat loss through the walls = 20,372 KWh = 1019 at 5 p/kwh So, in this case, the annual saving from water repellent treatment might be 27% of 1019 = 275 For the same house with a cavity wall at the same conditions then (see Appendix 2) Annual heat loss through the walls = 9,167 KWh = 458 at 5p/KWh So, in this case, the annual saving from water repellent treatment might be 13% of 458 = 59 Influence of latent heat of vaporisation As stated earlier, the evaporation of water could also influence energy demand. If we assume that the annual wind driven volume of water that is absorbed by the walls is 200 litres/m2/annum and with the latent heat of vaporisation of water of 2.3 KJ/g, then Amount of heat lost through vaporisation = 2.3 x 200 x 1000 = 460,000 KJ/m2/a (130 KWh/m2/a) If the house wall area is 100 m2, then, Total annual potential heat loss though vaporisation = 1,300 KWh. 4 P a g e

5 This figure represents 14% of the 9,137 KWh estimated through conductivity heat loss from the cavity wall case The conductivity and vaporisation contributions will not be purely additive some estimate of the real water loss through vaporisation is needed Conclusion The annual energy savings are estimated to be between for cavity or solid wall construction in the north of England. It is realised that this is a very crude estimate. The next step is to obtain a more accurate figure, perhaps through discussion with the BRE. 28/04/08 References 1. Masonry Drying and Cellar Rehabilitation F Frossel (Fraunhofer pub) K-M Roedder in Bautenschutz and Bausanierung No.3 p (1978) 3. A.W.Stupart and I.H.Murray BRE Scottish Laboratory Drying of bricks after Impregnation H M Kunzel and K Kiessel Bauinstandsetzen 2 p (1996) 5 P a g e

6 Moisture content % LABORATORY REPORT Appendix 1 : Portsmouth Conductivity Data (a) Results before treatment applied 14 Hours After Dry 1 4 Hours After wet Condition at 17 0C Wet Condition (24 C ondition at 17 0C Dry Condition (110 with 40% Hours water with 50% Sample Original Condition 0C ) for 24 Ho urs Humidity) Immersion) Humidity) Brick Brick Brick % Moist Brick Brick Brick C onductivity Brick Brick Brick (b) Results after treatment applied 14 Hours After 24 Hours After 9 Hours After wet Dry Condition Dry Condition at wet Condition at Condition at 20 Wet Condition (110 0C ) for C with 40% Original 18 0C with 49% 0C with 43% (24 Hours water Sample Hours* Humidity) Condition Humidity) Humidity) Immersion) Brick with Brick Crème C Brick Untreated Mortar Hours After 24 Hours After 9 Hours After wet Dry Condition Dry Condition at wet Condition at Condition at 20 Wet Condition (110 0C ) for C with 40% Original 18 0C with 49% 0C with 43% (24 Hours water Sample Hours Humidity) Condition Humidity) Humidity) Immersion) Brick with Brick Crème C Brick Untreated Mortar Hours After 24 Hours After 9 Hours After wet Dry Condition Dry Condition at wet Condition at Condition at 20 Wet Condition (110 0C ) for C with 40% Original 18 0C with 49% 0C with 43% (24 Hours water Sample Hours Humidity) Condition Humidity) Humidity) Immersion) Brick with Brick Crème C Brick Untreated Mortar wetting drying Brick with Brick Crème C Brick Untreated Mortar 0.0 Dry Condition 14 Hours After Original 24 Hours After 9 Hours After Wet Condition (110 0C ) for Dry Condition Condition wet Condition wet Condition (24 Hours water 24 Hours at 17 0C with 40% Humidity) at 18 0C with 49% Humidity) at 20 0C with 43% Humidity) Immersion) 6 P a g e

7 Name of House (for your reference) Data from Resurgence Calulator Particulars of the House typical semi Today's Date 01/05/2008 Type of House Semi-detached Number of Rooms 6 Rough Size of Rooms Medium (20 sq.m) Number of Floors 2 Floor area - interior 120m 2 Ceiling area - interior 120m 2 Floor area - exterior 60m 2 Roof area - exterior 72m 2 Wall height 5m Wall length 31m Exterior Wall Area - Gross 107m 2 Window & Door area ( 11% of walls) 12m 2 Exterior Wall Area - Net (less windows) 95m 2 Volume of house 276m 3 Temperatures Rough Internal Temperature (select) Specific (below ) OR enter Specific Internal Temperature Internal Temperature used Rough External Temperature (select) 20 degrees C 20 degrees C Northern UK OR enter Specific External Temperature External Temperature used degrees C 9 degrees C 7 P a g e

8 Sold brick construction; Construction & Materials Exterior Walls Exterior wall lining Brick/concrete - solid No lining Annual 5p/kWh 1019 Annual cost with insulated dry lining 155 Roof Insulation 300mm Annual 5p/kWh 50 Annual cost with 300mm insulation 50 Windows Wood/uvpc double glazed Annual 5p/kWh 142 Annual cost with low-e double glaze 96 Ground Floor Type Solid concrete 8 P a g e

9 Annual 5p/kWh 181 Annual cost with suspended floor 100 Air changes Moderate draughts Annual 5p/kWh 532 Annual cost with low draught house 266 Heat loss for house with existing insulation Summary Heat Loss (kwh) 5p/kWh Percentage Walls % Roof % Windows/Doors % Ground Floor % Draughts % Total % Cavity Wall Construction: Construction & Materials Exterior Walls Exterior wall lining Annual 5p/kWh Annual cost with insulated dry lining Roof Insulation Brick/concrete - air-filled cavity No lining mm Annual 5p/kWh Annual cost with 300mm insulation Windows Wood/uvpc double glazed Annual 5p/kWh Annual cost with low-e double glaze Ground Floor Type Solid concrete Annual 5p/kWh Annual cost with suspended floor Air changes Moderate draughts Annual 5p/kWh Annual cost with low draught house Summary 9 P a g e

10 Heat loss for house with existing insulation Heat Loss (kwh) 5p/kWh Percentage Walls % Roof % Windows/Doors % Ground Floor % Draughts % Total % 10 P a g e